Vox AC30 Reverb Model Possible Choke Problem

[Iron Works]

Oops I meant 60Hz. I just did it again by firing it up and blowing through a straw onto the tube socket (and the leads running to it). It hummed LOUDLY after a few seconds of blowing (OK I can hear the jokes in your mind about humming and blowing...). This might happen normally, given that it's quite humid here along the northwest coast (Vancouver, BC), and the amp did not hum on startup because I've had it running several hours today already (but it might tomorrow or the next day).

I'm in the process of replacing the nasty stinky wiring one piece at a time and that tube - the third gain stage of the Vibrato circuit coupled with reverb return -- has all new wires except the heater wires. The new wiring has not seemed to help, but rinsing and scrubbing the sockets does. I'm hoping not to replace tube sockets if I can avoid it as I've not seen good replicas of the 1960's Fenders and the shields these days are rather thin and cheap feeling. Anyway I put a phone-app spectrum analyzer on it and it's definitely 60Hz. Surprisingly, once the amp is warm or if I hit 2 or 3 sockets with a heat gun for 4 seconds, this amp is quieter than most of this era.

[EDIT: the humidity here today is reported to be 46% at 23 degrees Celsius]

[EDIT: I did replace the PI tube socket when I first began suspecting sockets. After removal, I used a Fluke 75III set to ohms to measure from lug to lug. The readings ran from "open" through 9M, 5M and even 3.7M, suggesting that there was conductance there, and that's without 200 VDC or 6 VAC and the current capacity of an amp. After an isopropyl bath and scrubbing on that socket I got "open" readings across all lugs. Now it's 4 days later and I still get "open" readings but the socket looks a bit greasy so might pass higher voltages/current across lugs.]

[Helmholtz]

Makes sense.

the humidity here today is reported to be 46% at 23 degrees Celsius

That's rather low humidity.


Seems that some hygroscopic deposit has covered most of your amp's innards. In this case it is very probable that also the board became conductive (won't inject heater hum but might cause other problems).

[Chuck H]

Seems that some hygroscopic deposit has covered most of your amp's innards. In this case it is very probable that also the board became conductive (won't inject heater hum but might cause other problems).

That's what I thought too. I've had plenty of trouble with those black paper boards. Never the sockets. I would say that whatever is in there must not only be hydroscopic, but also prone to conductivity when wet because there's not much moisture in breath and if it can burn off in five or ten minutes in a warm amp then it's not taking much moisture to cause the hum. This is a problem if the amp sounds amazing because major repairs (like replacing the board and all sockets) might alter the tone to some degree.