The fault every player dreads
You switch the amp on. It comes to life, the pilot lamp glows, everything looks normal. About ten seconds later it dies. No bang, no smoke, no drama; it just goes dark and stays dark. You check the fuse, find it blown, fit another, and get the same ten seconds all over again.
That was the story with a Vox AC30HW2 that came in recently. It's a horrible feeling as an owner, because it looks for all the world like the amp is dying in front of you. The good news, and the reason this one's worth writing up, is that it wasn't. It was a single worn-out part doing exactly what that part does when it reaches the end of its life, and the expensive, irreplaceable bits of the amp were never in any danger.
A quick word on what the AC30 is
The AC30 is one of the genuinely iconic British amps: the chime and jangle behind a huge amount of music you already know. This particular model, the AC30HW2, is the hand-wired version, built point to point rather than on a printed circuit board. That matters here for one reason: it's an amp built to be worked on and to last, and it uses a valve rectifier, which is part of what gives it its feel. That valve is also what failed.
What actually went wrong
Every amp needs to turn the alternating current from your wall socket into the steady direct current its circuitry runs on. In a lot of modern amps that job is done by a small solid-state part that rarely gives trouble. In an amp like the AC30HW2 it's done by a valve, the rectifier, and a valve is a consumable. Like the output valves that colour your tone, the rectifier wears out. It's a service item, not a permanent fixture.
When this one reached the end of its life, it started pulling far more current than it should as it warmed up. That's why the amp lasted about ten seconds each time: it ran just long enough for the valve to heat up, then drew so hard that it blew the fuse protecting the amp. The fuse was doing its job perfectly. It was the rectifier behind it that had gone.
The part that matters: what it wasn't
Here's the part that actually matters to anyone who owns one of these.
When an amp blows a fuse the moment it warms up, the frightening possibility is always the transformers: the heavy, expensive, hard-to-replace lumps of copper and iron bolted into the chassis. A failed mains transformer on a valve amp can be a serious, costly repair, sometimes enough to make an owner wonder whether the amp is worth saving at all.
So before assuming anything, I tested them. Both transformers were completely healthy. So were the amp's filter capacitors, the other part that often gets blamed and replaced on principle. None of the costly components had failed. The whole fault came down to one valve; a part that's meant to be replaced periodically, that costs a fraction of what those transformers would, and that takes minutes to fit.
I mention this because it's easy, faced with a dead amp and a blown fuse, to start swapping the big-ticket parts on suspicion. That's how a straightforward job turns into an unnecessarily expensive one. I'd rather prove what's actually wrong before touching anything, so that what gets replaced is the thing that failed and nothing else. In this case that meant a new rectifier valve and a clean bill of health for everything around it.
Back to full power
With a new rectifier fitted, the amp came back up properly: no more ten-second cut-out, full voltage where it should be, steady under load. I ran it up to full power and left it playing for a good while to be sure the fault was genuinely gone and not just hiding, then put a guitar through both channels. It sounded exactly like an AC30 should.
One small extra while it was here: a cabinet foot had worked loose and come off, so I refitted it with an additional washer so it won't shake free again. The kind of thing that's trivial on the bench and annoying at a gig.
The takeaway
If your valve amp runs for a few seconds and then dies with a blown fuse, don't assume the worst. It's a real fault and it needs sorting, but very often the cause is a consumable part reaching the end of its life rather than a catastrophic failure of something expensive. The rectifier and output valves in a valve amp are designed to be replaced from time to time; that's normal, and it's a world away from a dead transformer.
The trick is to find out which it is before spending money, not after.