'85 Vox AC30 Noisy Trem

[wizard333]

'85 was the bad old days for Vox in a lot of people's opinion, so I wanted to ask this before I go digging.

What I'm hearing is a fading in and out of a background hum in time with the speed setting and independent of channel volume. Also, when set to the extremes, the trem circuit actually makes its own neat little UFO noise effect with no input signal applied.

I've never seen one of this vintage so I'm not sure, but was this just a quality/design issue they had at the time? Or is it something that I should fix? The owner is going to trade it, so if its a factory design issue I don't want to get into elminating it even if I can because he doesn't want to keep it, but if its something that isn't normal he wants it in good shape before it gets traded.

[Alex R]

What you probably have is some preamp hum that the trem is working on to create your wierd noises.

One of the preamp valves governs that part of the circuit, amplifying the signal before the trem cuts it. I'm not sure which one it is on the PCB AC30s, but try pulling them out in turn until the channel goes dead. If the funny noises go away too then you have isolated the noise to that part of the circuit. If not then it's more complicated and ignore the rest of this post.

Plug in a new preamp valve to replace the one you pulled, and see if that cures it.

Next possible culprit is the 32uF filter cap that's on the feed to that preamp valve's plates. The treble boost models, which yours is probably a version of, have two of these - easily recognisable as they are the only 32uFs in the amp. Sorry but again I don;t know the PCB amp's layout so well. Those caps probably need replacing anyway, so put in new ones.

If that doesn't do it, change the plate resistors on that valve (pins 1 and 6 by the way).

Oh hey, one more thing - check that your input jack socket tip contacts are earthing out when the guitar is unplugged. If not then they will make a lot of noise.

[Alex R]

Originally Posted by Alex R

- easily recognisable as they are the only 32uFs in the amp.

...actually no they're not but they are the ones on the preamp side, the others are usually in a can outside the chassis. Anyhow you know what I mean... And check those jacks first - quick, easy and a very likely cause.

[wizard333]

Thanks for the reply!

Do you know off the top of your head whether these were the same circuit as the classic AC30 just with pcb and cheap xformers? I only have the schematic for the older AC30s but I'd always heard these were different. If its the same circuit with different construction that makes life easier.

[Alex R]

I've worked on a couple of PCB AC30s and was able to assume the circuits were essentially the same without encountering difficulties. I don't know for sure but I think you can assume they're very similar.

[wizard333]

Thanks again!