Gibson GA-8T 120hz.Hum & No Tremolo

[Helmholtz]

Ok, now try a 100k grid stopper directly at V1B pin 2.

[misterc57]

Ok, now try a 100k grid stopper directly at V1B pin 2.

That did not help.

Had to connect to pin 7 as the amp and schematic are crossed on V1. First stage grid is pin 2 and 2nd stage is pin 7. I am going to update my schematic. All connections look proper for V1A and V1B.

[Helmholtz]

Had to connect to pin 7 as the amp and schematic are crossed on V1. First stage grid is pin 2 and 2nd stage is pin 7.

Is that original wiring?

[misterc57]

Is that original wiring?

It appears to be

[Helmholtz]

Can you see the 40kHz oscillation at the V2A pin 1 with power tubes pulled?

[misterc57]

Can you see the 40kHz oscillation at the V2A pin 1 with power tubes pulled?

No oscillation with output tubes pulled. Also getting a nice clean audio signal on the scope (for the entire range of the VOL pot) up to that point.

[Helmholtz]

I still think the power supply including ground wiring is involded.
It might be necessary to add an extra node for the PI. Chuck H, what do you think?

But first I would try wiring an additional filter cap from the screen node to ground. Also try different grounding points for this cap.

It may also help to add 470R screen stoppers.

[misterc57]

I still think the power supply including ground wiring is involded.


A. It might be necessary to add an extra node for the PI. Chuck H, what do you think?

B. But first I would try wiring an additional filter cap from the screen node to ground. Also try different grounding points for this cap.

C. It may also help to add 470R screen stoppers.

Want to make sure I understand...

A. are you referring to the 22K/47K junction feeding V2A pin 9 ?

B. parallel the 10uF with another 10uF at the 470R/1W ?

C. at V2B and V3B pins 7 ?

When I first looked at this amp it had a 1.75K cathode resistor for pins 2 of V2B and V3B. If I recall the plates were running at 400V so I changed to the 270R per the schematic and the plates are at 255V. Could this have any relation to the issue?

Thank you!

[misterc57]

as far as grounds go I removed one that was likely causing the audible hum. This was the white wire coming from the pots and jacks. Since it is already at ground I did not reconnect it.

All filter caps and PT center taps are at the same ground point.

[Helmholtz]

A. are you referring to the 22K/47K junction feeding V2A pin 9 ? - Yes, but suspend that for later.

B. parallel the 10uF with another 10uF at the 470R/1W ? - Yes, but try different grounding points.

C. at V2B and V3B pins 7 ? - Yes.

When I first looked at this amp it had a 1.75K cathode resistor for pins 2 of V2B and V3B. If I recall the plates were running at 400V so I changed to the 270R per the schematic and the plates are at 255V. Could this have any relation to the issue? - I don't think so. 270R makes much more sense than 1.75k.

Comments included above.

[misterc57]

Added the 470R screen stoppers and a parallel 10 uF at the 470R/1W junction. Tried different grounds. Was seeing some strange behaviors such as

1. Every now and then the current would start pulsing, causing the oscillation to be there and not be there. I first noticed this when grounding to the same ground point as the vibrato circuit. But I also saw it when grounding directly to the chassis. It would come and go and I was not able to correlate any causes.

2. Oscillation amplitude and frequency would change with the VOL pot, then it would not.

Where I am at now is the oscillation no longer changes with the VOL pot. Amplitude is low (picture 1). When putting a 1K audio signal (VOL 5) into the amp the output looks bad (picture 2).

Both measured at the amp output...

[misterc57]

Interesting article on the same amp repair.

http://www.rru.com/~meo/Guitar/Amps/Gibson/GA8T/

"I stayed with a 20uF first stage to keep the sag characteristics I hoped were there, but used 47uFs for the other two caps, to reduce hum. Suddenly the little monster came alive!"

Looks like the power tubes cathode cap is 50 uF as opposed to 20 uF on mine. Also that floating black ground wire I had connects the ground side of the PS filter caps. Looks like all grounds are joined at pin 3 of the rectifier tube.

[Helmholtz]

What was the effect of adding the screen stoppers only?

Did you already clean/retension tube sockets?
Did you swap tubes?

[misterc57]

What was the effect of adding the screen stoppers only?

Did you already clean/retension tube sockets?
Did you swap tubes?

Screen stoppers only have the effect last reported. When I clip in the extra 10uF it very slightly drops that amplitude of the oscillation.

I have cleaned tube sockets and will check tension again. I did early on have to replace the broken V3 tube socket with a pull from a hammond organ chassis.

I just noticed that my 270R/2W power tubes cathode R has been cooking. It is still reading 270R

[Helmholtz]

Screen stoppers only have the effect last reported. When I clip in the extra 10uF it very slightly drops that amplitude of the oscillation.

So leave screen stoppers installed and try a larger cap.

I just noticed that my 270R/2W power tubes cathode R has been cooking. It is still reading 270R

Always a reason the measure voltage drop.