Gibson GA-95

[jpcar]

Hi,

Can anyone explain the purpose of the 250pf capacitors before the 1.5k resistors that continue to the grids of the power tubes in the signal path?
I guess they are filtering something out of the sound, but are they necessary?
I was having a hum problem and I replaced these caps and that seems to have sorted the problem but I think I might be lacking something in tone.

[Enzo]

Not filtering sound. They are there for stability.


Personally I never liked the sound of the old Gibson amps, but other guys like them.

If replacing them killed hum, one of several things come to mind. The amp was oscillating at RF, which often makes a gumming background. Or: .one was shorted thus unbalancing the output stage. Something like that. Or for that matter, when you are in there changing parts, you may have restored a poor ground connection in the circuit.

[jpcar]

Oh, that's good to know about RF.
That might explain the hum. I tested the old caps and they weren't shorting. They actually seemed not far off 250pf. Also, it might have been a poor ground connection as you say.

[Helmholtz]

A switchable 30µ cap (C17) shorting the output - sort of standby muting?

[jpcar]

That switchable cap (C17) is on the treble pot so it seems to be a bright switch.

I thought replacing the 250pf (c15, 16) caps had removed the hum but actually it's still there. It's a 120Hz hum. I attached a pic of it present at the speaker terminal.

Any thoughts what might be causing this?
I replaced all the big electrolytics (C29a, b, C30 ,31, C28a, b)

[Helmholtz]

That switchable cap (C17) is on the treble pot so it seems to be a bright switch.

Schematic shows a 30µF in series with a switch (S5) at the OT secondary. Must be a big foil cap.

Where did you connect the probe for the pic?

Are idle currents matched?

Does the hum stop when you remove V5?

[jpcar]

Schematic shows a 30µF in series with a switch (S5) at the OT secondary. Must be a big foil cap.

Yes, that's correct. The switch is part of the treble pot, as shown in the pic. When I switch it on (cap in circuit), this adds brightness to the sound. The original was a big foil cap. I replaced it with a small 33uF / 63v cap. Is that ok or does it need to be a big foil cap?

Where did you connect the probe for the pic?

The probe was connected to the speaker terminal (w/ the speaker connected)

Are idle currents matched?

Yes, the idle currents of the power tubes are pretty close and biased fine.

Does the hum stop when you remove V5?

Yes, the hum is gone when I remove V5.

[Helmholtz]

Yes, that's correct. The switch is part of the treble pot, as shown in the pic. When I switch it on (cap in circuit), this adds brightness to the sound. The original was a big foil cap. I replaced it with a small 33uF / 63v cap. Is that ok or does it need to be a big foil cap?

Maybe the schematic is wrong. But the 30µ cap I'm speaking of is wired to the OT secondary and not to the treble pot (such large capacitance would make no sense wired to a treble pot. Don't see what you mean with "as shown in the pic".)
It seems to be part of a muting circuit that shorts the output. If wired as shown in the schematic, you can't use a polarized ecap as speaker output is AC.
Why did you replace it?

Yes, the hum is gone when I remove V5.

This shows that the hum is not caused by the power stage.

Does the amp deliver full power?

When replacing filter caps, did you observe correct polarity?

[jpcar]

Maybe the schematic is wrong. But the 30µ cap I'm speaking of is wired to the OT secondary and not to the treble pot (such large capacitance would make no sense wired to a treble pot. Don't see what you mean with "as shown in the pic".)
It seems to be part of a muting circuit that shorts the output. If wired as shown in the schematic, you can't use a polarized ecap as speaker output is AC.
Why did you replace it?

Hopefully this pic is clearer. To the left is the OT secondary - red wire across to the cap and the switch on the treble pot on the right. The old foil cap is still in place (in the middle).
I didn't realize this wasn't polarized! Oops. I have just reconnected the original cap now and the hum is the same. I had replaced it in case it was causing the hum, even though it's measuring fine with a capacitance meter and ESR.

I think the schematic is right. It just doesn't show that the switch is attached to the treble pot. On the front panel it shows this switch is called 'Jazz'. (other pic attached). This is not mentioned in the schematic.

Does the amp deliver full power?

Yes, I believe so.

When replacing filter caps, did you observe correct polarity?

Yes I did.

[Helmholtz]

Hopefully this pic is clearer. To the left is the OT secondary - red wire across to the cap and the switch on the treble pot on the right. The old foil cap is still in place (in the middle).
I didn't realize this wasn't polarized! Oops. I have just reconnected the original cap now and the hum is the same. I had replaced it in case it was causing the hum, even though it's measuring fine with a capacitance meter and ESR.

I think the schematic is right. It just doesn't show that the switch is attached to the treble pot. On the front panel it shows this switch is called 'Jazz'. (other pic attached). This is not mentioned in the schematic.

Seems switch S5 is on the back of the treble pot (contrary to the notes on the schematic).
Otherwise the cap seems to be connected as shown in the schematic. Forget the small ecap.
Other than ecaps all foil caps are non-polarized, i.e. suitable for AC and DC.

How does it sound in "Jazz" position? I would expect zero treble.

Cap should have little effect on frequencies as low as 120Hz.

Now pull V1 and see if this stops the hum.

[jpcar]

How does it sound in "Jazz" position? I would expect zero treble.

Yes, it sounds duller with "Jazz" position on (which is the cap in circuit)

[g1]

The switch is physically mounted on the treble pot, but it is not electronically in the treble circuit. It does, however ensure that it can only be engaged (or disengaged?) at one extreme of the treble pot.
Is the 'click' (jazz position) of the switch at the 'full treble' end of the pot, or at the 'treble off' end? When 'jazz' engaged, is the switch open, or closed? (edit: nevermind, I missed post #11 while posting)

The instructions say that for a 'jazz' tone, you should reduce treble, engage 'jazz' switch and boost mids & bass. So I would guess that the 'jazz' position is full counterclockwise of the treble pot, and when switch in 'jazz' position, it is closed. That would engage the cap and further kill highs (more than just turning treble all the way down).

[jpcar]

The switch is physically mounted on the treble pot, but it is not electronically in the treble circuit. It does, however ensure that it can only be engaged (or disengaged?) at one extreme of the treble pot.
Is the 'click' (jazz position) of the switch at the 'full treble' end of the pot, or at the 'treble off' end? When 'jazz' engaged, is the switch open, or closed? (edit: nevermind, I missed post #11 while posting)

The instructions say that for a 'jazz' tone, you should reduce treble, engage 'jazz' switch and boost mids & bass. So I would guess that the 'jazz' position is full counterclockwise of the treble pot, and when switch in 'jazz' position, it is closed. That would engage the cap and further kill highs (more than just turning treble all the way down).

That's exactly right.
The "Jazz" position click is at the 'treble at zero' side of the pot. And in "Jazz" position the switch is closed, (cap in circuit).