Gretsch Chet Atkins preamp hum

[nickb]

The sound is full on with the wire fron the wiper of Vol 2 to pin 3 of V3 lifted at the socket. I tried redressing this wire, but it only makes it worse.

I don't understand what is being said here. Did you mean V3 pin 7 rather than 3? What did you use for a grid leak with the wire lifted? Does "sound full on" means there was a lot of hum?


Either the hum is after the Vol2 pot or not. Set the control about 1/4 way up and try shorting between the ACW and CCW pins of the vol2 pot. If it goes away the problem is before.

If before, first try putting a cap say 0.1uF in parallel with the 0.05uf ( I would have used a much bigger value but I guess this was a cost constraint as the time) on the supply to V2. Failing that work your way through the bass circuit first, grounding one point at a time to see what makes it go away. Same with treble. You can even ground the plates of V2 as the series resistance from the supply is high.

[Randall]

"I don't understand what is being said here. Did you mean V3 pin 7 rather than 3?"

Yes, that is what I meant, V3 pin 7. Sorry.

"What did you use for a grid leak with the wire lifted?"

I used nothing, I simply lifted the wire from V3 pin 7. As commented above, this is not surprising that it made a lot of noise.

"Does "sound full on" means there was a lot of hum?"

Yes, it does.

"Set the control about 1/4 way up and try shorting between the ACW and CCW pins of the vol2 pot. If it goes away the problem is before. "

Shorting these pins of Vol 2 has no effect. One side of the pot measures 0.7 ohms to ground. Shorting the oustside pins together, or shorting either to ground makes no difference. Shorting the wiper to ground silences the noise. But, didn't we already know this? Remember, V2 in or out has no difference.

[nickb]

Shorting these pins of Vol 2 has no effect. One side of the pot measures 0.7 ohms to ground. Shorting the oustside pins together, or shorting either to ground makes no difference. Shorting the wiper to ground silences the noise. But, didn't we already know this? Remember, V2 in or out has no difference.

No, we didn't already know this. V2 removed is not necessarily the same. We have learned that with a highish impedance on the wire it hums and with a low impedance (i.e control turned to zero) it doesn't. That suggests it's being picked up in the wire ( I know you tried dress). The easy way would be to try a screened lead, grounded at the vol2 pot end only.

[Randall]

"That suggests it's being picked up in the wire ( I know you tried dress). The easy way would be to try a screened lead, grounded at the vol2 pot end only."

Indeed! Running a shielded lead from Vol2 pot to V3 grid 7 cleaned it right up. Thank you nickb! However, the wire looks to be original. Unless it somehow got moved, why wouldn't channel 2 always have had this issue?

[nickb]

"That suggests it's being picked up in the wire ( I know you tried dress). The easy way would be to try a screened lead, grounded at the vol2 pot end only."

Indeed! Running a shielded lead from Vol2 pot to V3 grid 7 cleaned it right up. Thank you nickb! However, the wire looks to be original. Unless it somehow got moved, why wouldn't channel 2 always have had this issue?

Maybe something nearby got noiser / moved closer.

[nosaj]

No, we didn't already know this. V2 removed is not necessarily the same. We have learned that with a highish impedance on the wire it hums and with a low impedance (i.e control turned to zero) it doesn't. That suggests it's being picked up in the wire ( I know you tried dress). The easy way would be to try a screened lead, grounded at the vol2 pot end only.

Thanks Nickb for this explanation.

nosaj