SF Twin Reverb - fuse blowing

[lowell]

This is the 135 watt model w/ master volume. This amp blows the fuse every now and then. I cannot get it to blow on the bench. Customer says he has the amp fairly loud working quite hard when it happens. I'm thinking that an output tube is shorting at loud volumes with the vibrations maybe. What are people's thoughts on this? Oh, and there was a bad screen resistor on 1 tube.

[WholeToneMusic]

Did you try turning it on in the dark to check for arcing on the power tube sockets?

[Enzo]

What I hear when I read you r post is that you did NOT try a different set of power tubes. Occasional fuse blows is a classic sign of a failing tube. How old are the power tubes in it now?

And a bad screen resistor? There is only ONE thing that can cause a screen resistor to burn out: excess current through it. There is only one circuit path for that current, and that is through the tube. A burnt out screen resistor is a classic sign of a failing tube.

Starting to see a pattern?

[lowell]

Thanks guys that was what I thought too... and you confirmed it for me.