Marshll 1987 clone blowing fuse

[rhaigh]

I have a plexi clone I built about 6 or 7 years ago that i've never had any trouble with. Never blew a fuse and I been playing live with it just about every weekend since I built it. Now, when I turn on the power switch it blows the 2A fuse in the power section. This doesn't always happen. Sometimes it doesn't blow a fuse, and as long as I leave the amp on, I can use it fine all night. Since the standby is still on when it blows a fuse, I can't see the trouble being after the standby switch. Everything seems to be fine in the mains tranformer primary circuit including switchs, bulb, fuse sockets, and the transformer itself.

Any ideas?

Like I said, once I get the amp to power up (after going through 2 or 3 fuses), the amp seems to work perfect.

[rhaigh]

I forgot to mention that the bias circuit is NOT on the transformer side of the standby switch. In other words, the bias circuit is disconnected until you flip the standby off.

[Don Moose]

We're talking about the Mains fuse, not the HT fuse, right?

Does this happen if you power up with the STBY in PLAY mode?

Since it's before the STBY, that says it's gotta be one of:

- A problem with the standby switch itself ... arcing to the chassis?

- Primary filter cap beginning to fail at the elevated B+ it gets on standby (the load of the power amp drops it a good bit)

- A problem in some tubes' heater or the heater wiring

- A problem in the bias supply - but that's usually such a low-current rail, I can't see it being likely.

Hope this helps!

[Don Moose]

I forgot to mention that the bias circuit is NOT on the transformer side of the standby switch. In other words, the bias circuit is disconnected until you flip the standby off.

This seems like a really bad idea - I understand it hasn't been a problem up to now, but slapping the outputs with B+ and ZERO bias for several milliseconds while the bias filter charges sounds like a good way to hit that whole section with some punitive currents ... unless there's a lot more filter after the STBY so the B+ takes a few mS, too.

Hope this helps!

[Enzo]

How old are the power tubes? Once it is running, does rapping on the side of the power tubes do anything? It shouldn't.

Pull the power tubes and cycle it off and on. Does the fuse hold? THE MAIN reason fuse blow in tube amps is bad power tubes.

Is your HT fuse the proper 500ma (1/2 amp)?

For better or worse, his bias is wired just like the original Marshall.

[rhaigh]

Thanks for the help. The problem ended up being a faulty standby switch. The switch worked but must have been grounding out sometimes. Anyway, back in business!