Fender '94 Twin Amp Hum....

[acorkos]

There's significant hum (and static momentarily when i flip on the standby switch) in this amp with all front panel controls zeroed.

Plugging a guitar cable into the power amp in jack eliminates the hum...sounds normal.

pulling V1, V2, V3, V4 or V5....no change in hum.

pulling V6, V7, or V8....hum is gone

hum is heard in a second amp fed from preamp out jack

no hum is heard in a second amp fed from effects send jack.

i'm not sure what to check next....i've metered all the resistors associated with V6-V8 and they are fine. not sure about the caps. any ideas? thanks for your help!

[Chuck H]

It looks like V6-V8 are in series with the signal path. So V6 is the likely culprit. Not the tube, but the circuit. Since the front panel controls don't affect the hum and breaking the circuit between the power amp and V6 does I'd say that narrows it down. I'm going to guess it's a ground poblem with the reverb recovery circuit. Hum from the PT is getting onto the grid of the recovery tube. Probably a ground fault in the cable, tank jacks or amp chassis jacks. Do be sure it's a shielded cable in there and that it's routed correctly. Also check that the tank hasn't been replaced with the wrong part number since grounding scheme for the tank could be different from stock with a different part number.

[acorkos]

thanks...it hums whether the tank is connected or not

[Chuck H]

Hmmm... I'm still thinking ground fault in the reverb recovery.

[Stoffel]

hey, did you ever found the fault? Seems like I have the same problem here. Alreadeay changed some resistors and caps but no progress...

[acorkos]

hey, did you ever found the fault? Seems like I have the same problem here. Alreadeay changed some resistors and caps but no progress...

i eneded replacing all power supply filter caps (2 x 220uF, 7 x 22uF)...hum eliminated

[Stoffel]

Thanks for letting me know. I already ordered C38 and 39 (the 220µF) and C40 and 41 (47µF). I'll try to change this ones. If the problem is still there I will order the 22µ's and change them too.
I'll let you know!
(strange about my amp is that the hum was already there when I bought it in '95. Sended it back 3 times for this, but the dealer sticked by his story that this was normal for and amp like this. Never believed him but didn't know anything about schematics in that time.)

[rf7]

Chasing this same problem. I traced the problem to triode V6B. It seems to be independent of the input to the tube (shorting the grid and the cathode does not change it). I ran the heater of V6 with a dc supply and it still hums. Jumped the last filter cap feeding the plate of V6 with another cap and it did not help.

[Chuck H]

And I would like to itterate that it's entirely likely that a ground fault was corrected during the cap change.

[Stoffel]

Still have this problem!!

One year later and a little smarter about schematics I'm picking up this problem again...

I have already changed all caps and a bunch of resistors. The noise stops when I remove V6.

I isolated the preamp, the mixing part and the reverb part (by removing R54, R55, R63 and C25. Then I connected R65 (at the preamp side) to the ground. Noise persists. When I connect pin 7 of V6B to the ground the noise is gone.

According to these steps I suspected the resistors R65, R64, R66 and R67. Changed them all but the noise is still there. Don't now further what to do.

Any tips are welcome!!