• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Blown screen resistor

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Power amp with KT90s.

B+ fuse blew. Screen resistor on one tube also blew. (I use Radio Shack 1/2 watts, which act like fuses.) No further damage. No meltdown of cathode resistors.

KT90 is probably two years old, but has been in service only for a month or so. I hadn't pulled the old tubes for quite a while after I got the new ones.

Tube tests okay, shows just a little leakage from heater to cathode. No shorts far as I can tell.

I don't see anything else wrong. No sign of going off bias. Anything I should look for?

I haven't been working on the amp for quite a while. This just happened. No particular loud dynamics or anything like that.

Bad tube? Anything else I should be suspicious of?
 
Because I can't afford not to, I'm going with tube failure. Odd, given that the tube is so new, but it's also the only major change I've made in a while--in a rebuilt amp that has run trouble free for years. I cleaned the sockets and pins recently (and these are relatively new sockets), and I went over the solder joints when I had a problem a year or so ago.

Obviously, if this happens again, I'm going to have to rethink the "tube done it" answer.

Thanks, guys.
 
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