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Hello all,
I'm having an issue with a humming Vox AC30 6TB. It's one of the reissues from the 90s with a tube rectifier and vib-trem switch. The amp has several known design issues which increase its noise floor. I've performed the modifications described in the following video:
https://www.youtube.com/watch?v=Kfoj4ft_yuY
except I haven't elevated the heaters, I left the center tap connected to ground as in stock. So I have rewired the heaters with tightly twisted wire; cut the traces to the grids of V8 (the first stage for the normal and brilliant channels) and replaced them with shielded wire under the board, and moved the input grid stopper resistors (R30, R31, R33, and R34) to the end of this shielded wire and tied directly to the pins of V8; and reworked the grounds such that the input section is grounded separately from the power section. I also replaced a very noisy tube; I think it was V8. All of these improvements have decreased the noise floor of the amp, but it still hums very loudly with everything turned down and nothing plugged in.
I think it's a 120 cycle hum. It hums with all preamp tubes except V6 pulled. Pulling V6 stops the hum, as does switching the amp to standby. I've tried moving around my new heater and ground wires and tapping on things to no effect. I can definitely hear the power transformer physically humming as well. A known issue with these amps is that the power transformers start to hum and eventually fail, in some instances injecting that hum into the rest of the circuit which then becomes audible through the speaker. I wonder if that's what is happening to mine as well? I'm posting to see if anyone has any ideas for other things I can check before I replace the power transformer, and if not to make sure that theory makes sense.
I'm having an issue with a humming Vox AC30 6TB. It's one of the reissues from the 90s with a tube rectifier and vib-trem switch. The amp has several known design issues which increase its noise floor. I've performed the modifications described in the following video:
https://www.youtube.com/watch?v=Kfoj4ft_yuY
except I haven't elevated the heaters, I left the center tap connected to ground as in stock. So I have rewired the heaters with tightly twisted wire; cut the traces to the grids of V8 (the first stage for the normal and brilliant channels) and replaced them with shielded wire under the board, and moved the input grid stopper resistors (R30, R31, R33, and R34) to the end of this shielded wire and tied directly to the pins of V8; and reworked the grounds such that the input section is grounded separately from the power section. I also replaced a very noisy tube; I think it was V8. All of these improvements have decreased the noise floor of the amp, but it still hums very loudly with everything turned down and nothing plugged in.
I think it's a 120 cycle hum. It hums with all preamp tubes except V6 pulled. Pulling V6 stops the hum, as does switching the amp to standby. I've tried moving around my new heater and ground wires and tapping on things to no effect. I can definitely hear the power transformer physically humming as well. A known issue with these amps is that the power transformers start to hum and eventually fail, in some instances injecting that hum into the rest of the circuit which then becomes audible through the speaker. I wonder if that's what is happening to mine as well? I'm posting to see if anyone has any ideas for other things I can check before I replace the power transformer, and if not to make sure that theory makes sense.
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