Four Terminal Pot?

[stokes]

Thats why you always have to take the results of a tube tester with a grain of salt.Quite often the tube will appear fine at the lower volts in the tester and then arc internally when hit with full voltage.It sure sounds like you just kept putting the bad tube in a different socket.

[apehead]

That's what I had suspected. Wow, I took another look at the amp due to another, unrelated problem (see the thread "A different Kind of Power Tube Arcing"), and I made a very stupid mistake . I hooked one of the screen jumpers the pin four (the actual screen) instead of pin six (an empty pin used for a soldering post)! Schematic of mistake. And it still ran!

[Enzo]

To add to what others have said, tube testers will verify a tube as bad, but they cannot tell you a tube is good.

Look at it this way, that screen resistor connects to a power supply and the tube. That's it. The only path for current through that resistor is through the tube. Since the screen is not normally connected to anything else inside the tube, the only way the screen resistor can burn up is if the tube is shorted inside.

[apehead]

That makes sense, and this is why this forum is so great. I thank you all for helping me to better understand my equipment.