Rebuilt Gibson GA-8T Loud Hum at 120hz.

[One10]

The 22µ caps are ok, all 10µ caps are borderline regarding ESR.
From the measurements there's no reason why the old caps should work better.

But there's an additional cap parameter, namely leakage resistance (or leakage current) which can't be measured with an ESR meter.
Leakage often improves with re-forming.
That can be achieved by running the amp (with the new caps) on a bulb limiter for some hours.

Hello Helmholtz, thank you. I ran them at full operational voltages for at least 4hrs. not knowing that there may be problems with them.

1.) I had to remove them anyways, they started to fall out of the amp, the expensive silicone glue and automotive grade double-sided tape lost their grip after only 2 weeks.
This is the first time silicone caulking and/or expensive automotive grade 3m tape failed to hold the capacitors.
The capacitors have a clear plastic wrap that nothing sticks to very well. Unfortunately they printed on it, so I can't remove it.

2.) Yes, I would like to know the leakage current / rate.

3.) I had a bulb limiter somewhere around here, perhaps I can go to the Big Box store and find parts to make another then give it a try.
I never knew you could use it for that.

(Does it matter what wattage? I might have had a 300w incandescant in the original, would a 100w also work?)

[Helmholtz]

[COLOR=#2980b9][I]. I ran them at full operational voltages for at least 4hrs. not knowing that there may be problems with them.

That should have caused some brute force re-forming and I don't think running on the bulb limiter will result in further improvement.
Normally I would recommend a 60W bulb to limit the current.

I had to remove them anyways, they started to fall out of the amp, the expensive silicone glue and automotive grade double-sided tape lost their grip after only 2 weeks.
This is the first time silicone caulking and/or expensive automotive grade 3m tape failed to hold the capacitors.
The capacitors have a clear plastic wrap that nothing sticks to very well. Unfortunately they printed on it, so I can't remove it.

I don't understand. Why is it necessary to hold the caps with glue or tape?

[One10]

I don't understand. Why is it necessary to hold the caps with glue or tape?

In the Gibson GA-8T, the chassis is vertically orientated, just imagine trying to place capacitors on a wall in your house. They would fall immediately to the floor. Same thing. Have to use glue, or dbl. sided tape. The original was multi-cap can & had a clamp, can't re-use it, it is permanently riveted to the old cap.

[Helmholtz]

In the Gibson GA-8T, the chassis is vertically orientated, just imagine trying to place capacitors on a wall in your house. They would fall immediately to the floor. Same thing. Have to use glue, or dbl. sided tape. The original was multi-cap can & had a clamp, can't re-use it, it is permanently riveted to the old cap.

Can you post a picture?

[One10]

Can you post a picture?

It's the third photo in post #12.

This is an example of how it looked originally, my original condition photos are inaccessible (hard drive crash). See how it is a vertical chassis.



Not a whole lot of space to mount them properly.

[Helmholtz]

No pictures visible in post #12.

[One10]

No pictures visible in post #12.

Probably the browser you are using?, or this forum program isn't supporting Firefox or Safari browsers. I only see all of them while in Google Chrome.

[Helmholtz]

I guess what you're seing is a local link to your own computer.
Happened to me and others as well.
You probably didn't check the "upload to server" button in the small upload window.
Was easier with the old forum software.

[One10]

Ok. Helmholtz, I have tried it yet another way. Can everyone see them? Sorry all these procedures confound me, I am getting too old.

[Helmholtz]

Works .

[One10]

Helmholtz, see how the chassis is vertical? That is why my caps have to be glued-in.

[One10]

Hahaha, I replaced the capacitors yet again, this time with some N.O.S. Spraque 'blue' Atoms TVA1906, 20µf x 500v for another test - I had bought around 1996 (too old to use them in anything now anyways).

The 120hz. hum still is at -41 to -45dB w/the sprague, or new Kemet caps.

What is odd though, now the 'other' hum at 59hz. dropped to -89dB with the spraque atoms!

I am going to rest now, as I do not posess sufficient technical knowledge to know what the hay is going on.

Thanks for everyone's help!

[Enzo]

Just reacting to the photo in post 41 just above, that black cap will be a lot smaller in modern parts. And that firecracker cap strapped to the chassis? Were it mine, I would mount a couple little terminal strips and mount individual caps stretched between them. Very secure.

[The Dude]

I think terminal strips is what he did if I'm seeing correctly.

[Enzo]

Duh, you are correct, I looked at the one photo which he used to show the mounting problem, and I looked no further. Seems he did exactly what I was suggesting already.