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Amp Volume Fluctuation/Squeal Modular Synthesizers
Old 17th December 2010
Here for the gear

Amp Volume Fluctuation/Squeal

Hey, about a year ago I bought a used Genz Benz El Diablo tube amp. For a few months everything was great, then I noticed in the middle of gigs the amp volume would reduce pretty drastically. Pretty soon during practices I would turn it off standby and get an extremely loud, high-pitched squeal.

I decided to change out all the power and pre-amp tubes and the amp was fine for another few months it seemed, then I started having the same problems all over again. I've looked all over the internet trying to figure out what this might be but I couldn't find anything. I figured if anybody knew, it would be someone on gearslutz. Any ideas?
Old 17th December 2010
Gear Maniac
brad347's Avatar

sounds like you might have a parasitic oscillation.
Old 17th December 2010
Lives for gear

Take it to someone who knows how to fix valve amps as a 'valve swap' only partly cured it there is something else amiss. You may have 'trashed' the new output valves (if unlucky) or it will be something else causing the problem and the outputs may still be OK.
Matt s
Old 18th December 2010
Here for the gear

Thank you both for the help. I guess I'll just have to take it to someone to fix it. Oh well, can't blame me for looking for an easy fix, especially when I'm so strapped for cash.
Old 18th December 2010
Lives for gear

If putting new valves in it 'fixes' it for a while you could continue doing that but it will probably work out MORE expensive than having it fixed. Having it checked over should ensure it will last a lot longer as you may possibly damage other parts.
Bit like driving your car with the handbrake half on, wastes petrol and sluggish performance, eventually trashing the brakes.
Matt S
Old 19th December 2010
Lives for gear

If you know someone whos a tech, they need to look at the solder joints on the bypass capacitors. especially ones that bypass on the stage that global negative feedback is appied. ( hint, hint). what is sounds like to me that it works fine till it heats up. this is usually caused by capacitors "drifting in capactance when heated" but even bad solder will do this or even a crack in the circuit baord to the cap to cause the cap to electrically open due to temperature expansion..

.... either way, it sould be looked at by some body so that they can for board damage as well as bad (oxidized) solder. and to see if there is bypass caps are touching bypass resistors (a big no, no in construction) or any other passive device that heats up.
Old 19th December 2010
Gear Maniac
brad347's Avatar

that's a really great idea/observation, which leads me to another one...

Are the tube sockets mounted directly to the PCB?

If so, I'd look there and reflow ALL those connections before taking it to a ($$$) tech.

Tube sockets mounted to a PCB is poor design in a guitar amp. What ends up happening is that every time a tube is inserted or removed, those solder joints flex. Eventually they crack, and become intermittent.

I've fixed a stack of Hot Rod Deluxes, Devilles, Blues Juniors, Pro Juniors, etc. of this problem.
Old 20th December 2010
Here for the gear

Hey thanks for the suggestion. Now that you mention it, it does look like the tubes go right into the PCB; and the tubes are quite a pain to pull out. I'll definitely need to take a look at the solder joints on the board.

The amps design makes it quite a pain in the a to get into, but it should be well worth the time if I can easily re-solder it myself. Just because I'm not too familiar with taking amps apart myself, are there any things I should avoid touching so that I don't die? I'm pretty sure the capacitors are are separated into the two black boxes on the left and right, but other than that I should be good right?

I also just realized my clean channel doesn't seem to work at all. I don't know what that's about.
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