motorboating guitar amp

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motor boating on a twin

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Post by Curranproducer » Tue Feb 17, 2015 12:07 pm

Re: motor boating on a twin

Post by Jana » Tue Feb 17, 2015 12:43 pm

Post by Curranproducer » Tue Feb 17, 2015 4:22 pm

Post by sluckey » Tue Feb 17, 2015 6:07 pm

Post by Stevem » Wed Feb 18, 2015 11:59 am

Post by tony hunt » Wed Feb 18, 2015 12:34 pm

Post by Curranproducer » Mon Feb 23, 2015 1:31 am

Post by tony hunt » Mon Feb 23, 2015 6:26 am

Post by Firestorm » Mon Feb 23, 2015 8:12 am

Post by Mark » Mon Feb 23, 2015 9:11 am

Post by Stevem » Mon Feb 23, 2015 12:31 pm

Post by Curranproducer » Mon Feb 23, 2015 12:55 pm

Post by tony hunt » Mon Feb 23, 2015 2:37 pm

Post by Structo » Mon Feb 23, 2015 3:29 pm

Post by pdf64 » Tue Feb 24, 2015 2:09 pm

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and follow my . Following it will minimize damage due to a miswired amp.

the easier they are to troubleshoot, so keep learning.

--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. . You must before doing any work inside an amplifier chassis. See more .

Power tubes usually wear out quicker than preamp tubes but as they say, "tubular morghulis," "all tubes must die." --they can cause all kinds of problems including weird noises. Reverb driver tubes often have over 400 volts on their plates and die quicker than preamp tubes.

including complete loss of signal, red plating due to loss of bias voltage, weird noises, static, crackle & pop, weakening or thinning of signal output, loss of tube heat, higher than normal plate voltage, zero voltage on the cathode of a cathode biased amp--almost anything. Sometimes you can gently wiggle a tube and listen for noise. You can clean the socket by spraying contact cleaner on a tube's pins and inserting it into the socket a couple of times. See .

.

. Old weak filter caps can loosen up the bottom end and increase voltage sag and note bloom. A 30 year old, weak 25uF cathode bypass cap that has deteriorated to 1uF will trim a lot of low frequencies. To most of us that seems like a problem that needs to be fixed but these "defects" may be the cause of the "perfect tone" this amp gives its player.

With old classic amps I like to discuss this with the client. Do they want the amp brought back to original specs or do they want to keep the current amp tone? This is especially true for well known guitarists. You don't want someone famous telling everyone you ruined their favorite amp.

. No lights, no sound, no speaker hiss.

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. Lights are on or you can hear speaker hiss.

.

.

.

.

.

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(no speaker hum or hiss at all) then the problem can be just about anywhere in the amp but you should suspect a bad tube, blown fuse or the power supply in that order.

Try a new rectifier tube.

Try plugging in a guitar cable, turn up all the amp's volume, gain and master volume controls up a little and touch the tip of the guitar cable's other end. You should hear loud noise. If you hear noise jump to

If the pilot light does not light you can remove the bulb and test it for continuity across the bulb terminals using a multimeter's continuity or "beep" function. A blown bulb will not show continuity.

If none of the tubes show any heater glow (dimming the room lights can help you see the tube glow) the problem is probably with a blown fuse or the power supply.

. They can look good but still be blown.

Many amps have internal fuses mounted on the circuit board or on wires in in-line fuse holders. Any one of these fuses can make the amp silent. Again, test all the fuses for continuity with your multimeter. You should remove a fuse to test it because if left in-circuit you can get a false "good" reading.

If after replacing the fuse the amp blows the new fuse see the section. 

and power it up. A allows you to power up a new or problem amp and limit damage to the amp from miswiring or other defects. If an amp is blowing fuses the limiter will allow you to power up the amp, not blow a fuse and troubleshoot. The limiter keeps the current flowing through the amp low enough to not blow the fuse. .

. If you can't find a 100 watt bulb for your limiter a 150 is the next best thing. You can not use an LED or fluorescent bulb.

if available. If you don't have a standby switch jump to .

I which is normal, .

then a problem is in the power cord, power transformer, heater wiring, fixed bias circuit (fixed bias amps only) or rectifier. These are the only things usually powered in standby.

then the rectifier tube or socket is the problem. Try another rectifier tube and inspect the rectifier tube socket.

check the rectifier tube socket and its wiring. .

. If the bulb glows dim (which is normal) then jump to .

if voltage is over 30v before doing any work inside the chassis.

and tape them off individually so they can't short to the chassis or make contact with one another.

Power up the amp, if the light bulb doesn't glow at all or glows dim then jump to the next paragraph. If the bulb glows bright then the power cord or transformer winding is shorting out internally. Drain the caps and disconnect all three power cord wires, wrap each one in electrical tape and plug in the amp. If the bulb doesn't light then the transformer is shorted internally and must be replaced. If the bulb glows bright the power cord is shorting so try a new power cord.

downstream in the rectifier, 6.3v heater wiring, 5v heater wiring or bias circuit.

Power down the amp and . . Power up, if the light bulb doesn't glow then its downstream wiring is OK. If the bulb goes bright you know there is a problem downstream of the connection in either the rectifier tube socket wiring or in amps with a solid state rectifier the problem can be in the rectifier, filter caps, output transformer or tube socket connections. If your bias circuit is powered by the high voltage taps then it is a suspect too. Unsolder the connection from the last filter cap that supplies the preamp. If the bulb doesn't light the preamp wiring is OK, disconnect the power tube screen connection and test. If it's ok then you probably have a shorting capacitor. Unsolder them and test.

If the high voltage connection tested good power down, drain the caps and connect the wires. Power up and if the bulb doesn't light the heater wiring is OK. If the bulb burns bright then there is a wiring problem in the heater wires, heater artificial center tap or tube sockets. Using a magnifying glass carefully inspect every heater wire connection. If you don't find a problem and your amp has an artificial center tap then replace both resistors.

If the 6.3v circuit tested good power down, drain the caps and connect the that power the tube rectifier. If your amp has a solid state rectifier skip this step and move to the fixed bias circuit. Power up, if the bulb doesn't light then move on to the bias circuit. If the bulb goes bright carefully inspect the rectifier tube wiring--there's a short somewhere.

If the 5v circuit tests good and your amp has a , power down, drain the caps and connect the fixed bias. If the bulb doesn't light the circuit is OK. If it goes bright there is a problem in the bias circuit. Carefully inspect the circuit looking for shorts to ground. If your fixed bias circuit runs wires under the circuit board then that wire may be shorting to the chassis. Test the bias circuit components.

You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads (this is low enough resistance to beep during a continuity test). Your power transformer should measure somewhere near these values and not show an open (mega ohms) or shorted (0 ohms) connection.

. Power down the amp and insert the tubes one by one, starting with the rectifier, then preamp tubes, then power tubes. After inserting each tube, power up the amp and check the light bulb.

(normal) pull that tube and insert the next tube (don't remove the rectifier tube, it is needed to power the other tubes). Repeat until the bulb goes bright.

then power down, it means you have found the problem area.

Replace that tube with a known good tube. with the new tube you had a bad tube and should be good to go.

with the known good tube then there is a short in the circuit near that tube. Closely inspect the tube socket with a magnifying glass for anything that would short two pins or short a pin to ground. Use your meter to check for continuity from socket pin to ground. Read the ohm rating color stripes of all the resistors in the circuit to verify their value. Closely inspect the components that are connected to that tube socket on the circuit board and check them for continuity to ground.

If you can then you know the mains fuse, power transformer, power tubes and output transformer are powered and at least partially working. The problem is probably between the input jack and phase inverter (or driver for single ended amps).

If the amp powers up but is quiet or sounds bad . You should have a spare set of known good tubes on hand so and see if that fixes your problem.

.

If one channel works and the other doesn't you know the problem is in the bad channel between the input jack and where the two channels join.

. Insert a jumper or guitar cable from FX Out to FX In to see if that cures the problem. If the jumper cable does fix your problem then try cleaning all the jack contacts, especially the switch contacts. If that doesn't fix it replace the bad jack.

a very common "no guitar sound" problem is incorrect input jack wiring. , with the amp off, insert a guitar cable into the input jack, then measure the other end of the cable from tip to sleeve for resistance. The Hi jack should measure about 1 megaohm which is the standard value for the input resistor which is connected across the input jack's tip and ground terminals. The Low jack should measure around 136k.

. Visually confirm that the tip and switch terminals separate when a plug is inserted. You can usually bend the terminals so that they close with no plug and open with a plug inserted.

. That will bypass the gain stages and circuitry upstream. If that works you know the problem is upstream of the Return jack.

. Many Fender amps ground out the signal if you plug only one speaker into the Aux speaker jack.

to verify its tone and function. You can also , if the amp sounds good you know you have a bad speaker.

at both ends of the speaker cable. Slide-on spade connectors can make intermittent connection and cause buzz, static and audio drop out. I do not like spade connectors--solder that speaker cable to the speaker terminals because you can blow the output transformer and/or power tubes if that connection breaks.

.

 

Start by pulling the first preamp tube, powering up and seeing if the noise is still present. If it is then you know the noise is being generated downstream of the tube you pulled. Power down, pull the next tube and continue until the noise goes away, then you know the noise is coming from between the last tube you pulled and the previous tube.

You can sometimes identify the problem by waiting for it to occur then carefully spray to cool amp components and solder joints. If spraying a part or solder joint causes the issue to go away or come back you've located the problem.

Weird noises, including squealing, wooshing, clicking, static, honking, motor boating, etc. can be caused by:

so swap out the tubes one at a time for known, good tubes.

can cause rattles, buzzes, rings and other strange noises in combo cab amps. If you connect your amp to an extension cab and disconnect the internal speaker and the noise goes away it's probably caused by a microphonic component or possibly a cab rattle. You can gently tap the tubes with a or wooden pencil with the amp on and listen for excessive noise. You can also put light pressure on each tube using a chopstick or gloved hand and then play the offending note to try to identify the noisy tube. High temperature o-rings placed on preamp tubes can help in combo cabs but metal tube shrouds won't fit over them.

 

. If wiggling the tubes causes noise then put some contact cleaner on the tube's pins and insert them into the socket two or three times. You can also for solid pin contact.

--both ends of the speaker cable. Slide-on spade connectors can make intermittent connection and cause buzz, static and audio drop out. I do not like spade connectors--solder that speaker cable to the speaker terminals because you can blow the output transformer and/or power tubes if that connection breaks.

Try sitting on the cab while playing or have someone put pressure on the baffle board to identify the problem area. Try a separate extension cab to verify it's a cab issue. You can glue and screw small braces to tighten up loose joints. To help troubleshoot you can use a signal generator (or phone ap) connected to the amp and play with the signal frequency to find the resonant tone that causes the rattle or buzz. Once you get a continuous buzz it will be easy to find the rattle using your hands to dampen different parts of the cab. You can also use a to put pressure on amp components to look for a microphonic component like a tube or capacitor.

can be caused by tube arcing or loose grounds. You can sometimes locate an actual arc by opening the chassis and playing the amp in the dark. Look for little flashes of light when the "zap" sound occurs. Look for loose, broken or unsoldered joints, especially on the ground bus and where the bus ties to the chassis.

. Insert a jumper or guitar cable from FX Out to FX In to see if that cures the problem. If it does fix your problem try cleaning all the jack contacts, especially the switch contacts. If that doesn't fix it replace the jacks.

. Dirty pots can cause intermittent contact, signal dropout and static. Most pots have a hole in their shells so you can squirt contact cleaner into them. Run the back and forth through their full travel very quickly several times right after you squirt them.

from the negative feedback circuit. When you start up a new build amp with an NFB loop or replace an output transformer you have a 50% chance of getting a loud squeal from positive feedback (usually not affected by the volume control). Swap the output transformer's primary wires to make the feedback negative. The primary wires run from the output transformer to the power tubes. You can usually swap the secondary wires (speaker jack wires) instead but you can not swap them if you have a multi-tap secondary (4-8-16 ohms for example). . In my 5F6A Bassman it just caused strange noise layered on top of all the notes.

causing capacitive coupling which can cause hum, noise, squeals and oscillation. the amp. You can use a non-conducting wooden chopstick to move wires around with the amp operating and volume full up to find the source of noise. It can also be used to apply pressure to components and solder joints to identify weak or broken components and joints.

(above human hearing) can cause lower than normal amp voltages, lower than normal (or no) output volume and temporary signal dropout. Use chopsticking to find lead dress issues that can cause oscillation. Snubber caps can be used to filter above human hearing frequencies. Temporarily alligator clipping a 1nF (1000pF) or 500pF snubber cap across the preamp tube plate load resistors, one at a time, can help find the problem.

can cause hum, oscillation, squeal and motorboating, especially in high gain amps. You can temporarily alligator clip a new cap in parallel with an existing cap. If the hum or oscillation goes away then replace the old cap.

can cause constant static, crackle and pop noise. Sometimes putting some pressure on the resistor with a chopstick or spraying it with freeze spray will change the noise and help you identify the bad resistor. I like to use 2 watt resistors for all preamp plate resistors to lengthen their lifespan and reduce amp hiss.

A bad 1M input resistor (usually mounted on an input jack) will allow the first gain stage grid to float in the short time between the jack shorting switch opening and the cable plug making full contact. You can test this by opening the input jack switch, if it squeals the input resistor is bad. Measure the resistance across the resistor and if it measures good re-flow its solder joints and check the input jack ground.

is sometimes caused by a weak component or bad solder joint that is affected by heat. You can sometimes identify the issue by waiting for the problem to occur then carefully spray to cool amp components and solder joints. If spraying a part or solder joint causes the issue to go away or come back you've located the problem.

(amp sounds like a boat engine) is usually caused by a weak or damaged power supply filter cap. You can temporarily alligator-clip an extra filter cap in parallel with the caps, starting with the first filter cap. If the temporary cap fixes the problem then replace the original cap. Also verify the power supply voltage dropping resistor values.

are caused by a noise mixing with a note to create a harmonic note that shouldn't be there. Hum, oscillation and other noise can cause them.

can cause a multitude of problems. Many classic Fender amps from the '50s to the '70s were made of a material that will hold moisture and conduct electricity from one eyelet to another. This can cause all sorts of problems and noise. Some old circuit boards were coated with wax to fight moisture but after 60 years the wax isn't holding up. You can sometimes measure voltage from a high voltage eyelet to the circuit board immediately around the eyelet to check for conduction. You can use a hair dryer to dry the circuit board for a temporary fix and troubleshooting. Replacing the circuit board is the best long term fix.

Nasty sounding guitar audio can be caused by many things including:

It can be caused by a dying tube but the typical culprit is an interaction between two wires. Ultrasonic oscillation (above human hearing) can block out guitar audio and silence the amp. Chopstick the amp wires, especially the tube socket wires and separate the grid and plate wires. Do this while the amp is in oscillation to see if you can get it to stop--if it's ultrasonic oscillation then feed a signal into the amp so you'll know when the oscillation stops. Use a "filter cap on a stick" (500pF cap with a ground clip on one end) to filter all the tube grids to see if you can kill the oscillation. When you find the guilty tube you can add a filter to the grid or bypass its plate load resistor with a cap. Use the smallest cap that solves the problem for the least side effect.

so swap out the tubes one at a time for known, good tubes.

. Insert a jumper or guitar cable from FX out to FX in to see if that cures the problem. If it does fix your problem try cleaning all the jack contacts, especially the switch contacts. If that doesn't fix it replace the jacks.

. Dirty pots can cause intermittent contact, signal dropout and static. Most pots have a hole in their shell so you can squirt contact cleaner into them. Run the pot back and forth through its full travel very quickly several times right after you squirt them.

. Applying pressure with a chopstick and hearing noise can identify a bad joint.  can also help identify bad solder joints by cooling them and causing a change in noise.

are caused by a noise mixing with a note to create a harmonic note that shouldn't be there. Hum, oscillation and other noise can cause them.

. The speaker voice coil rubs its surround. Typically happens during loud, low notes. Try another speaker to identify a bad speaker. Sometimes mounting the speaker upside down will stop the cone from rubbing. You may have to have the speaker re-coned to get rid of cone rub.

--both ends of the speaker cable. Slide-on spade connectors can make intermittent connection and cause buzz, static and audio drop out. I do not like spade connectors--solder that speaker cable to the speaker terminals because you can blow the output transformer and/or power tubes if that connection breaks.

For a new or recently repaired amp using the can cause low output or funky audio. Verify the value of all resistors and caps.

. Component leads can break and wires can pull loose. Resistors can crack yet look perfectly normal. components and wires can help identify the loose connection or bad component.

can cause a multitude of problems. Many classic Fender amps from the '50s to the '70s were made of a material that will hold moisture and conduct electricity from one eyelet to another. This can cause all sorts of problems and noise. Some old circuit boards were coated with wax to fight moisture but after 60 years the wax isn't holding up. You can sometimes measure voltage from a high voltage eyelet to the circuit board immediately around the eyelet to check for conduction. You can use a hair dryer to dry the circuit board for a temporary fix and troubleshooting. Replacing the circuit board is the best long term fix.

:

. This is a common cause of a weak sounding amp. When a power tube blows it can short the plate and screen and cause the screen resistor to burn and blow. A blown or cracked resistor can look normal--gentle pressure from a chopstick can help find a bad resistor. Measure the resistance across the resistor.

. A leaking cap can cause scratchy pots and affect preamp and power tube bias. Measure the DC voltage on tube grids and look for anomalies.

. Measure the resistance across the resistor. A shorted resistor will show very low or even 0 ohms.

. Applying light pressure to the resistor and hearing noise can sometimes identify a bad resistor. Measure the resistance across the resistor. An open resistor will show a very high resistance well beyond the resistor's rating.

If the output transformer shorts between windings it can lead to no output, weak output or funky sounding output. You can test the transformer by measuring the resistance between its leads. and remove the rectifier and power tubes before making the following resistance measurements:

For push-pull transformers you should see approximately the same resistance between each secondary wire at each power tube and the center tap--typically somewhere between 10 to 200 ohms.

A will have much lower resistance compared to the other winding and typically measure at less than 10 ohms.

The will often measure less than 1 ohm between all the secondary leads so it is difficult to detect a shorted secondary.

If a transformer (break) it will show a very high resistance between the primary leads or between the secondary (speaker) leads (typically 500k or higher).

A will typically show less than 10 ohms resistance between the primary and secondary leads.

Measure the resistance between all the leads and chassis ground--low resistance of less than 10 ohms indicates a .

For comparison my healthy Hammond 125C push-pull output transformer measured: red center tap to brown 150 ohms, red center tap to blue 116 ohms, brown to blue 268 ohms. All secondaries measured .2 to .6 ohms between them. A Hammond 125GSE single-ended output transformer measured 53 ohms between the two primary wires. All of the secondaries measured .2 to .3 ohms. Both transformers measured an open circuit (no connection) between their primary and secondary wires and between all wires to the transformer outer metal shell.

If any of the above faults are present your best bet is to just replace the output transformer. For valuable vintage transformers it is possible to have them rewound.

  .

because noise is often caused by dirty power (perhaps caused by a refrigerator's compressor motor) or radio frequency interference (RFI) caused by a cell phone, noisy light dimmer, fluorescent lights or other electrical equipment. Before taking the cover off your amp try it in another location, preferably in another building.

An input jack whose switch tab isn't making contact when no guitar is plugged in can cause loud hum or buzz (see jack picture above). Try pushing the switch tab closed and see if it kills the noise. amp noise.

Try sitting on the cab or putting pressure on different parts of the cab, speaker baffle and speaker frame (it helps to have a friend play while you prod).

can cause 50 or 60Hz hum from heater-cathode leakage but can also cause 100 or 120Hz hum so swapping out every tube should be done before going any further. .

After trying different tubes the next step is to Buzz has a sharp tone to it where hum has a smooth sound. A "buzz" is caused by noise that can be seen on an as a waveform with sharp spikes.

In the United States hum and buzz usually comes in two frequencies, 60 and 120Hz (Hz means cycles per second). If your power runs at 50Hz like in Europe you will have 50 and 100Hz hum and buzz. Determining the frequency of the noise will help you track down the source. See at 2:15 for samples of both types of hum. 50 and 60Hz hum usually comes from the power transformer circuit, fixed bias circuit or heater wires. 100 and 120Hz hum comes from the power supply after the rectifier.

. You can clean them by spraying contact cleaner on a tube's pins and insert it into the socket a couple of times.

. Touching the tubes gently with your chopstick or a gloved hand while playing something that makes it buzz can help you identify the bad tube.

--both ends of the speaker cable. Slide-on spade connectors can make intermittent connection and cause buzz, static and audio drop out. I do not like spade connectors--solder that speaker cable to the speaker terminals because you can blow the output transformer and/or power tubes if that connection breaks.

. The tape will need to make good contact with the chassis for it to act as an RFI shield. For the 5E3 Deluxe and other tweed amps the tape would be applied to the inside of the wooden back panel to cover the chassis opening. For more modern Fender amps you would apply the tape to the underside of the top of the cab.

by minimizing pulsing 120Hz ground return current.

If your amp has an unused triode you should connect the unused plate, cathode and grid to ground.

You can use a non-conducting wooden chopstick to move wires around with the amp on and the volume up to find the source of noise and identify lead dress problems. On my first amp build I had the V1A plate and grid wires lying on top of one another which created a moderate hum. Simply moving those wires apart made the amp almost silent. A chopstick can also be used to apply gentle pressure to components and solder joints to identify weak or broken components and joints.

and signal generator to track down the source of a difficult to identify hum or buzz. See the for more info.

50 or 60Hz is sometimes caused by power line noise and can be addressed by applying a small, high voltage filter cap across the high voltage rectifier input wires. This works well for this. I usually solder one across the rectifier tube socket where the two high voltage power transformer wires connect. For a solid state rectifier solder the cap across the high rectifier inputs.

Buzz can also be caused by AC components. I had a nasty 60Hz buzz that was caused by the placement of a 120v pilot light too close to the amp's signal wires.

Radio Frequency Interference or RFI can sometimes be heard as a 50 or 60Hz buzz. Adding copper or aluminum foil tape to the back of a chassis cover can help reduce RFI. Moving sources of RFI like cell phones, light dimmers and fluorescent lights away from the amp can also help.

100 or 120Hz buzz can be caused by a bad tube so swap in a new set of tubes. Buzz can also be caused by a noisy rectifier. This can usually be eliminated by placing in parallel with each rectifier diode. If the rectifier is inside a housing then running caps from the two rectifier inputs to the + and - terminals usually works well.

50 or 60Hz usually comes from the power transformer or its wiring, the fixed bias circuit, the tube heater wires or from external RFI caused by fluorescent lights, dimmers and other sources.

Assuming you've tried the amp in a different location, 50 or 60Hz hum must be generated by the power transformer circuit or the fixed bias circuit (if your amp has one). Keep as much distance from the power transformer's wires and the amp's wires and circuitry as possible. Heater wires should be twisted and the untwisted wire that goes to each tube pin should be as short as possible with no big loops. Signal wires should cross the heater wires at 90 degrees to minimize coupling.

. If your heater voltage is higher than approximately 6.6v it would be a good idea to reduce the heater voltage to 6.1 to 6.5 volts. See this link for information on .

If your amp's power transformer has no 6.3v heater center tap then the amp needs an . Verify there is an artificial center tap installed and check the resistance of its two resistors. A missing 6.3v center tap or a burned out resistor will usually cause loud hum.

To troubleshoot the fixed bias circuit verify its ground connections are good and you can check the caps by temporarily alligator clipping another cap in parallel with the circuit's caps. If adding the cap reduces hum then replace the cap.

. A Humdinger pot allows you to adjust the resistance between the two 6.3v heater lines to ground to achieve minimum hum. You adjust it by ear with the amp turned up to max to hear the hum best. You can also connect a Humdinger's ground connection (wiper) to a cathode biased amp's cathode resistor to elevate the heater ground reference for even more hum removal. See this link for .

Push-pull amps should have their power tubes wired in phase so the tubes can use common mode noise rejection. For in phase wiring each heater wire should connect to the same heater pin on the two power tubes. This is why it's a good idea to use two colors of heater wiring so you can keep the phase correct.

Parallel power tubes should be wired out of phase to help cancel hum between the power tubes.

so swap out the tubes for known, good tubes.

An that doesn't ground out when nothing is plugged in. If the jack's shunt switch does not make good contact with the jack's tip connector you will get loud hum when no guitar is plugged in but the amp will sound fine when you plug in a guitar.

. Electrolytic filter caps have a typical lifespan of 15 to 20 years. . If the new cap reduces hum then replace the original cap. If your amp is fixed bias don't forget the fixed bias circuit filter cap(s).

. Loud hum can be caused by components that should be grounded but aren't. Forgetting to solder a volume pot's ground wire or a cold solder joint on a cathode resistor's ground are two common causes of loud hum. probe all the amp's ground connections to see if I hear an improvement. Touch the ground probe to all volume pots' ground terminal, the ground side of preamp cathode resistors, etc. .

. If your input jacks are grounded to the chassis and you also run a ground wire between them then a ground loop is formed (chassis is one side, the wire is the other) which can act as an antenna and pick up RFI noise and hum. You will also form a ground loop if you use shielded cable inside the amp and ground both ends. You should only ground one end of shielded cable.

(wires too close to one another causing capacitive coupling) the amp's wires around with the amp on and the volume up to see if you can decrease the hum. Pay close attention to the tube grid and plate wires--keep them as far apart from one another as possible. My first amp build had a loud hum because the first preamp stage's grid and plate wires were sitting on top of one another. Separating the wires silenced the amp.

After I finished testing and tweaking the prototype in bare circuit boards I mounted them in a Hammond blank chassis. When I fired up the newly mounted amp I had a nasty buzz at max volume that wasn't there before I put it in the chassis. It didn't sound like smooth 60Hz heater wire hum or 120Hz power supply ripple hum so the investigation began.

The amp with no guitar cable plugged in was absolutely silent at max volume. The buzz could be controlled by both the volume and master volume controls. The amp's tone control could almost entirely eliminate it when turned full down. . Hmmm. The buzz didn't change with guitar movement or when touching the guitar's strings or grounded metal bridge.

--somewhere between the guitar and the volume pot.

. I tried another cable--no change. I tried my --no change. I tried a cable with no guitar connected and got the expected crescendo of noise.

that light the basement workshop. I turned them off and the amp looked cool in the dark as it spewed forth The Buzz.

. I diligently moved the amp to another part of the house and tried again--no change.

Time to get into the chassis. My first thought was a so I alligator clipped a wire to the chassis and carefully probed all the ground connections in the amp to see if I heard an improvement but found no success.

. I did find that the input jack wires that ran by the power switch and across both circuit boards were picking up some 60Hz hum from the power switch. I replaced the input jack and headphone jack wires with RG174 coax which eliminated the hum but had no effect on The Buzz.

. I injected a 500Hz .1v signal into the amp's input jack. I connected the oscilloscope's ground probe to the preamp ground and connected the probe to the input jack tip. I zoomed in on the 500Hz signal and played with the tone control to change the volume of The Buzz but couldn't see any signal artifact change with the guitar tone change. I jumped to the speaker output jack and tried again with no success. Next I tapped into V1A's output just after the coupling cap (to avoid high voltage DC on the probe) and repeated the tone change and again I couldn't see any change on the oscilloscope. . I zoomed in the scope's display and confirmed it was THE BUZZ.

I used the scope's horizontal scan knob to move the waveform left and right so I could measure the time between spikes--16.5 milliseconds. I took the reciprocal of 16.5ms (1/.0000165) to get the frequency and the answer was--60Hz, it was wall power related after all, or so I thought. I pulled out my trusty H&K Tubemiester 5 and plugged it into the same wall power socket and fired it up--no buzz. I figured the H&K might have some power line filtering built in so that test proved nothing.

I have seen some amps with a .01uF 3KV (3000V) disk capacitor across the rectifier tube socket's high voltage input pins to pre-filter high freq noise so I gave that a try. It did help a little but THE BUZZ still lived.

I sat there staring into the amp's chassis wondering what it could be. I noticed the 120V pilot light (not a standard 6.3V light) sitting next to the input jack. This was my first build using a 120V light so I had no experience with them. I hauled the amp back to the solder station and de-soldered the light's neutral wire and fired up the amp-- . It was that damn noisy bitch of a pilot light.

The fact that the amp was silent when no guitar cord was plugged in was a big clue here that I missed. With no cord plugged in the circuit from the input jack to V1A's grid is grounded so THE BUZZ was being shunted to ground. I should have focused the investigation on the input circuit from the start. 20/20 hindsight and all that. . .

When I stuffed the prototype B9A boards into the chassis I put them to the far left to make room to mount the power transformer on the far right. After getting the boards mounted I realized I had no front panel real estate on the left side of the chassis away from the inputs and preamp stages. actually and I had to pay a penance to atone for my sin.

The little prototype is now nice and quiet and just sounds killer.

 

--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. . .

and flashlight to help spot bad or broken solder joints.

. Dimming the room lights can help see the glow. Most amp pilot lights are powered by the heater circuit, but not all so don't assume a lit pilot light means the tubes have heat. If a tube isn't lit measure the AC heater voltage from heater-wire-to-heater-wire, it should be around 5.7v to 6.9v which is 6.3v +/- 10%.

My next step is to starting at the rectifier output and filter caps (B+1, B+2, B+3). It's good to have a baseline voltage chart for your amps so you know what voltage to expect. A higher than normal B+ reading can be caused by a non-functional tube that's not drawing current from the power supply. A lower than normal reading can be caused by a tube that is pulling too much current which can be caused by a short, bias problem or in new build amps an incorrect component value.

there is a break in the power supply. Back up toward the power supply and find where there is voltage and where there is no voltage and look for the cause. A blown power resistor between the filter caps is a common cause of a break in the flow of voltage.

Next measure the voltage at the power tube socket closest to the power transformer. For octal power tubes (8 pins) I look for 5.7 to 6.9v heater voltage between socket pins 2 and 7.

: The power tube plates can have over 500 volts DC on them.

The power tube pins 3 (plate) and 4 (screen) should have high voltage DC and pin 5 (grid) should pop when probed with the meter. For fixed bias amps the grid should show a negative voltage but cathode biased amps will indicate near 0 volts on the grid. Pin 8 (cathode) will show 0 volts in fixed bias amps or show voltage between 10 to 25 volts DC in cathode biased amps. Nine pin power tubes like the EL84 have different pin functions so Google their data sheet to see their pin functions.

. . Next measure the voltage at every pin of the troubled tube's socket for clues to the problem.

can be caused by no electron flow through the tube. This can be caused by no heater voltage or a disconnect between the tube cathode, the cathode resistor (if used) and ground. High voltage on the plates can also be caused by a higher value cathode resistor like using a 15k resistor instead of the specified 1.5k.

is caused by too much current flow through the tube. This can be caused by an incorrect bias voltage on the grid. A leaking coupling cap can let DC voltage through to the grid and heat up the bias current. A lower value cathode resistor can also allow too much current to flow through a tube.

. If there is voltage on the plate but the cathode is zero this can be caused by a bad cathode resistor connection or no heater voltage. With the amp off measure the resistance from the cathode pin to ground. It should equal the cathode resistor value.

should show the cathode as connected to ground and show a negative voltage on the grid, usually between -30 to -50 volts DC. An incorrect grid bias voltage can make an amp sound bad.

Most preamp tubes should show no voltage on their grids (12A*7 tubes' pins 2 & 7) except tubes that function as phase inverters or cathode followers where voltage on the grid is normal. If the preamp tube's grid leak resistor is tied directly to ground it should not have any voltage on it. If there is voltage on the grid it's usually caused by a leaky upstream coupling cap (that must be replaced) or grid current. To verify the source of grid voltage you can pull the tube and see if the voltage on the tube socket grid pin goes away. If the grid voltage is still present then it's coming from a leaking capacitor or a wiring mistake. No voltage on the grid pin means the unwanted grid voltage was being generated by grid current. Try another tube and look for incorrect component values or wiring mistakes associated with that tube circuit.

. When a power tube blows it often shorts the tube's plate to the screen which results in a blown screen resistor. A blown screen resistor will normally fail open with infinite resistance. The amp can operate with one power tube but it will sound weak and funky. When you replace the blown tube it will still function poorly because the blown screen resistor will not allow any voltage to the screens so the amp will sound different than with a blown tube but still not sound good due the huge output imbalance between the good tube and the tube with no screen voltage. Verify the screen voltage and the resistance across the screen resistor.

If the first power tube checks out then which will be another power tube for push-pull circuits or the driver tube for single ended amps. so you can listen for a pop when probing the preamp tubes' grid.

. . Carefully check the voltages of that tube to find clues to why it's not working.

4 and 9, and 5 and 9 and verify you have voltage on pins 1 and 6 (plates), 3 and 8 (cathodes). . Nine pin tubes should have 0 volts on their grids unless the triode is being used as a phase inverter, cathode follower or uses grid bias.

If you have unexpected voltage on a tube's grid you may have a leaky coupling capacitor upstream allowing DC voltage through to the grid or a bad tube generating grid current.

. Too many electrons pound on the metal plates, heat them and make them glow red.

A bad tube can red plate so try another tube.

.

The tube's bias voltage, which is the voltage difference between the cathode and grid, controls the current flow through the tube.

If your amp has adjustable bias then measure the voltage on a power tube grid (usually pin 5 for 8-pin tubes) and adjust the bias pot for the largest possible negative voltage (usually around -60v DC) for the coolest bias. If this solves the red plating problem then and see if the problem remains solved.

If the power tube cathode has a connection problem the amp will stop passing current so the cathode usually doesn't cause bias and red plating problems so .

The fixed bias circuit is pretty simple so we want to closely inspect every solder joint and component with a magnifying glass.

Fixed bias amps need between -30 to -50 volts DC on the grid pins.

With the power tubes removed, power up the amp and measure the power tube grid voltages on all the tube sockets. This is pin 5 on typical 8-pin tubes.

If your amp has adjustable bias then set the highest negative voltage possible, usually around -60 volts. This will give us the coolest bias possible.

If you can't set a bias voltage between -40 to -60v DC then you may have a just upstream of the power tube grid. Un-solder one end of the coupling cap and see if you can set a bias voltage of -40 to -60v. This is especially true if you get different bias voltages on the power tube sockets. If nothing changes after lifting the coupling cap then the cap isn't the problem.

In the bias circuit we should have AC voltage where the power transformer connects to the circuit. We should have negative DC voltage after the bias diode. Move downstream and follow the negative DC voltage all the way to the tube socket. If you find a break in the voltage then look for the cause.

A bad solder joint, cracked resistor, dead, disconnected or reversed filter cap and bad bias pots can cause bias problems. A short to chassis will also kill the bias voltage. Gently push on bias components with a chopstick and look for movement due to loose solder joints or component cracks.

A loose socket grid pin can prevent bias voltage from getting to the tube which will cause instant red plating. Re-tension the tube socket pins.

The reverb driver tube is pushed very hard with high voltage in many amps so the tube tends to burn out sooner than all the other small tubes. A bad tube can also cause excessive hum in the reverb. with known good tubes and test the amp.

and cause hum, pops, crackling, weak or no reverb. Clean the connectors (male and female) with contact cleaner. Spray the male connectors and insert them into the female connectors several times. If the connectors are heavily corroded a toothbrush, small brass brush or steel wool may be needed to get them clean.

. They can cause intermittent signal drop out.

Mixing up the reverb input and output cables will result in . Swap the cables at the tank and try again. If it doesn't help or lowers reverb volume then swap the cables back.

and they are difficult to repair. Replace the tank if any of the wires are broken. I'm a fan of the MOD tanks for around $20.

If you with the power on and reverb turned up and hear the reverb spring crash you know the reverb tank's springs, output transducer and reverb recovery amplifier are functioning (the second half of the reverb circuit).

You can and bang on the amp again. If you hear the spring crash (it will be quieter this time) you know the input transducer is working. After this test you will know the problem is upstream of the tank, which is the reverb driver circuit including the reverb transformer. Swap the tank cables back to normal.

by verifying the voltage on the driver tube plate pins. For blackface and silverface amps the voltage should be around 400VDC (volts DC). If there is no voltage on the plate then try replacing the reverb driver tube. If that doesn't fix the problem then replace the reverb transformer.

by connecting an 8 ohm speaker to the reverb transformer secondary wires. When you play through the amp you should hear a fairly loud clean guitar signal through the speaker. If not, then the problem is probably the reverb driver tube or the reverb transformer.

. If they are loose put some glue on them to hold them solid or just replace the tank. MOD replacement tanks are my favorites.

can help cure reverb problems, especially those that occur at high volume and high reverb settings.

can sometimes quiet a noisy reverb. See .

due to positive feedback between V4A and V4B (Vibrato channel third gain stage).

A bad pan ground can cause the (add dirt). Check cables and cable plugs and sockets.

Try turning the reverb pan 180 degrees to reduce interaction between the power transformer and the pan's output transducer.

There are many reverb pan grounding schemes and an incorrect set of cables can lift the pan's ground or cause a ground loop. Look at your amp's schematic to verify what part of the tank and cables should be grounded and what shouldn't.

Temporarily clip a ground connection to the tank and see if that helps with the hum.

See for more info.

" the Amp

. On my first amp build I had the V1A plate and grid wires lying on top of one another which created a moderate hum. Simply moving those wires apart made the amp almost silent. . You can also lightly tap on tubes to identify microphonic tubes. Don't forget to chopstick the speaker cable connections.

Intermittent problems are sometimes caused by a weak component or bad solder joint that is affected by heat as the amplifier warms up. You can sometimes identify the issue by waiting for the problem to occur then carefully spray to cool amp components and solder joints. If spraying a part or solder joint causes the issue to go away or come back you've located the problem.

For really tough troubleshooting tasks I and inject a 500Hz 100 milliamp (.1 amp) signal at an amp's input jack and by probing the tube grids. Start at the input jack and work towards the speaker and watch for the wave shape to disappear or change shape. There are tone generator apps for your phone but you will need an adapter with a 1/4" mono TS plug to plug into the amp's input jack. You will risk your $600 dollar phone every time you plug in though. You can play a single string on a guitar to generate a waveform but it's kind of a pain in the butt and a guitar's harmonics and timbre will make it harder to spot anomalies on the scope.

I also use a dummy load when doing this test so I don't have to listen to the damn tone coming through the amp's speaker. unless you use a high voltage rated probe. The grids normally have zero or low DC voltage so I take my signal sample from the grids unless I really need to examine the plate signal.

. It's safest to connect the scope probes to the amp chassis with the amp's power off.

Examine the amp from amp input to output and look for the tone generator or guitar signal to change shape (distort) or disappear (loss of all sound).

To examine the guitar or tone generator signal before amplification clip the scope probe onto the input jack center conductor and the probe ground clip to the preamp ground bus. This will be the smallest, lowest voltage signal you will encounter. Amplification through the amp will make the signal larger (higher voltage).

Adjust the scope time and voltage setting knobs until you can see at least one full wave on the screen. If you set the scope's trigger line to just above zero volts the signal wave should stabilize on the screen (the trigger line needs to touch the on screen signal to stabilize the signal for viewing).

To see the signal at the first gain stage grid move the probe to the tube socket grid pin. Tube grids are usually at zero volts DC (phase inverters and cathode followers can have up to 80v DC) so grids are safer to probe than plates which can have over 500v DC on them.

Move the scope probe to the second gain stage grid and increase the scope's voltage setting knob to shrink the signal to fit on the scope screen.

When probing the power tubes move the scope probe ground clip to the power amp ground.

Pay attention to the max AC & DC voltage ratings of your scope and probe. This is especially important when probing tube plates with high voltage DC on them.

Set the scope to AC coupling. This inserts a cap between the probe and scope circuit to keep high voltage DC out of the scope.

Set the X10 switch on the probe if needed for higher voltage signals. With the X10 switch on a 100v signal will show as 10 volts on the screen.

Clip the ground clip to an appropriate ground point with the amp turned off. For example use the preamp ground bus or last filter cap ground when probing the preamp and use the power amp ground or first filter cap ground when probing the power amp.

When chasing an oscillation or other signal artifact it can be helpful to disconnect the global negative feedback because NFB will try to correct and minimize the oscillation or artifact.

 

The very strong tremolo oscillator signal can induce ticking into the amp's signal stream. .

Another option is to use a shielded cable like RG174 for the tremolo driver plate wire (V5 pin) to stop the transmission of the tremolo signal. This is almost a 100% fix for tremolo ticking. Ground the cable shield at only one end (either end) to prevent a ground loop.

If the above don't work then adding a .02uF cap to the tremolo roach should stop it.

.

To test a diode make sure the circuit is powered down (drain the caps) and measure the diode's resistance both ways across it (measure, then reverse the meter probes, measure again).

A good diode will show resistance of 7 to 15 ohms in one direction (forward resistance) and almost infinite resistance in the other direction (reverse resistance).

Reading an open circuit (infinite resistance) both ways means the diode is open.

A zero resistance reading in either direction means the diode is shorted.

A diode with a resistance reading below 7 ohms should be replaced.

Measuring a series of diodes in a row should show about 12 ohms of resistance per diode in one direction and infinite resistance in the other.

The striped end of a diode is the cathode end, the other end is the anode.

donation to

 

Merlin Blencowe, , 2nd Edition.

 

Merlin Blencowe,

 

Morgan Jones, , 4th Edition.

 

Richard Kuehnel, , 3rd Edition.

 

Richard Kuehnel, , 2nd Edition.

 

Richard Kuehnel,

 

Robert C. Megantz,

 

It's fairly technical but it's the only book written specifically about guitar amplifier overdrive. It includes many graphs to help make the material easier to understand.

 

T.E. Rutt,

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• ► LM380 guitar amp has a pulsating sound at high volume level only, how can I fix?

LM380 guitar amp has a pulsating sound at high volume level only, how can I fix?

Started by Jasonmatthew911, April 09, 2011, 10:14:55 PM

Jasonmatthew911

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• Jason B. - Rep. of Panama
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Quote from: Gurner on April 11, 2011, 04:32:54 AM PRR has hit the nail on the head....motorboating is the problem (google it - nautical hits aside, there are a hole heap of articles on how to cure it) Bottom line you need to ensure an ripple on the PSU rail caused by your output stage .....doesn't find its way back into your preamp stage (ie via the same PSU rail) - else you've got a positive feedback loop going on (eg  output stage draws large current at higher settings, ripple on supply rail increases, this is seen by the preamp psu rail (since it shares the same rail as the output stage), this psu ripple then 'bleeds' into the audio signal coming out of the preamp, which then gets amplified by the preamplifier itself & then the power stage again.....ripple increase on PSU rinse, reapeat.) a Low pass filter between your output stage PSU rail and your preamp PSU rail is the likely solution....but if you want to get quick and dirty.....ramp up those 100uf & 2,200uf caps to the largest you can lay your hands on!

Quote from: PRR on April 09, 2011, 11:32:12 PM > a 10 ohms resistor .... cap into the pin of the LM380 chip Change that 10 ohms to 470 ohms. See if preamp supply voltage falls much. See if it motoboats less.

Quote from: Jasonmatthew911 on April 13, 2011, 01:46:08 AM What about putting a 10 ohm resistor in parallel with a ferrite at the output of the power amp, after the 2,200uf cap that goes to the speaker, could this help maybe?

Quote from: Gurner on April 13, 2011, 05:16:25 PM I've just had a look at the datasheet (I'm not familiar with the LM380)... "The output is short circuit proof with internal thermal limiting ." ...what that means is, if the chip starts getting to hot, the chip will self protect - your heatsink arrangement is not the source of your problems (nor would I be going and adding ferrites to the output of the amp as per your post a couple back) I don't know what more to say...clearly the problem is motorboating...the only way to solve it is to revisit your circuit's PSU rail configuration. When I mention an LPF earlier, I meant something like this... http://www.techlib.com/electronics/audioamps.html (look at the diagram called "4-Transistor Amplifier for Small Speaker Applications" - the 470 Ohm resistor & the 10uf cap to the left of it are an LPF ...the purpose is to take out any ripple generated by the high current draw of the output stage finding it's way back into the preamplifier stage.

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Quote from: Jasonmatthew911 on April 14, 2011, 03:39:45 PM Quote from: Gurner on April 13, 2011, 05:16:25 PM I've just had a look at the datasheet (I'm not familiar with the LM380)... "The output is short circuit proof with internal thermal limiting ." ...what that means is, if the chip starts getting to hot, the chip will self protect - your heatsink arrangement is not the source of your problems (nor would I be going and adding ferrites to the output of the amp as per your post a couple back) I don't know what more to say...clearly the problem is motorboating...the only way to solve it is to revisit your circuit's PSU rail configuration. When I mention an LPF earlier, I meant something like this... http://www.techlib.com/electronics/audioamps.html (look at the diagram called "4-Transistor Amplifier for Small Speaker Applications" - the 470 Ohm resistor & the 10uf cap to the left of it are an LPF ...the purpose is to take out any ripple generated by the high current draw of the output stage finding it's way back into the preamplifier stage.

Quote from: thedefog on April 15, 2011, 10:01:17 AM I was just wondering if you were using a star ground and had some sort of shielding going on with your build. I had a similiar problem with my Tiny Giant build that was due to a poor grounding scheme at the power supply input that cleared up after I redid the ground network.

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• Merlin, Lancashire UK
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Quote from: Jasonmatthew911 on April 12, 2011, 01:37:22 AM Here is exactly what I have  in the supply right now, starting from where the power goes into the pre-amp: ... then goes into 4580 IC and continues to power section, starting with .1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 2200uf electrolytic to GND, then goes directly into LM380....

Quote Ok, I tried the 470 ohm resistor on the power supply rail between the pre-amp stage and going into the power amp stage, the other leg of the resistor going into the (+) of the 10uf cap, and the (-) to GND...

Quote from: merlinb on April 15, 2011, 04:32:56 PM Quote from: Jasonmatthew911 on April 12, 2011, 01:37:22 AM Here is exactly what I have  in the supply right now, starting from where the power goes into the pre-amp: ... then goes into 4580 IC and continues to power section, starting with .1uf cap to GND, 10 ohm resistor w/ ferrite in parallel, 2200uf electrolytic to GND, then goes directly into LM380....

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Motorboating Silvertone 1484 Guitar Amp

• Thread starter micajah
• Start date Mar 31, 2014

• Mar 31, 2014

Ok all you guitar amp people - Trying to finish my 1484 refurb. When I got it it was missing the output transformer. I replaced it with a new Weber 1484 direct replacement transformer. I then replaced all of the electrolytic caps including the multi can (re-stuffedit to save space) and the drifted resistors as well as a couple of the caps that looked suspicious. I added a grounded 3-pin AC cord and disconnected the unnecessary ground switch. Replaced a bad 8 pin socket. Deoxed the other sockets as well as all the pots. Replaced the input jacks with switched and isolated jacks - grounding them back to the preamp tube with shielded wire. Also added a switchable 4/8 ohm output jack. Now I have a motorboat oscillation that varies from sub-audible to treble depending on the tone controls. This occurs on both channels when I turn them up. I can hear the amplified input but the motor boating is very strong overwhelming. Is it possible that the choke is bad? - it's the one thing I didn't check so far. Im going to go over the work I did one more time to make sure I didn't get something wired incorrectly. Any suggestions where to start? Thanks in advance.  

Slowly Speeding

micajah said: When I got it it was missing the output transformer. I replaced it with a new Weber 1484 direct replacement transformer. Click to expand...

junk junkie

ditto RWood's suggestion. sounds like the leads are flipped. Also verify the secondary connections. If the lead thats supposed to be grounded is actually the positive where the feedback connection is, that will do the same thing. all this assumes that it does in fact have a feedback connection. Got a schematic ?  

nashvillebill

Super member.

• Apr 1, 2014

No global NFB on the 1484: http://bmamps.com/Schematics/Silvertone/silvertone1484.pdf  

Epic Member

I've even seen some comments in guitar amp build/rebuild threads that replacing standard wire with shielded can cause problems in some situations due to the change in capacitance.  

"Hard At Play"

greetings micha, A wise suggestion will require you to varify your preamp, top to bottom, which can be known to be the culprit of the motor boat phenomenon. Performing a thoroughly analyzed evaluation which is an absulute must when proving your preamp stage, and a standard testing procedure through out the rest of the circuit stages within the amps total circuit path. It's also an absolute must to verify your power supplies Filiment & B+, making quite sure that your caps & resistors are up to snuff, and that the all-mighty-magical-rubber-ducky is alive still and well.... These are standard procedures that will assure that you did not miss a single misshap, thus thwarting any possible migrains that might arise. Cheers...Zandru....  

Fish fingers and custard!

You may have created a ground loop with your shielded wires. Usually motorboating is caused by the supply being modulated by the audio signal. To check that, you need to look at the supplies with an oscilloscope to see if they are moving around with the motorboating. If you don't have a scope, try lifting the grounds on one end of the shielded cables. Lift them on the output of the cables.  

dr*audio said: You may have created a ground loop with your shielded wires. Usually motorboating is caused by the supply being modulated by the audio signal. To check that, you need to look at the supplies with an oscilloscope to see if they are moving around with the motorboating. If you don't have a scope, try lifting the grounds on one end of the shielded cables. Lift them on the output of the cables. Click to expand...

• Apr 2, 2014

Ok- retensioned and cleaned the 9 pin sockets - got rid of the snap crackle pop when I tapped the pre-amp tubes but motorboating is still there. Moving on.  

Also tried reversing the OT primaries - made no difference.  

Is the motorboating affected by the tremelo controls?  

OK - tracked the motor boating problem down - When I bought this amp it was missing the OT. And it was obvious that someone else had done some fixin' - some of the caps were already replaced etc. I ASSumed that they were all replaced correctly. Well on V1, the electrolytic between the cathode pins 8 & 3 was hooked up not to a ground via a terminal strip, but to the strip connector next to the grounding connector which was a leg of the power supply. Wont ASSume anything is correct again (maybe that was why the OT want bad.) Switched it to the correct connector and now I have no more motor boating. In fact the amp is quiet. No hum or buzz. Too quiet. Another problem to track down. Input one can just barely be heard turned all the way up with the guitar all the way up. This is obviously not right. Input two is much louder - but not what I expect when I turn it all the way up. Seems like it should be louder. Also seems to breakup real easy. But on the positive side - I can hear and adjust tremolo. But no reverb. Though before when it was motor boating I could swear if I turned up the reverb knob and knocked the tank I could hear it. I switched tubes thinking it may be the prob. No change. So back to the circuit tracing.  

• Apr 3, 2014

Have you replaced the filter caps? Also, have you pulled or disconnected the "death cap" C31? If the PT is not getting hot, I would not hastily jump to the conclusion that it's bad.  

Regarding the main supply voltage, pull the output tubes and measure the plate voltage. If it goes up to 475V, the PT is probably fine. Check the rectifier diodes. Put a 1 Ohm resistor in series with the cathodes to ground on the output tubes and use that to measure the current. Maybe they are running hot. For the mismatch on the plates of V1, check the plate resistors and the grid resistors. There is no feedback to prevent any offset, so it has to be either the tube or a resistor. Have the coupling caps been replaced?  

Checked the wiring on the rectifier/cap stacks and the positive side of C27 was mis-wired and that's where the voltage drop was. Now it's back to about 475V. I knew it wasn't the Cap can C25 - I replaced all those. Also I put in a 3-wire AC cord and removed the unnecessary death cap and disconnected the ground switch. Now the plate voltages on V1 are 107 and 118. Bias on the output tubes Pin 5 are V7 - -32.42, V8 -32.53. Now all I got to do is trace down a big hum on channel 1. It's silent when the volume is down but very discernible when you turn it up. Channel 2 is great - tremolo works, but the signal is not being fed to the reverb. If I knock on the tank with the reverb up I can hear the spring through the speakers but no feed from the output. Thanks for all your input.  

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Owner/ Master Luthier

David Hosler has more than 40 years of experience designing, building and repairing stringed instruments. His Luthery experience is combined with playing lead guitar for many touring bands throughout the 80's and early 90's.

David first began working and building guitars in Florida.  In the early 80's he moved to Greenville, South Carolina where he opened the Guitar Workshop in Travelers Rest, SC building and servicing guitars.  His reputation and skills brought David to the attention of Taylor Guitars which soon offered him a position with the iconic company.  In the fall of 1996 he and his family moved to join Taylor Guitars in Southern California. During his time at Taylor Guitars, his talents and experience moved him up in the company, soon earning the title Vice President of Customer Service, Repair and Quality Control.   David's creative insight and experience also led him to design and create Taylor Guitars’ first proprietary pickup system, "The Taylor Expression System."  Legendary audio designer, Rupert Neve , worked together on the project and the original Expression System won numerous awards, including the coveted TEC award from the Mix Foundation.  In the following years David led the development of the Taylor T5, T3 and Solidbody line of Taylor Guitars.

In 2014 David brought another industry changing pickup design to Taylor Guitar by creating the highly acclaimed Expression System 2.

Retiring from Taylor in June of 2015 David moved back to his home state of Florida to open and operate

SEVEN C MUSIC in St. Petersburg with his son Joel.

Hunter Allen

Master Luthier

Hunter Allen is a native Texan, born and raised on a ranch in central Texas.  In the early 1980's he moved from there to the Country Music Capital of the world, Nashville, TN. Once there he apprenticed at a small guitar shop in Spring Hill TN. learning instrument repair and building as a trade.  It wasn't long after he was hired on as one of the head repairman at Glasers Instruments in Nashville. Hunter spent 30 years at Glasers working on instruments for many legends in the music business such as, Emmy Lou Harris, Clint Black, Willie Nelson, Tony Joe White, Dwight Yokum, Keith Urban, Ricky Skaggs, Joe Bonamassa and many more... 

In 2016 Hunter moved to St Petersburg, FL enjoying the beach lifestyle and the local music scene.  Not long after moving to St Pete he joined the SEVEN C MUSIC family. With his experience and knowledge, Hunter is a great addition to the shop and a perfect compliment to the service we offer.

THE WORKSHOP

World Class Service

The heart of the Seven C Music Workshop is offering service and repair at a whole new level.  Acoustic or electric, our luthiers & craftsman have over 100 years of combined experience

repairing and restoring local and professional musicians instruments to perfect playability. 

Take a moment to look through our service page to discover a new and better way

to take care of your instruments.

We offer warranty service for

Taylor, Martin, FENDER, Collings, Waterloo,

Guild, Duesenberg, Fano & Eastman guitars.  

Instrument Setups

A proper professional setup on a stringed instrument can take it from unplayable to enjoyable and inspiring!

Like other services, there’s a technical part and an experience part of doing a professional setup that makes the difference. Our luthiers have over the experience to make sure your instrument is playing its absolute best

when it leaves our shop.

If your instrument needs a fret dressing or refretting, we’ve seen and done it all on every kind of guitar. 

Our fret services are some of the best in the industry; from stainless steel to evo gold, our unique process of pressing frets, get you the perfect fret job every time. If you've had a bad fret job done or maybe your frets are just worn out, we can get your instrument playing perfect again. 

PICKUPS & ELECTRONIC

We work quickly and efficiently to make sure your new pickups are installed professionally and cleanly. Acoustic or electric we offer the highest quality pickups from  L.R. Baggs to Seymour Duncan. 

If you have a dirty pot or sticky switch, we use only the quality replacement parts from companies like Switchcraft, CTS, and Emerson in our electronics repair and upgrade services.

We can get your instrument sounding like you need it to.  

Whether your vintage guitar needs a neck reset or your instrument was dropped and damaged, our craftsman can reset the neck to the precise angle needed or repair the damage to almost look new again.  

We use the highest quality glues and techniques to be sure your instrument hold together for the rest of its life.

Finish Work

From vintage Nitrocellulose Lacquer to modern UV Polyester we use tried and true techniques used by Martin, Taylor & Collings. 

Whether you dinged your guitar, have a blemish or just want your guitar polished to its original sheen, we can get your instrument looking new again. 

535 22ND STREET SOUTH 

SAINT PETERSBURG, FL 33712, USA

GENERAL INQUIRY- [email protected]

BOOKING INQUIRY- [email protected]

727-290-6741

SHIPPING INSTRUCTIONS

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Huge Selection Always Buying

Florida’s #1 guitar buyer.

Always buying clean used and vintage guitar gear. What have you got? Trades & Layaways. New, used and vintage guitars, amps and everything for guitarist. REPAIRS & RENTALS by the day, week or month!

Call us at 727.318.0986 or email us at [email protected]

Guitar Repair

Explore our inventory, about st. pete guitars.

Best Prices • Great Value Huge Selection • New & Used

What an amazing journey…

25 years ago, ‘Stevie B’ opened his first guitar shop, Stevie B’s Total Guitar on US 19 N in Clearwater, FL on March 16, 1996.

He was an ambitious young man, ready to ‘make his bones’ in the guitar business. Signing a 1 year lease, Stevie B took a giant leap of faith.

Today, Stevie B is proud to be operating his 5th and newest guitar shop, St. Pete Guitars on America’s #1 Beach, St. Pete Beach located at 6630 Gulf Blvd.

From all of us. Thank you to all of our friends and customers who helped make it happen. 25 years later and we’re still making guitar dreams come true since 1996.

Local Artist Spotlight

Country Great and Gretsch Devotee Len Tabor playing our brand new Gretsch 6620 Nashville Tiger Flame Maple Guitar!

Stevie In Action

Guest Appearances

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