Peavey VTX Classic repair

[dobbhill]

Amp repair for a buddy:
Went to replace tubes on this amp and the B+ fuse blows on High Power setting after about 10 seconds of playing. It was blown when I got it. I know nothing of cathode drive tube output sections and less about those silicon thingys (transistors). Attached is the 100 watt version schematic. Is this something you guys could help me with, or is it better to take this to the local (40 miles away) shop?
thanks,
D

[CaseyJones]

Oh, happy happy, joy joy.

These things are about as much fun to work on as a Line 6. Crack that sucker open and there's a printed circuit board in there completely encrusted with modern marvels of technology. There's a warning on the back that there's no user servicable parts inside and for the most part they mean it...

So, as always start with the basics. The Classic is an EL84 amp, right? That means there's no way to break a reference lug off a tube and stuff it in there the wrong way so we can rule that out. Next step... fortify yourself for a confrontation with rocket science and have a look inside. Look for anything burnt or anything the previous owner messed with. Those are the usual suspects, something was slightly wrong and someone made sure it became very wrong. If you're lucky it's just a burned resistor or a scorched diode in the power supply.

Something is drawing more current than it should which is what blows fuses. The excess current draw will leave clues in cooked parts.

[dobbhill]

Thanks for the reply, Casey.
Nothing burned, except one of the previous tubes' paint was darkened by what I assumed was overheating.
The owner has been advised to consider buying another amp......
D

[Karl]

You can check Q6-Q9. Desolder them and get a datasheet. Measure with diode setting on your DVM. Red probe on the base pin and black probe on the other pins, if the transistors are ok you should measure 0,5-0,7 from base-collector and base-emitter.

Transistors work like two diodes. NPN is two diodes with the cathodes pointing to the base and PNP is two diodes with anodes pointing to the base.

[dobbhill]

Thanks Karl.
I'll try that.
D