Blew a screen grid resistor. Tube issue?

[Anonymous]

The tubes are old in this amp (Fender Twin Reverb clone). I had the volume nearly maxed out, and then a pop and blue light as I race to flip the power off. I guess the chances are pretty low that this just happened under normal operation? It was a 5W cement resistor.

I should probably just replace it and retube? I'd kind of like to know for sure it was a tube that caused it, but I guess it would be stupid to put the same set back in to see and risk blowing another resistor, or something worse?

 

[Third Age Amps]

If the fuse blew, it's probably the tube. If the fuse is still good, it's probably just the resistor.

The good news is that you know the resistor is bad, so change it then turn it on for a short time with the old tubes in it and see if it blows a fuse.

If it does, a power tube is probably shorted. If not, i'd bet the resistor had just worn out. That's what sucks about those resistors. If they go bad they can take out a tube and if the tube goes bad, it can take out the resistor.

 

[Anonymous]

If the fuse blew, it's probably the tube. If the fuse is still good, it's probably just the resistor.

The good news is that you know the resistor is bad, so change it then turn it on for a short time with the old tubes in it and see if it blows a fuse.

If it does, a power tube is probably shorted. If not, i'd bet the resistor had just worn out. That's what sucks about those resistors. If they go bad they can take out a tube and if the tube goes bad, it can take out the resistor.

Thanks for the info! Fuse was good. In fact, after dissecting it, so was the resistor, and so was the tube.

When I built the amp, I used needle-nose pliers to bend wires. Apparently, I wasn't careful enough. When I cranked the amp, it must have been enough to vibrate a B+ wire into just the right position to short it to a nut bolting a tube socket in (I'm assuming there was a small hole in the wire's insulation). Not to mention all the current flowing through it. Changed that out, and now I'm all set!