#1
I picked up a Vox AC15 C1 a little while ago and I've been really impressed with it so far. The previous owner upgraded the speaker to a Celestion Blue and it's amazing! But yesterday it started making a "whooshing" wind-like noise. Everything else on the amp seems to be working fine (no pops, crackles, scratchy knobs, or anything else).

When I flip it off standby the noise appears if I'm plugged into the top boost channel (or if I don't have a guitar plugged in at all). But if I plug into the normal channel the sound goes away. I can play through both channels, but in the top boost channel the "wind noise" is always there while I play.

Any ideas what might be up with it? My initial guess is one of the tubes might be going. Is there a specific tube I should check first since it only happens on the top boost channel?

Also, why would the sound be there when there's no guitar plugged in? It goes away as soon as I plug into the normal channel. Could that be a sign of some kind of grounding issue? Any help would be much appreciated!
Current Gear:
2002 Gibson Les Paul Standard
'57 AVRI Fender Stratocaster
MIJ Fender Jaguar Special HH
Marshall JVM410
Vox AC15 C2
#2
I personally would suspect a preamp tube but I'm not an amp tech. The fact that it only happens in Top Boost mode says a lot. I wonder if that amp has a dedicated tube for that boost. Edit: I see there are only 2 preamp tubes and technically one is a phase inverter which means it is part of the power section.

Also what happens if you turn the top boost volume down and the master volume up?

What I would do first is swap the two preamp tubes and see if that changes anything. You could also maybe try putting a dummy 1/4" patch cable in your FX loop if you are not running anything there.
Anyone have any better ideas.
#3
Quote by 311ZOSOVHJH
What I would do first is swap the two preamp tubes and see if that changes anything.

This.

The V2 (the right preamp tube, if looking at the rear of the amp) is the 2nd gain stage only utilized by the top boost channel. If you swap these, and notice the same thing happening on the normal channel, then you have a bad tube and simply need to replace it. If it doesn't fix it, then take the amp to get looked at by an amp tech.
#4
Cool, that seemed to fix it. Also, it looked like the tubes didn't want to sit all the way down in the current layout, but when I swapped them I was able to get them seated firmly. And I noticed the input jacks were both very loose so I tightened those as well.

So anyway... some combination of the above fixed the problem. Thanks!
Current Gear:
2002 Gibson Les Paul Standard
'57 AVRI Fender Stratocaster
MIJ Fender Jaguar Special HH
Marshall JVM410
Vox AC15 C2
#6
Quote by 311ZOSOVHJH



Glad it worked and thanks for coming back to notify. Your next step should be to buy 1 or 2 decent (JJ, JAN, or NOS) preamp tubes and some Dioxit contact cleaner for the tube pins.

Thanks! The tubes in it are actually gold pin JJ's. The previous owner swapped out the stock tubes shortly before he sold it. They sound great with the Alnico Blue

But I will look into the contact cleaner. That would be a good idea to keep around anyway...
Current Gear:
2002 Gibson Les Paul Standard
'57 AVRI Fender Stratocaster
MIJ Fender Jaguar Special HH
Marshall JVM410
Vox AC15 C2