Bogner Shiva Blowing Main Fuse

[Neutronarmy]

Hello,

I have a Bogner Shiva on the bench that is blowing the main fuse. Upon replacement of the main fuse, the removal of the power tubes, and gradual AC applied via a variac, all appears well. The amp pulls 500mA idle (off of standby), the bias supply is consistent, plates are consistent at 528V and the amp will run seemingly without issue. I also checked the screen grid and control grid resistors and both meter properly.

Guessing that perhaps the Svetlana EL34s had some sort of failure only present under high voltage/high current (they tested fine for shorts and emission in a tester), I replaced the pair with a set of JJ E34Ls I had lying around. I made certain to lower the bias voltage to the lowest setting (roughly -54V vs. the -47V it was running stock) before slotting in new tubes. The same behavior was exhibited: it idled for ~1 minute at 800mA before increasing rapidly. I managed to cut the power before a fuse blew, but the behavior happens consistently when power tubes are inserted.

Unfortunately, from what I've read online Bogner refuses to supply schematics. This model has a traditional power transformer - from what Google Images showed me a SMPS was later added to certain Shiva models. Any help in troubleshooting this problem would be greatly appreciated.

Thanks,
Chris

[g1]

With power tubes out, monitor the bias voltage at the output tube sockets for awhile. Should remain stable. Check both sockets.
Sounds like you may be losing bias after a minute or two.

[Neutronarmy]

With power tubes out, monitor the bias voltage at the output tube sockets for awhile. Should remain stable. Check both sockets.
Sounds like you may be losing bias after a minute or two.

g-one,
Thanks for the input - I really appreciate it. I've monitored pin 5 of both EL34s with a multimeter for roughly five minutes each and neither side loses bias voltage (consistent at -54V +/- 200mV). I also tried to move the pins slightly while metering to see if there was any change in reading from questionable contact and no luck varying the bias. This is definitely a confusing problem.

[Neutronarmy]

Quick note to anyone who happens to read this: I may have solved it. Underneath some of the heatshrink around the output tube sockets was some sketchy looking solder. I cleaned it all up and it seems to be working. I'll give it a run through its paces and if the issue doesn't pop back up, chock this problem up to mechanical stress on the tube socket connections.