Red Knob Twin hum until V9 removed

[01redcobra]

Red Knob Twin came in with a crackling noise on notes below the A string. Pulled the chassis and found R129 open and R128 burned badly, but still showing resistance. Replaced both of those. Once in the cabinet , I found the noise to still be there. With the chassis on the bench, I started a tap test to see what was reacting to the vibrating. Found that C107 was the culprit and replaced that. Now it seems that I have a hum that only gets worse when the reverb tank is hooked up. Also, the reverb can be heard even with the level all the way down. But the hum is there with the tank disconnected. With the volume all the way down, I still have the hum. If I pull V9 it will go away. All my voltages are right in line. I've tried a couple of the coupling caps, but nothing is helping. What am I missing?

[g1]

What happens if you ground the reverb pot end of R113?
Sounds like the reverb pot or related connection is at least part of the problem.

[Jazz P Bass]

What I would look at is the current that the amp is pulling from the mains.
I use a Kill-A-Watt meter.
That amp should not pull more than 80 watts at idle.

I would suspect V8.
Pull it & see if the hum stops.

[01redcobra]

I pulled V8 and that made no difference. My kill-a-watt is reading 120 at idle and 115 without V8. So far V9 is the one that stops the noise (other than V3+4 of course). If I take a voltmeter probe to any of the leads from the volume pot or the resistors (R113 +112) the hum increases a lot. It also goes berserk if I try to probe pin 7 of V9 for voltage. Is that normal?

[Jazz P Bass]

I would expect a quad of 6L6s to pull 120 watts.
My guess is it is biased way hot.
The hum could be caused by excess Vac ripple on one or more of the filter and/ or decoupling caps.
Have you tested for that.
A bad output tube could also do it.
(Kill a Watts rule, don't they?)

[g1]

I would expect a quad of 6L6s to pull 120 watts.
My guess is it is biased way hot.

It is a quad of 6L6 and is pulling 120W.
Did you mean you would NOT expect 120W draw?

01redcobra: Have you tried grounding the wiper of the reverb pot (or R113)? It does not make sense that you can't kill the reverb with the reverb pot, or that removing V9 kills the hum but turning down the reverb pot does not.

[01redcobra]

It's a brand new quad of JJ's. I biased it as per the Fender bias ports on the back. I didn't check for the ripple yet. I bought the Kill-a-watt on your recommendation in an older post. Thanks!! I'll check out more later tonight. I don't remember the noise being there until after I changed that bad cap at C107, but I could be wrong.

[Jazz P Bass]

It is a quad of 6L6 and is pulling 120W.
Did you mean you would NOT expect 120W draw?
.

The odd numbering got me.
V5, V6 oh & V10, V11.
120 watts at idle is good.

[01redcobra]

Didn't try grounding the wiper yet. I still have the tank disconnected too. The hum is there with V9 in and gone when it's out. When the amp was in the cabinet, no matter where the channel select for the reverb was, you could still hear the reverb. Not a lot, but still not a dry signal. The amp is loud as heck running a guitar thru it, but the hum is just enough without a signal to bother me. The guy wants to record with this amp and I know the engineer is not going to appreciate that hum. Running into the loop with a guitar also quiets the hum down.

[Jazz P Bass]

Do this.
Measure the hum.
Use your meter, set it to read Volts ac & hook it up to the speaker.
At idle, with no input & the volume down.
4mvs is good.
Anything above 9 says something is amiss.

[01redcobra]

Jazz, the tube numbering got me disoriented too. seeing V9 next to V3 was just not right. Also I found that there is no C116 on this amp. I guess a production change, there's a spot for it, but there was never anything in there.

[01redcobra]

It is a quad of 6L6 and is pulling 120W.
Did you mean you would NOT expect 120W draw?

01redcobra: Have you tried grounding the wiper of the reverb pot (or R113)? It does not make sense that you can't kill the reverb with the reverb pot, or that removing V9 kills the hum but turning down the reverb pot does not.

Grounding the wiper makes the hum much worse. Ugh.

[01redcobra]

Do this.
Measure the hum.
Use your meter, set it to read Volts ac & hook it up to the speaker.
At idle, with no input & the volume down.
4mvs is good.
Anything above 9 says something is amiss.

Yes, it's reading 36-42mv at idle.

[01redcobra]

Yes, it's reading 36-42mv at idle.

Of course with V9 removed it's back down to 5mv

[Jazz P Bass]

Is there any measurable Vdc on the reverb pot side of C103? (V9B plate cap .0022uf)
If there is, C103 may be at fault.