One of our AC30s is bust

[DannyP]

One of our AC30s gave out in soundcheck last Friday - no power-on light or standby light. Luckily we had a spare.

It's a far-east jobby (CC?), it has normal and top boost channels, but only one of each (unlike our spare AC30 which has two inputs for normal and two for top boost).

We checked the fuse that's in its own little torpedo-bay and that was fine.

We checked the fuse under the kettle-lead socket and that was blown, but when we replaced it, the amp still wouldn't work.

We checked the kettle lead and that's fine.

Anyone have any ideas what might be wrong with it?

[ICBM]

If it's a CC (I think so from just having two input jacks), almost certainly a blown rectifier valve. They do this because of a combination of crap modern valves and the standby switch being in the wrong place in the circuit.

If you're going to replace the rectifier with another valve, avoid Sovtek and possibly Ruby - JJs are better, and I've heard good things about TAD but not used them personally. Or, replace it with a solid-state plug-in if reliability is more important than the fairly small difference in tone and dynamics.

If you really want the valve rectifier sound, avoid using the standby switch - leave it on permanently. This might shorten the life of the power valves a bit, but no more than that.

[DannyP]

Brilliant, I'll get on to that!

So I'm just swapping out the one GZ34? Should be straightforward - fingers crossed.

Thanks so much for the advice

[ICBM]

Forgot to say - if it is the rectifier, you'll most likely need to replace the mains fuse again since the new one will have blown as soon as you turned on.

[DannyP]

Righto, cheers.

[musophilr]

If it's a CC (I think so from just having two input jacks), almost certainly a blown rectifier valve. They do this because of a combination of crap modern valves and the standby switch being in the wrong place in the circuit.

I've had a blown GZ34 in my CC2X. It it an easy fix to rewire the standby switch to where it "should" be? IIRC did you say in another thread it should be AFTER the smoothing caps not before?

[ICBM]

I've had a blown GZ34 in my CC2X. It it an easy fix to rewire the standby switch to where it "should" be? IIRC did you say in another thread it should be AFTER the smoothing caps not before?

If I remember right, it wouldn't be that easy to move it to the best place - after the first filter cap but before the feed to the OT - but would be reasonably easy to move it to the second best place, feeding the choke. This would still leave the OT and the power valve plates powered up, but would shut down the screens and so kill the valve current.

Some amp manufacturers (eg Peavey) do it this way on purpose, in fact - possibly to reduce the chance of arcing in the standby switch, since the current being switched is much lower.

[DannyP]

Just to conclude this story....

We decided to ignore the problem and use our spare AC30 for a bit.

Then a couple of weeks later, exactly the same problem happened to the other original AC30 two days before a gig.

Suddenly I find myself having to source two rectifier valves locally and ended up paying through the nose (£50 for two Mesa gz34s - ouch!).

We've ordered two sovtek solid state plug-ins for £15 a pop for future use.

Moral of the story - keep spares, make sure you back-up gear is maintained and don't be a tight, lazy sod like me!