Overheated Vox AC30 CC2 late 90s/early 2000s edition - repair help

Hey audio technical wizards,

My brother and I need some help. We're trying to repair his VOX AC 30 which was left on for a couple of days on Standby by mistake. We're not near any dealers at the moment to drag it in, so trying to see who here can help us diagnose the problem.

We got a new fuse and replaced it. We could have blown the transformer, but I thought I would try to replace the tubes first. I talked to one guy in LA who can service it when I get back and he said replacing the tubes isn't going to matter at this point. So, my question for you guys is:

Can the tubes be overheated? Is it possible this is an easy fix or unlikely?

He also mentioned if I put in new tubes and the fuse didn't blow then it would confirm the transformer. But if the fuse blew with new tubes then it would be a tube problem. Did I hear that right or do I have that backwards? I've never played with the internals on his VOX and I have a Fender Twin, so I wanted to get some advice from any geniuses here who may have some troubleshooting procedure we can go through to solve where the issue lies and any temp fixes we can do during the holidays. He can play his Blues Deville through the holiday but if we can get the VOX going for the shows it would help immensely.

Any advice?

well,

You haven't actually described the problem.

It's going to be hard for any of us to comment without having some idea of the symptoms you are experiencing.

"Standby" implies that the heaters in the tubes were being heated but that the plates and grids did not have high voltage on them.

 With the amp is standby the amp shouldn't be as hot as when it is not in standby mode... and leaving a amp on for a few days is not typically something that will hurt it... although the original Vox line did have a reputation for running hot and overheating, the reputation is largely attributed to lack of ventilation. Maybe the reissues are accurate. ;-)

 The point I am trying to make is that you probably have some routine part failure. A burnt out transformer is NOT routine... where as a burned out resistor, bad tube, or dead capacitor is common.

 You might try replacing some tubes... and they might fix the issue... but if it was a bad tube the tube will probably have seemed bad...  internal sparking, red glowing plates melting into Salvador Dali shapes, fire, smoldering black soot on the sockets... that sort of thing.

 If it's something else that killed one of your tubes... it will kill your new tubes too.... so you'll want to know what really happened.

 Take the amp to your tech... or at least pull the chassis and look for burnt out parts before placing a fresh tube in.

 all the best,
mike

one thing none of us has stressed

DONT TOUCH ANY THING INSIDE YOUR TUBE AMP... EVER!!! UNLESS YOU HAVE THE SKILL TO CHECK THAT THE POWER CAPS ARE DISCHARGED or YOU HAVE THE SKILL TO WORK ON A HOT AMP.

A TUBE AMP HAS DEADLY VOLTAGE AND AMPERAGE RUNNING INSIDE IT... THE DEADLY VOLTAGES SOMETIME REMAIN AFTER YOU HAVE TURNED OFF AND UNPLUGGED THE AMP. IT MAY KILL YOU IF YOU TOUCH THE WRONG THING.

So, just look and do not touch... got it?

Honestly, that is sort of like saying that burning a light bulb continuously will make it too hot. These tubes were designed to be banged around in portable radio sets... and these tubes were also designed to be run 24/7 in stationary radio sets. It may be a tube... it's surely something simple. One bad power tube will not result in silence... you'll get distressed and reduced power.

One bad preamp tube will not usually result in complete silence... it may switch out the input signal but you can check for self/system noise in the amp. Is it totally quiet? The fuse between the amp and speaker will trip if the speaker connections are shorted. If the speakers shorted... it didn't do it by itself ;-) and so I imagine we would have been entertained with stories about how much fun that was. :-) Check that fuse if the amp is totally silent.

The most likely candidate for a completely "dead" amp is a power supply filter capacitor. Does the amp appear completely dead?

If a power capacitor went down in a blaze of flaming glory... it is possible it took out a power transformer... but modern amps are usually protected from that exact scenario. So the cap may die but the transformer will be ok.

If you have a bad power transformer you probably will be blowing fuses or circuit breakers on the AC line. The windings in the transformer could short to open and then not trip again after a reset... but that usually takes some dedication... meaning a few resets and more shorting until it's burned through. Most people would remember and mention that. Did that happen? :-) Also this usually happens when you suck too much power through the power transformer. Only a terribly built tranny is going to melt and short while sitting at idle with no demand. So, I think it is unlikely it happened on this Vox.


It can also be a simple parts failure or bad connection of any number of small coupling capacitors or undersized resistors (which ARE susceptable to over heating problems). Symptoms for these issues can be all over the map.

What are the symptoms?

best,
mike

oh man, I never answered this one.......whoops.

I believe it was something internal in the capacitor. The tubes were fine. I'll have to ask him again. It's been awhile since we dealt with that one.

I find this hard to accept / understand. If you cant safely leave an amp on overnight - how have guitar amps historically survived doing 4-6 hour gigs for years on end without a problem?
 
I just left a stereo tube amp in my studio on for 3 or 4 days last week - with no problem. This amp is functionally no different than any tube guitar amp except that the tubes probably have better ventilation.
 
I would be very dissapointed in such an amp if any of the internal components cooked just by leaving it on overnight - especially if it was on standby. This is not normal.
 
HOWEVER, one thing I have encountered recently is bad rectifiers. The AC30 is supposed to take a GZ34 - if it is one of the tube rectifed versions. Most of the currently manufactured GZ34s are pretty crappy compared to NOS - and on top of that, JJ was known to have had a bad run a few years back and the Chinese versions are said to just be bad in general. This year I had 2 different JJ's GZ34's die on me within 2 weeks. That said, my opinion is that they sound pretty good (for those that can stomach the idea of a rectifier having a sound).
 
My bet, for the guitar amp troubles described by the OP, would be on a blown rectifier. It is not that horrible to leave a tube amp on overnite. Current manufacture rectifiers are often defective tubes that would have been weeded out by QA at the factory in the old days. (Someone said that tube customers perform the QA these days :)
 
Fortunately, when tube rectifiers go, they usually dont hurt the rest of the circuit.
 
If you dont have a tube tester, you probably shouldnt bother trying to find remedies to a blown amp unless it is something obvious like a fuse or burnt resistor (but ofter these can be symptoms of another problem.) BTW - when a rectifer blows it usually takes the fuse with it.  A few trips to the tech for tube swaps and other simple repairs at bench charge and higher ($50 and up) and a tube tester starts to look reasonably priced.
 
Mike
 
Mike