1963 vox ac30 troubleshoot

justinwelson · 2011-10-15T22:57:22+00:00

Hi everyone Im new to this so im really hoping for some help regarding my amplifier. I have a vintage 1963 vox ac30 that has been giving me some problems of...

1963 vox ac30 troubleshoot

makepeace

I guess the first thing to ask is how long its been since its been re-tubed? After, that it could be anything from stale capacitors to a popped solder joint.

vic

Justin, my guess.... could be a blown rectifier valve....the big one, next to the mains transformer (on the right looking from behind). Or one of the pre-amp valves (check the last 2 on your right, again looking from behind the amp). You'll see whitetish silvery stuff on the inside of the glass wall of the valve if it's blown, and of course such a valve won't glow when the amp is switched on.
Unplug from wall socket.....Slide the amp out, remove the aluminium shield around each pre-amp valve and switch in on for a few seconds, checking for glowing valves. If these are OK but still no sound, switch off and take it to a Tech.
Take care not to fiddle inside when the power is on !! (always unplug from the wall socket please)
Good luck.

justinwelson

Thanks alot guys, with regards to the tubes I replaced them with some second hand tubes last year but they were from a friend who is paranoid about changing is amps tubes all the time. They all seem to light up fine. What I have done now though is left it to stand while being on, and it seemed to have no problems. My thoughts are whether its an over-heating issue or not, because it only seems to happen when I jam on it.

vic

justinwelson wrote: Thanks alot guys, with regards to the tubes I replaced them with some second hand tubes last year but they were from a friend who is paranoid about changing is amps tubes all the time. They all seem to light up fine. What I have done now though is left it to stand while being on, and it seemed to have no problems. My thoughts are whether its an over-heating issue or not, because it only seems to happen when I jam on it.

It could be a cap/s as well then....but I still think it could be a bad preamp tube (ECC83)...it's one of the signs. but rather get it checked out...you don't wanna mess up a transformer or part of the circuit.

makepeace

Vic wrote: Take care not to fiddle inside when the power is on !! (always unplug from the wall socket please)

Vic is right, be very careful. You don't ever want to fiddle with the insides of a tube amp, even when its not plugged in. The power supply filter capacitors store lethal DC voltages.

Considering the age of this amp, I would take it to a tech to replace tubes and do a general once over. Based on your explanation of the problem, replacing the filters is a must (if they haven't been replaced already, which is likely) and definitely check the solder joints.

By the way, if you have any DIY inclinations, its relatively easy to do these type of things yourself if you do a bit of reading on the subject and ask questions on this forum and others.

Good luck

Zulublues

Hi Justin

If you have no sucess, try Glen Turrell. If you need his number PM me. He has done some great repairs for me in the past and his prices are very reasonable. He is based in Westville.

Ciao

nick

justinwelson wrote: Thanks alot guys, with regards to the tubes I replaced them with some second hand tubes last year but they were from a friend who is paranoid about changing is amps tubes all the time. They all seem to light up fine. What I have done now though is left it to stand while being on, and it seemed to have no problems. My thoughts are whether its an over-heating issue or not, because it only seems to happen when I jam on it.

When you swapped tubes did you rebias the amp?

ezietsman

nick wrote:
justinwelson wrote: Thanks alot guys, with regards to the tubes I replaced them with some second hand tubes last year but they were from a friend who is paranoid about changing is amps tubes all the time. They all seem to light up fine. What I have done now though is left it to stand while being on, and it seemed to have no problems. My thoughts are whether its an over-heating issue or not, because it only seems to happen when I jam on it.
When you swapped tubes did you rebias the amp?

AC30 is cathode biased, no biasing can be done.

justinwelson

*To make peace*
Im always interested in learning more about these amps. What do you mean about replacing the filter caps? Is that something that should be done on a regular basis?

vic

Justin,looking from the back, try swopping around the first two LH -side pre-amp valves (vibrato circuit) and the last two RH-side preamp valves. See if the problem persists. Let us know please.

AlanRatcliffe

justinwelson wrote: What do you mean about replacing the filter caps? Is that something that should be done on a regular basis?

Regularly, but fortunately not often. Electrolytic caps perish and generally need replacing every 15-20 years.

nick

ez wrote:
nick wrote:
justinwelson wrote: Thanks alot guys, with regards to the tubes I replaced them with some second hand tubes last year but they were from a friend who is paranoid about changing is amps tubes all the time. They all seem to light up fine. What I have done now though is left it to stand while being on, and it seemed to have no problems. My thoughts are whether its an over-heating issue or not, because it only seems to happen when I jam on it.
When you swapped tubes did you rebias the amp?
AC30 is cathode biased, no biasing can be done.

I didnt know that :-[

justinwelson

Vic wrote: Justin,looking from the back, try swopping around the first two LH -side pre-amp valves (vibrato circuit) and the last two RH-side preamp valves. See if the problem persists. Let us know please.

When the problem first happened, i swapped out all the tubes with a bunch I had lying around and the problem never went away. Still had no volume from when i turned it back on. Changed the tubes back, turned it on the next day and it was fine. My concern is the inconsistency, dont want to be jamming with it somewhere and it just cuts on me.

vic

justinwelson wrote:
Vic wrote: Justin,looking from the back, try swopping around the first two LH -side pre-amp valves (vibrato circuit) and the last two RH-side preamp valves. See if the problem persists. Let us know please.
When the problem first happened, i swapped out all the tubes with a bunch I had lying around and the problem never went away. Still had no volume from when i turned it back on. Changed the tubes back, turned it on the next day and it was fine. My concern is the inconsistency, dont want to be jamming with it somewhere and it just cuts on me.

mmmmh....better sell that amp ( to me) ? just kidding. Rather take it to a tech then and get it sorted out. Keep us posted.
Post some pics of the amp please . What colour is the control panel ?