Electronics > Beginners
Tracking back 100Hz hum in a tube guitar amp
floobydust:
It's an awkward circuit because the output pentodes are AC driven filaments, with a bridge rectifier and filter caps for the preamp triode DC driven filaments, all driven off a single transformer winding.
The whole point of R18/R19 and R29/R30 are to keep balance between the AC and DC filament loads.
Look for symmetry, so the same voltages across R18 and R19 as well as between HTR GND and GND HDC should be 6VDC with little ripple.
I think you found a problem with 12.4V and 2V instead of 6VDC.
Most likely, one output tube might have a heater-cathode short/high leakage current there, which is imbalancing HTR GND.
I would check R18, R19 are the same value, same for R29, R30 and that C11 is OK. A hard heater-cathode short can cause current to flow through the DC bridge and near R4, C13 is mystery part NT2 which might be for protection against that. It would all cause hum and upset bias at R8/C21 as well.
The last Vox I was in, an antique AC30 with cracked coupling capacitor, covered in nicotine and booze. That was a weird repair.
eblc1388:
--- Quote from: Andreax1985 on February 02, 2019, 06:07:59 pm ---... the AC voltage across R18 and R19. Well, I read 12.40V across R18 and only 2.0V across R19. Is this normal?
--- End quote ---
It isn't, with such a big difference. Pull both EL84 to see if the voltage returns to approximately half the filament supply voltage. But judging from the large difference, it probably won't help much.
Look more closely to the schematic to see if there is any unexpected connection to the GND on this low voltage part of the circuit that breaks the symmetry.
floobydust:
CAREFUL
Pulling the output tubes leaves almost no load on the HV power supply and B+ will rocket up.
There are 350V and 450V caps, careful they do not get overvoltaged with little or no load there due to tubes being pulled.
The two power transformer primaries are in series?
Also, after removing power there will be dangerous HV stored in the filter caps, I don't see any bleeder resistors and the JFETS might take a while. Use your multimeter to check the voltages on the caps before going back in.
eblc1388:
That's a very valid point.
Andreax1985 can you confirm whether the unit has only one or two separate power transformers. If there are two, pulling the EL84 tubes would cause voltage increase of B+ and should not be attempted.
Andreax1985:
Hi! I can see only one power transformer.
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