Gibson GA40T amp starts thumping in 30 second intervals

[rackflot]

Hello all. Just thought I would ask before i started in on it.

Even in Standby Mode, the amp will just send out a thump every ~30 seconds. It takes longer when it is cold.

I have the print from gibson and will start soon to repair, any ideas on where I should start?

I have attached the print to this post.

[Chuck H]

That amp has power to the output tubes but not the preamp when in standby. I suppose this isolates the problem to the output section or the power supply. If the amp still has any old electrolytic caps I would start by replacing those. It could be voltage or bias fluctuation causing an audible report. I've also known oxidized tube pins or dirty sockets to make intermittent noises. Of course if the power tubes are old they would also be suspect. Once you get this problem fixed... This isn't to say the preamp is fine. It's just not causing this problem. Old electrolytics in the preamp should be replaced also. Switches, jacks, pots and tube sockets should be cleaned and re-tensioned as applicable. Resistors should be checked for drift and film caps should be checked for leakage. No point waiting for the amp to break when it could be a good reliable piece of gear again instead of an old novelty.

[pdf64]

Yes, the multisection cap C26 is the most likely suspect. That could be confirmed by doubling up one or more of the sections with known good caps, start with the screen node cap, C26C.
If the problem is in C26, it's best to totally remove it, rather than leave good caps paralleled up with its sections. Old caps can develop insulation failures that cause problems. Pete.

[rackflot]

Hello again. started working on it. I found where I had replaced a part with 4 resistors in series of 250 ohms, hi wattage type, had dis-attached from the bottom of V6 tube.

I also noticed that one of the smaller caps looks like it may have been damaged at some time. I am wondering what kind of cap to replace it with, electrolytic or tantalum?

I replace C26 about 20 years ago, it looks ok but now I am looking at C27 as it looks all waxy and nasty. I will check it with a cap checker but there could be an issue when under high voltage. Is there a place i could order one of these 3 in one caps or should i just use separate caps?

Here are some pictures of the caps that I want to replace. I would not mind going through the entire amp and replace all of them since it would be a cheap upgrade.

the first is a 20 MFD which I am assuming is microfarad cap. it is polarized so I have to get a similar one with the same if not higher voltage.


The second are the disc caps. Is there a type or manufacturer that I should use for these?


Thanks for you help, I am getting motivated now as it appears that now that it is apart, wholescale replacement is possible.

[rackflot]

Capacitor Identification

Yes, the multisection cap C26 is the most likely suspect. That could be confirmed by doubling up one or more of the sections with known good caps, start with the screen node cap, C26C.
If the problem is in C26, it's best to totally remove it, rather than leave good caps paralleled up with its sections. Old caps can develop insulation failures that cause problems. Pete.

I would like to go through and do a whole scale replacement of all of the caps in the amp.

What i need to know is what kind of caps i should use for the

black 20mfd 25vdc caps?

Ceramic disk or something better? Orange drop? metal styrene?

The other big multitap electrolytic I will just replace as it is bleeding wax through its skin.

I attached some pix to the original post, been looking at places to buy them, not that expensive.

once I know, i will order...

[Jazz P Bass]

The 20uf/ 25Vdc capacitor is an aluminum electrolytic cap.
It is a polarized cap.
Ceramic disc, Orange drop & styrene are not polarized.
Plus they are usually very small values.
The "modern" value is 25uf.

[rackflot]

Ok, the electrolytics are cheap and easy.

Should o use the disc ceramic or like an Orange drop? What is recomended?

[Chuck H]

Ceramic caps are are not usually advisable in the signal path. They are more prone to nonlinearities, temperature instability, proximity sensitivities and microphonics. Use film caps whenever possible. Mica caps are nice for values below 1000pf. From 1000pf up "I" like polypropylene caps. They're more temperature stabil. So your amp won't sound different between cold and warm. Orange drop 715 series caps are polypropylene. Polyester is another popular film cap. Most guys like them because it's the type used in nearly all vintage amps. I've never had any problems with Mallory 150 series polyester caps or Vishay Orange drop 225 series polyester caps. Mallory 150's are especially nice to work with on PTP, eyelet and turret board type construction because they're axial lead caps.