Gretsch Chet Atkins preamp hum

[Randall]

I got this 1950 something Gretsch 6160 rat's nest in for an overhaul (customer has the matching Country Gentleman guitar). I got everything up to par except for a pesky loud hum in channel 2. It sounds like a guitar cord plugged into the input with no instrument plugged in. The sound follows Volume 2 control, and goes away at full off. The sound is there with V2 removed, so I am concentrating on V3 at the moment. But I don't see what is causing this. 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 have 118v on pins 1 and 6 on V3, 58mV on pin 2 (vol 1 wiper), 2.8v on pin 3, 5 mV pin 7, and 2.0 on pin 8.

Anyone see what I am missing?

Channel 1 is quiet enough.

https://el34world.com/charts/Schemat...etsch_6160.pdf

[nosaj]

I got this 1950 something Gretsch 6160 rat's nest in for an overhaul (customer has the matching Country Gentleman guitar). I got everything up to par except for a pesky loud hum in channel 2. It sounds like a guitar cord plugged into the input with no instrument plugged in. The sound follows Volume 2 control, and goes away at full off. The sound is there with V2 removed, so I am concentrating on V3 at the moment. But I don't see what is causing this. 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 have 118v on pins 1 and 6 on V3, 58mV on pin 2 (vol 1 wiper), 2.8v on pin 3, 5 mV pin 7, and 2.0 on pin 8.

Anyone see what I am missing?

Channel 1 is quiet enough.

https://el34world.com/charts/Schemat...etsch_6160.pdf

What are the voltages on Vol 2?

nosaj

[Randall]

"What are the voltages on Vol 2?"

20mV top of Vol 2
0mV on wiper, but goes into squealy osc. depending where the meter lead is as vol is increased.
0mV on ground side

[Steelwitch]

What kind of “hum” is it? 60hz? 120hz?

Have you confirmed that the input is grounding properly?

[Randall]

As stated, it is more of a sound like an unconnected guitar cable, one side into input other side not connected. It does it with the first preamp tube removed, so I don't think it is the input grounding.

[Steelwitch]

Just from looking at the schematic, there’s no way it could be beyond V3 if channel one does not have this issue. I would still go through and confirm that all of the ground points up to there are good. At least it will eliminate a poor ground connection and you can move on to inspecting other possibilities. I have come across the type of 60Hz hum you are describing and it has almost always been a poor ground connection. You may want to swap out V2 and V3 for known good ones to eliminate the possibility of a faulty tube obviously

[Randall]

Tube substitions was the first thing I tried. Verified input grounds are good.

[Randall]

Dang, this thing is beating me!

[g1]

What are V2 DC voltages?
I assume with V2 removed you can still kill the hum with Vol.2 ?
If you disconnect the grid wire at V3 and leave it floating, I expect there to be big hum there, so that is probably not related.

[nosaj]

Dang, this thing is beating me!

logically I would try this ground pin2 V1a hum no hum? move on to next
Ground Pin7 V1b hum no hum?
Ground Pin2 V2a hum no hum?
Ground pin7 V2b hum no hum?
Ground pin2 V3 hum no hum?
Ground pin7 hum no hum?

Somewheren we should narrow this down.

nosaj

[nosaj]

"What are the voltages on Vol 2?"

20mV top of Vol 2
0mV on wiper, but goes into squealy osc. depending where the meter lead is as vol is increased.
0mV on ground side

Probably cause your meter lead is acting as an antenna.

nosaj

[Randall]

Voltages on V2 with no tube are:

1: 288vdc
2: 0vdc
3: 0vdc

6: 289vdc
7: 0vdc
8: 0vdc

Noise is there under this condition, but it does go away with Volume full off, and get louder as volume is turned up.

[nosaj]

Voltages on V2 with no tube are:

1: 288vdc
2: 0vdc
3: 0vdc

6: 289vdc
7: 0vdc
8: 0vdc

Noise is there under this condition.

Ground pin2 V3 hum no hum?
Ground pin7 hum no hum?
nosaj
Then we're up to V3

[g1]

I wouldn't pay much attention to anything going on with the tube out. May be normal to hum with tube out.
DC voltages with tube in. And tests nosaj mentioned but with tube in.

[Randall]

I have already done all of this, but for the sake of discussion:

With V2 in:

V1 pin 2: 0vdc, noise when grounded (this is the preamp tube for the other channel, which is quiet, not sure how this is relevant)
V1 pin 7: 0vdc, noise when grounded

V2 pin 2: 0vdc, noise when grounded
V2 pin 7: 0vdc, noise when grounded

V3 pin 2: 0vdc, noise when grounded
V3 pin 7: 0vdc, noise is much louder with meter clipped in, quiet when grounded.

There is no change in behaviour with V2 in or out. This is what is bolluxing me. Looking at it on the scope, I get a 120 Hz sawtooth at speaker out.