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I recently bought a Gibson BR-6 not too long ago. After much deliberation about upgrading to a three-prong plug, which I eventually did, a new problem arose. I'm not sure if I ever checked on this when the amp came or not, mostly because I played around 3 or 4 for the clean tones at living room volume. It was good enough for me.
Yesterday I turned on the amp, and I put it to around 5. I felt gaslighted by this amp, because I was hearing mostly the acoustic volume. I turned down the volume on my guitar all the way, and I heard a very subtle difference. I put my ear to the amp speaker, and turns out it was playing at a really weak volume. I thought maybe the outlet I was plugging into was causing this so I tried another outlet, and another, until I noticed that the amp worked fine until I turned the volume above 4. It sounded scratchy as I turned the volume knob, and over 4, it seems to drop to an almost inaudible sound.
What might be the cause of this. Is this an easy fix?
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01-30-2022 11:11 PM
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The first thing that I would try is spraying some DeOxit contact cleaner into the volume pot, turning it a few times, and seeing if that solves the problem. It may just be an old, dirty pot. For that matter, you could turn the volume knob through its entire range of motion 20 or 30 times and see if that fixes it. If not, it may just need a new pot.
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Yep, sounds like a bad volume pot. You can try to clean it, but it might not respond. If it still doesn't work, replace it. If you don't have the ability to do that yourself, it's a quick and easy job for someone with the skills. Potentiometers are just variable resistors, and the old ones were not all that reliable.
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Sounds to me more like parasitic oscillation. Bad pots almost never totally cut out, they might be scratchy, lose volume but that’s a pretty unusual problem for a volume pot.
but it does sound like a parasitic oscillation problem where as you increase gain, the amp is oscillating at frequencies above human hearing. It’s actually working really hard but you don’t hear it. This is due to a wire lay out that allows coupling between two area that shouldn’t (than are in phase with each other) and reinforce each other into a feedback loop.
anyway, pretty tough to cure without a scope and knowing what you’re doing.
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You might be right. I tried using contact cleaner on the pot, and nope, same issue. It's a bit crackly between 1 and 4 - after 4 it cuts out "completely", but not really. It just drops down to a really really really low volume.
Originally Posted by 6v6ster
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You might try emailing or calling Skip Simmons: Vintage Amp Repair, Expert Fender Amp Repair | Skip Simmons Amp Repair. He has a lot of experience with vintage amp repair and restoration.
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Almost is the magic word. These are old wirewound pots. If a turn of the winding has corroded almost through or is broken and the fractured ends are oxidized and barely touching, the effective resistance between rotor and the end of the “good” side of the winding will be quite high as soon as it jumps the damaged winding. This is a distinct possibility. A replacement pot may be the simple answer. Originally Posted by 6v6ster
This is just one example of why old amps like this should be checked completely when returned to service after years of closet living.
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Well I guess it wouldn't hurt to try. Can you help point me into the direction of the pot I should buy? I saw there's different types and I wouldn't wanna go out and buy the wrong one. Originally Posted by nevershouldhavesoldit
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Sorry, but I don't know the orignal spec and would be guessing. If there's a legible marking on the original, you should be able to find out exactly what it is. I suspect you could use a modern pot, but wirewound are still available and shouldn't be a problem to find on line. There may even be a few owners on the JGO forum who could look into theirs and see what it is. Originally Posted by Loreto
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As a diagnostic test, what about jumpering the input and output tabs on the volume pot? Meaning, zero resistance from the pot.
The amp should then go to 10. Make sure whatever you plugged in is turned down or you'll blast yourself (you hope).
Or, connect a spare pot in parallel.
Don't do this until somebody that actually knows something approves it.
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FWIW, if you google "Gibson BR6" you find plenty of information, video how to service, blogs ...
Even a schematic :
Gibson_BR-6F.pdf
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Does it make much of a difference if I use a wirewound one or not? Originally Posted by nevershouldhavesoldit
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I suspect that depends on whom you ask. To me, an Ohm is an Ohm - when set to the same resistance and completely intact and clean inside, I can’t hear a difference in a guitar amp. But I’m sure there are many who think they can, and maybe they really can. Eric Johnson claims he can hear differences between brands of battery in effects pedals. To the best of my knowledge, no one’s ever tested him to see if he’s right. Originally Posted by Loreto
There are 3 basic kinds of pots - wire wound, carbon film, and plastic film. Good WW are electrically quieter than cheap carbon when turning them, if they’re kept clean and turned through their full range a few times every month or two to prevent oxidation buildup. Some will take issue with this interval, and I have no data to justify it - but it seems to work for me. Pots also differ greatly in quality, from the fineness of the windings to the quality of the materials, eg good ones turn smoothly with just the right feel. You can buy an Alpha Control 500k carbon pot with aluminum shaft for $3.50 and you can pay $10 for a CTS carbon pot with brass parts. Or you could pay $25 for a Dunlop 500k wirewound pot (all prices from Stew-Mac). All will sound fine when new. CTS carbon are now the standard for quality pots in most better guitars and amps - I doubt that any big manufacturer uses wire wound pots any more.
The schematic says it’s a 500k pot. I don’t think there were any pots except wire wound when that amp was made, but the circuit doesn’t care what kind it is - all that matters are its resistance value and its voltage rating. You could always stick in whatever kind of 500k pot you can find into the circuit instead of the old one. If it works, you have a diagnosis and a cure at the same time.
This is not how problems should be solved. You should disconnect the wires from one end tab and the center tab, then measure the end to end resistance of the pot to see if it’s 500k or much higher. If the latter, a winding is probably bad.
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OK, wasn't the pot. This tech tells me it's likely the 6SN7 tube. Thoughts?
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Try a new one and see
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Gonna take a month to get here, but I already ordered one! Originally Posted by wintermoon
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Without a scope all you can do is trial and error by replacing parts that you suspect……
If you have a multimeter you could chart voltages at crucial points and see if anything looks suspicious.
The fact that it seems to work fine untill 4 and then cuts out/goes way low volume does sound like an oscillation problem. But then it probably would have been present in this amp for all this years? Or has it been modified and wiring was changed?
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If it’s parasitic oscillation and it’s bad enough to cut the volume dramatically, there should be intense output tube glow when it happens. The extraneous signal hijacks the amp and drives it to maximum output, even though it’s inaudible because it’s high frequency. So a simple glance at the tubes as the volume control is turned up should tell that tale. Originally Posted by Little Jay
If it’s truly oscillation and the amp has not been modified, a visual check of wire routing and integrity (including insulation and solder joints, which should be shiny and not dull) may identify the culprit. If a wire in the front end (ie anywhere between the input jack and the connections between preamp stage and output tube) has come loose and is running directly adjacent to another wire (power or signal): it can cause oscillation. This can also be caused by finite resistance in a ground connection. All ground connections should have unmeasurable resistance, so inspect each ground point (using the schematic to find them all) for physically tight, intact, shiny soldering. You should be able to pull on any solder joint without disrupting it. Use a nonconductive tool like a wooden chopstick. And you don’t have to disconnect a grounded wire to check the resistance of its connection - just touch the leads of a resistance meter of any kind (VOM, VTVM, SS multimeter etc - it doesn’t matter here) to the chassis and the other end of the wire wherever it’s connected - you should see 0 ohms if the ground connection is good.
Does it only happen with a guitar plugged in? Does it happen with the input shorted (hot and ground leads connected together)? Does it happen with a high impedance line level signal like an iPod or preamp out connected to the input? Make sure the amp has been unplugged for at least 24 hours before doing any of this, and don’t touch anything inside with your bare fingers. You should discharge the power supply cap(s) for maximum safety - Google this for the many ways to do it safely.
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