Oy...troubleshooting power section

[greiswig]

So I'd been tinkering with my KT-88 build, which has only one pair of KT-88s. I'd tried cathode biased mode for a bit, but decided that it was too much trouble. I returned it to fixed bias mode, and changed the screen resistors to 1.2k from 600 ohm. I also installed a choke instead of the "RF mod" 500 ohm resistor.

I also changed the bias feeders to 100k each, versus the 56k they were at before.

Powered up and played fine...for awhile. Then it started humming. Turned off standby, but something was still smelling hot. So I unplugged everything, waited for voltages to drop, and started poking around. It was one of the 1 ohm bias measurement resistors. Strange, too, is the fact that it still gets hot even when the amp is powered up but standby is still engaged (no B+ applied).

Any suggestions as to where to start looking for my screwup?

[greiswig]

Okay, it follows the tube. If I switch sockets, the symptom follows the tube. So the tube must have gone south somehow.

But it strikes me as pretty coincidental. What might I be doing that is pushing the tubes too hard and causing them to fail? Any thoughts? Plate voltage is running right about 520, with the screen 5-6V lower. Biased the tubes at around 41ma.

[Cliff Schecht]

That can happen, but KT88's are pretty damn tough cookies. Do you have another amp or even a tube tester to shove it in? KT88's are barely breaking a sweat at 520V..

Put the tube by your ear and rattle it. Shouldn't sound like much is moving around inside (you can compare the good one to the bad one for reference). I've taken out a few power tubes by dropping the amp they were in. Did you drop your amp?

[greiswig]

Did you drop your amp?

I vaguely remember throwing it out the back of the truck so that I could catch it, but I wasn't there to catch it, and it wasn't a truck, it was a cliff, and...

No, I don't think I dropped it. But I'll check the tube for rattles and report back. I had a second set of tubes, and the amp seems fine with them.

Thanks!