[DIY] OT: Tube amp disaster

[Tony Allgood]

Thank you for all the help in this matter. Crate UK are non existant,
since the demise of Washburn UK. But I managed to get a copy of the
schematics from Crate in the US which they sent to me free of charge.
This is what I found so far.

The smell was caused by two cathode resistors belonging to both sides of
one of the pre-amp tubes, a Sovtek 12AX7. The tube appears to have a
blown heater. There is also a dead jFET, which switched in a capacitor
across one of the resistors when the channel settings are changed. Now,
I'm going to replace all the dead stuff, and power it up again with my
variac and see what happens. The schematics are fairly useful because
they also have test points with PDs to measure and verify. But, I have
no idea why a blown heater could cause the cathode current to go
skyward. The tube tests open in the right places. And this was no slow
heat up, it was an explosion, the jFET is just a shell, and bits of the
resistor lie about the PCB.

I think the 8 ohm into the 4 ohm thing probably was a bit of a red
herring.

But this amp, a Blue Voodoo 60 is a terribly designed piece of
equipment. As the amp warms up, the PCB warps, and breaks contacts with
the front panel mounted pots/sockets. It is no suprise to me that they
no longer sell this item.

Regards,

Tony Allgood  Penrith, Cumbria, England

Modular synth circuits, TB303 clone and Filter Rack
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan