Sovtek Mig-60 blowing fuse

[Fartrider]

Hey I have a Mig 60 that's been messing with me for a while.
The problem was initially bad/weak sound from the preamp. It sounded good at first but after 20 minutes of playing things starts to sound very weak and fuzzy. Now it's like that all the time, and last week ater i changed the output tubes and rebiased, I played a bit with the chassis still open and then i saw a flash and it was silent. I looked inside and found out that one of the fuses was blown.

It's the "F1" 500ma fuse, here's a schematic: http://web.aoct.org/marshall/Mig60.jpg

Oh and the amp is way more noisy now than before.
Is it maybe a filter cap problem?

Maybe its time to change those, I mean the amp was built in 94 and I don't think anyone has changed those yet. Those 100+30uf caps seem a little hard to get by though. Would 50+50 work?

cheers

[torcamaniac]

Try a better fuse. 630mA or more just to test.
Also check all the bridge rectifiers. You could replace these with higher current rating, maybe 1 to 2 Amperes more than the original.
Also measure heater voltages to preamp tubes.
It could also be a faulty switch.

[Enzo]

And never assume that because the power tubes are new, one of them cannot be bad. Even new tubes can be bad.

[Chuck H]

What is the bias current?

Noise + HV fuse popping usually means filters or power tubes. You replaced the tubes so I would naturally suspect the filters. But see Enzo's post. Since the amp got noisier after the new tubes were installed that's a good reason to suspect the tubes. You may also have a bias condition that didn't demonstrate itself when you were setting it. The bias supply also has electrolytic caps in the circuit and those should be replaced any time the HV filter caps are replaced. Looking at the design I think the 50/50 caps would be fine.

[g1]

Date check on OP, aisle 3.