Vox AC30HW hums

[xtian]

Client brought me his nice AC30HW 2x12 combo. It hums badly when tubes start to conduct. I pulled out ALL the tubes except rectifier, and it still hums. I tried a new replacement GZ34, same hum. I'm getting only 250vDC at B+, though the ripple is only 1.5vAC, not too bad. So I suspect the reservoir filter, or the OT.

I'll get back to this job later in the morning, but would be happy if anyone had previous similar experience. Thanks!

[xtian]

OK, no trouble, friends. I clipped in a new reservoir cap over the existing one, and the noise vanished, and B+ was much higher.

[Colossal]

Nice. Out of curiosity what did B+ climb to at what mains input?

[xtian]

Mains are about 118 today, and B+ was about 300, subtract 10v across the cathode resistor.

[Colossal]

Awesome, thank you. 300V is perfect DC for an AC.

[xtian]

Man, 200mA for four EL84s is HAWT. But I guess that's the thing.

[Roe]

Man, 200mA for four EL84s is HAWT. But I guess that's the thing.

yes a cooler bias doesn't sound as good unfortunately

[peckjed]

Interesting to hear! I have an AC4HW that also hums. Out of curiosity did you replace the reservoir cap or put another in parallel/series? A value if you have if!

[xtian]

Interesting to hear! I have an AC4HW that also hums. Out of curiosity did you replace the reservoir cap or put another in parallel/series? A value if you have if!

I replaced the reservoir with same value. (Don't remember what value...too many lyrics in that memory space.)

[peckjed]

Interesting to hear! I have an AC4HW that also hums. Out of curiosity did you replace the reservoir cap or put another in parallel/series? A value if you have if!

I replaced the reservoir with same value. (Don't remember what value...too many lyrics in that memory space.)

That’s interesting. I wonder why replacing the capacitor with the same value would cause the B+ to increase and hum to go away.

[chief mushroom cloud]

Man, 200mA for four EL84s is HAWT. But I guess that's the thing.

yes a cooler bias doesn't sound as good unfortunately

I think Keith Richards would argue otherwise....#Joker