Vox AC30HW60 with excessive hum

[Arbutt]

Hi People
Thanks for looking
This one has got me scratching my head there is something i'm overlooking here

The reported fault was turn on the sounds dropped in and out again and smell of burning

Chassis check can see one grid stopper 100R burnt up
Replaced for same and \ fitted new set of power tubes.
Replaced fuse and turned on... excessive hum , Tested all channels working ok
I removed the PI tube ( V7) ? No hum, If you remove all pre amp tubes apart from V7 and V5 Hum is present
Remove V5 hum is still present but greatly reduced
Checked with customer and prior to the fault , was working well and had zero background hum
Stolec_AC30-TB (1).pdfac30hw60.mp3
Replaced GZ34 and fitted all new pre amp tubes
Checked all grid stoppers and 1.5k all ok
Checked with another set of power tubes ( new to be sure)
All Filter are F/T caps look ok and cant see any ripple on them
I have piggybacked new caps on the filter caps still nothing
If i turn the bass fully off and treble up full i can reduce it
V5 pin 1= 273v
pin 6 & 2 =172v
pin 3 =173v
Pin 8 = 1.18v
cathode bias =10.4v around 80% Plate d
EL84 grid =321v
El84 anodes =323v
PI tube pin 1 & 6 = 232v
pi tube R71 = 52v
Amp was purchased new and never been repaired til now about 5 years old

The schematic is not the 100% version , but all seems to be the same

I think the fault is coming from the V5 circuit ??? or am i missing something
i was hoping to put up some scope pictures but mobile won't take a decent one

[Helmholtz]

Does the amp have a heater balance pot (humdinger)?

[Jon Snell]

50HZ or 100HZ? (60 & 120 in some countries).

[Arbutt]

Does the amp have a heater balance pot (humdinger)?

Hi Helmholtz. It does not . Looks like it has an artificial CT with 2 X 220r resistors..

[Arbutt]

Does the amp have a heater balance pot (humdinger)?

Morning i decided this morning to disconnect the resistor network for heaters and fit a humdinger temporary
no change
The picture below are from 56k on V5 and from the output speaker jack

[Helmholtz]

Ok, so we have a 50Hz fundamental. Doesn't look like a ripple/filter cap issue.

[Arbutt]

Ok, so we have a 50Hz fundamental. Doesn't look like a power supply issue.

Thanks. I just can fathom it. i know valve amps have hum, but this is far to excessive ,
I'm running out of brain thoughts for this one

[django420]

Did you find a solution to the hum issue? I'm having the same thing going on with my AC30HW60.

[Jon Snell]

Did you find a solution to the hum issue? I'm having the same thing going on with my AC30HW60.

Is yours 50HZ? If so, check for heater cathode leak in one of the valves as the Dallas version of the AC30/6 or AC30HW60 sometimes didn't ground the heater supply.
If 100HZ check the EL84s are ballanced as they should be.

[django420]

Is yours 50HZ? If so, check for heater cathode leak in one of the valves as the Dallas version of the AC30/6 or AC30HW60 sometimes didn't ground the heater supply.
If 100HZ check the EL84s are ballanced as they should be.

Yes the fundamental seems to be 50Hz (see screenshot). Interesting enough, when I unscrewed the supporting brackets for the power transformer, the measured SPL amp front went down from 58dB to 51dB. I screwed back everything and it stayed at 51dB. It's still a bit annoying compared to my AC30/6 TBX that doesn't have any hum and only measures around 42dB, which is a more tolerable level to me.

Any clues on this? Still haven't checked for heater cathode leak. This is above my knowledge and would have to bring it to an amp tech for this.

[django420]

Ok did some more troubleshooting. Removed all the preamp tubes. "Dead" quiet. No hum. Put back one tube at the time and it seems to be something around V2 causing the problem/hum. Replacing the tube for a new one didn't help. Already tried to replace all tubes before anyway. Clues?