Peavey Classic 400 screen supply

[Randall]

A Bohemoth Classic 400 comes in with no sound whatsoever. Damn, this thing is heavy, it is all I can do to get it out of the box and onto my bench. No sound out of the speaker or preamp out. Also, no sound feeding the power amp in jack. OK. I find both 16A screen grid fuses are open (page 1). This also feeds the preamp plate supply, that makes sense. Nothing looks damaged, and I find no obvious shorts, so I popped in a couple of 12A fuses I had on hand, and it powers up and makes music at relatively low volume. It's played several songs from an ipod so far.

So leaves me wondering, why would both fuses go, and why is it operating now on 12A fuses. I'm wondering what to do to from here?
, other than crank it? I guess I will run the tubes thru the tester and look for shorts. Anyone have any thoughts?

http://www.hartsafire.com/classic400..._service-2.pdf

[The Dude]

I'd suspect a bad output tube and it may not show as shorted on a tube tester. My experience is that the mains fuse blows before those fuses blow, so you should check that someone didn't put a 30 amp car fuse or tinfoil in the main fuse holder.

[Leo_Gnardo]

SIXTEEN AMP screen grid fuses ??? Hokey skokies! I can only imagine the proper size fuses - at least an order of magnitude smaller - blew due to shorted output tubes, then Johnny Amptech slotted in the biggest fuses he could pry out of his '72 Pinto's fusebox when he changed the output tubes.

edit: AND what The Dude said ^^^.

After a blink at the schemo, SG fuses are s'posed to be 3/16 amp. An unusual value to be sure. Quarter amp fuses I'm sure would suffice. Johnny Amptech must have not seen the 3/ - or maybe figured any value from 3 to 16 will do. Who knows... There's an awful lot of dumb out thar.

[Randall]

YIKES! I contributed to most of that dumb. I read that schem as T3 16A, what a maroon! I'm lucky I didn't smoke something. The ones that were in there are marked 312 3AG 1A 250V. I'll get some 1/4A slo blo.

As for the 8 tubes, 1 is dead, 2 are loose in the sockets, 1 came up slowly to about where the rest were and then jumped up considerably higher, and then dropped back down, and 2 are a different brand, so a recommendation for a new octet is in order.

[Leo_Gnardo]

YIKES! I contributed to most of that dumb.

And here I was blaming Johnny Amptech... Na'e worry it's all good, seems you have the beastly beast under control.

[The Dude]

I thought Randall just forgot some decimal points in the initial post. Yikes is right!