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

[misterc57]

Watching the scope, does chopsticking wires affect the oscillation?

No

[SoulFetish]

I made a stupid mistake above

That's kind of my thing around here, Helmholtz. But since it's your first time, I think I can let it slide this time

[Helmholtz]

But since it's your first time, I think I can let it slide this time

Thanks, most regrettably not my first time, though .

I do remember the most severe ones and that should prevent me from repeating them.

[misterc57]

Tremolo issue turned out to be crossed wires between V3A pins 1 and 9.

Amp is sounding good IMO although there is still the HF oscillation. Not audible, does this need to be addressed?

No input connections, TREM pots at zero, VOL and TONE pots at 5. This signal varies with the VOL control.

Thank you

[g1]

If the oscillation is there when it is delivering signal, then it is using up power and putting extra hours on the tubes. It could also cause crackling or other odd noises when signal is present.
I assume you tried shorting out the input jack as suggested. Check if the oscillation is present when delivering signal into a load.

[misterc57]

My input jacks are shorted jacks and I confirmed continuity to ground on both jack tips.

I put a 1K signal into the amp, VOL and TONE at 5. The resulting scope image does not look good. Could unbalanced output tubes cause this?

[Helmholtz]

Could unbalanced output tubes cause this?

No.
I think you have a power supply wiring problem. Best make a drawing as is and compare with the schematic.

It is special to this amp that the rectifier output does not connect to a filter/reservoir cap but rather to a 250R/4W series resistor feeding the 20µ reservoir cap.
The OT center tap must be connected after the power resistor to the 20µ cap and the node between the 250R and 470R resistors.

[misterc57]

No.
I think you have a power supply wiring problem. Best make a drawing as is and compare with the schematic.

It is special to this amp that the rectifier output does not connect to a filter/reservoir cap but rather to a 250R/4W series resistor feeding the 20µ reservoir cap.
The OT center tap must be connected after the power resistor to the 20µ cap and the node between the 250R and 470R resistors.

I may be wrong but it appears all connections are where they should be. OT center tap is connected properly.

V4 pin 2 connects to the 250R/4W instead of pin 8. Would this matter?

Schematic shows 10 wires from the PT but there are actually 11. The mystery wire does not seem to have continuity to any other PT wires and is connected to ground, for a total of 3 wires connected to ground.

I have drawn a picture showing the wire locations.

[Helmholtz]

V4 pin 2 connects to the 250R/4W instead of pin 8. Would this matter?

No.

Schematic shows 10 wires from the PT but there are actually 11. The mystery wire does not seem to have continuity to any other PT wires and is connected to ground, for a total of 3 wires connected to ground.

I guess it's a shield between primary and secondary and should go to chassis ground.

Can you post a pic that clearly shows how the 250R/4W resistor is wired?

[misterc57]

Hard to get a clean shot, please see below pics.

250R/4W connected at pins 2 and 7
22uF connects to pin 7

470R/1W at pins 7 and 5
10uF connects to pin 5

OT center tap connects at 22uF where the black and red wire meet in pics.

[Helmholtz]

Thanks, unfortunately no clue yet.
So that strange white thing is the 250R resistor.

Sure no one added global NFB?

[misterc57]

Thanks, unfortunately no clue yet.
So that strange white thing is the 250R resistor.

Sure no one added global NFB?

The only connection to the OT secondary is the speaker or dummy load.

[misterc57]

The oscillation rises, expands, contracts, falls with changes in the VOL pot setting. At zero and 10 there is no signal on the scope. Below are some pics

Could this indicate a problem with the VOL pot? Audibly the VOL pot sounds fine from zero to 10.

Also, the 1K signal feed is funky looking from the very first stage, see last pic.

Below pics are volume at zero, 3, 6, 9, 10

[Helmholtz]

Try adding another 10µ or 22µ filter cap to the preamp.
Connect its positive end to a point where the plate resistors meet and its negative end directly to the grounded side of the first or second cathode resistor.

[misterc57]

Try adding another 10µ or 22µ filter cap to the preamp.
Connect its positive end to a point where the plate resistors meet and its negative end directly to the grounded side of the first or second cathode resistor.

Tried a 22uF, no difference.