vox ac30,1972tb intermittent drop in volume?

[rickmac]

hi folks,
while playing for some time (amp warm etc)there is quiet a noticible drop in overall level using the brilliant channel?then it returns to normal again for a while,then repeats its little performance?any ideas guys?
cheers rick

[Prairie Dawg]

hi folks,
while playing for some time (amp warm etc)there is quiet a noticible drop in overall level using the brilliant channel?then it returns to normal again for a while,then repeats its little performance?any ideas guys?
cheers rick

If this amp has not been serviced in a while it's best to start by developing a baseline of known good parts. That would mean a set of known good tubes and a thorough evaluation of the electrolytics and replacement as necessary.

Once you have those matters in hand you can then proceed to start looking for trouble. The fact that it only happens in the brilliant channel is significant, and it simplifies things to a certain degree. Perhaps the preamp tube that serves the brilliant channel is calling it quits. Easy to check. You can swap it with another and see where the problem goes.

Something is heating up and taking itself off line for a breather but that can be nearly anything. What you might try is to get the chassis out and opened up-connected to your speakers of course, and let it heat up until it starts misbehaving. Then you can take the can of circuit chiller you bought at Radio Shack-or if you're lucky, a can of real freon from Dad's workbench-and squirt anything that looks vaguely suspicious that is in a place where it could get kinda hot.

On the other hand, getting the chassis out where it can breathe may cool it off enough so that the problem does not replicate itself. If this is so you can always try the opposite, using a hair dryer to see whether you can induce a breakdown.

All in all, an interesting problem. Let us know what the resolution was and best of luck.

[HTH]

+1 on swapping out the tubes - I've pulled my hair out on some recent repairs looking at everything EXCEPT the tubes and it turned out to simply be bad tubes (doh!).

If I was looking at an amp that old, I'd be looking at putting new filter caps in there and also testing the signal caps to make sure nothing is leaky.

[R.G.]

The first step in every tube amp repair is to eliminate tube problems; swapping tubes with known good replacements is often the simplest way to do this.

There's a reason those things are in sockets, not soldered in.

[Prairie Dawg]

The first step in every tube amp repair is to eliminate tube problems; swapping tubes with known good replacements is often the simplest way to do this.

There's a reason those things are in sockets, not soldered in.

Have you ever seen the ones that were used in proximity fuzes? You wanna talk about a short lifespan...

[rickmac]

vox 72,drop in level

hi guys,
thanks for the responses,ill let you know the outcome asap
take care
rick

[Prairie Dawg]

hi guys,
thanks for the responses,ill let you know the outcome asap
take care
rick

Sounds like a plan. My old crew chief always believed that you should start from the outside in, and work from the simple to the complex. This does two things. First, it eliminates many of the more common trouble spots, and second it enables you to focus in on the more complex stuff with confidence.

After you've been working on the same product for a while you get to know your way around and you can often go straight to the problem area without the preliminary/intermediate steps but this requires considerable experience.

This method of processing and applying information was acquired during 12 years of working on aircraft turboprop engines, primarily Garretts.

But it's good for anything, really.

[Robert M. Martinelli]

Hi rickmac,
You stated your amp to be a '72. If so, it should be one of the first built on Bakelite PCBs (by Stolec, AKA George Stowe Electronics). Due to this, as a footnote to the above excellent suggestions, and if swapping the tubes doesn't fix your problem, I'd advise you to check all the amp's solder joints for cracks, especially the ones on the PCB, which is known to be pretty fragile. A bad/cracked solder joint can give rise to the symptoms you're dealing with.

All in all, I'd say that redoing all the solder joints is an advisable practice anyway.

Hope this helps

Best regards

Bob