VOX AC15 virtually no output (volume) even with volume all the way up

[timatkinson8]

I bought a vox AC15 C1 in October 2014 and it has never failed or showed any signs of failing, but when I used it this morning very little sound came out of the amp even when the volume was fully turned up. I usually only have the master volume and volume only a quarter of then way up as I mostly use it in my room.
The tubes are glowing orange, which I assume is normal. Is it likely to be the tubes or the circuit that has caused this problem.

Many thanks.

[nickb]

Is it likely to be the tubes or the circuit that has caused this problem.

Many thanks.

No one can tell you. You have to diagnose the problem. Start with you guitar and leads - try different ones. Check the settings on amp and instrument. Is the hum and noise from the speaker any different from normal? How do the controls affect the sound? If you have spare tubes try them. Failing that you'll have to do some electrical testing inside. Do feel comfortable with the circuit side of things? Any doubts means take it to someone you trust.

[52 Bill]

The tubes are glowing orange, which I assume is normal. Is it likely to be the tubes or the circuit that has caused this problem.

Welcome to the place.

Your problem could be any number of things, a bad tube, a broken connection, a bad part, etc. Did you do anything differently this time? Check the speaker plug and jack and also the speaker impedance switch. Try plugging and unplugging the speaker cord a few times to clean the contacts. Also switch through the different positions of the speaker switch to clean those contacts.

You could try giving the amp a good thump with your fist to see if it causes any reaction.

[The Dude]

Welcome to the place. I've only had a couple of these in the shop, but by my recollection, both had bad tubes. I'm certainly not saying that is the case with your amp, but I'd probably start with tube substitution. Easy enough to do.

When you fire up the amp, is there any detectable hiss/noise coming from the speaker that might let you know if the output section is working? Another first check would be to hook up a known good speaker and see if that works. It could be something as simple as a shifted speaker gap.

[timatkinson8]

When you fire up the amp, is there any detectable hiss/noise coming from the speaker that might let you know if the output section is working?

No there is no hiss, however I can still get a quiet distorted sound from the amp when the master volume and volume are turned all the way up.

[oc disorder]

I agree with The Dude .. get a new pair of output tubes and try them first.. if no different you have a spare set that one day you can use.
It sounds a bit like there is a short on the speaker line.... I'm not sure how the speaker connects in the AC15C1 , either there is an internal plug and socket or it may be hard wired in the cabinet. I had an amp once that had the remnants of a jack plug in the extension speaker jack socket. I had to flick it out from the inside.
That provided a path (short circuit) that was zero ohms so not much got to the speaker ! Yours may not have this fault but it's an example of the weird and wonderful ? things that can happen and are hard to diagnose from long distance !

[Mick Bailey]

You say the tubes are glowing orange. With the smaller tubes (12AX7) they're two triodes in a single glass envelope. Each tube has two heaters. Sometimes one can fail. Check that they all have both heaters glowing.

[nevetslab]

Little or no output on Vox AC15C1

If after swapping out power and/or preamp tubes and you still have little or no output, is the amp vibration sensitive? Meaning, if you bang on the top or side, does it come back to life briefly, or make sounds. Does gentle tapping on the tubes excite it (wooden pencil or chop stick)

A couple months ago, I had a steady client's AC15 in for service...bad input jacks was the complaint. It was far worse than that. Getting the amp free from the cabinet, with all the torn Tolex preventing extraction was no picnic, but once free and on the bench for scrutiny, the input jacks were suffering from fractured solder joints. But, so were most of the controls on the main PCB. I didn't write down WHICH version of AC-15 this was...it wasn't a CC1X or C1X like we have in our rental inventory. The main PCB was held into place with the pot's threaded bushings, and with nylon snap-in standoffs...a real PITA to release to finally extract the board, allowing enough room to deal with getting pots removed, and full inspection of all the solder joints. The amp was very microphonic, had a lot of hum, and made a lot of racket when you banged on it. I found the filter caps on the main board with broken solder joints. De-soldered & re-soldered those, as it my normal procedure in curing solder fractures....with folding the component leads over for mechanical stability before re-soldering them. Poor layout job on the PCB design didn't help....all solder pads at default settings, pads too tiny being the result.

Found fractured solder joints on the power tubes and other preamp tubes during the scrutiny of the power supply/tube board, as well as solder joint fractures on the speaker jacks., as well as fractures on the Reverb in/out jacks.

Once all those were cured, it was no longer microphonic, hum virtually gone, totally insensitive to vibration, and output was strong and solid.

So, it's also possible your low or no output is related to these sorts of ailments.

When I dug into the AC-15 amps in our rental inventory, it was a totally different mechanical design, neither at all similar to the client's amp.

This may be beyond your skill level to tear into it this deep. If so, you're better off taking it to someone who's skilled at this kine of service. But, always worth the effort, as they're a great sounding amp, once cured!