Eventually blows fuse

[ricach]

I've got a Randall RG50TC (tube amp). It works fine for me for hours on end, but when my son's friend uses it for practice, he gets the main fuse to blow after about 10 minutes of playing. Obviously the amp is being played more agressively, but he doesn't recall doing anything in particular the moment it blows. Any suggestions on where to start troubleshooting this problem?

[g1]

Can you verify it has nothing to do with the location? Is he using it somewhere else or at the same place you use it? Maybe not the most likely problem but it would be good to be able to rule it out.

[jrfrond]

Aside from AC power issue, to wit, high voltage out of the wall (an unlikely cause), it might be a runaway power tube or a bias issue, i.e power tubes biased too hot. Since it takes a long time to happen and is intermittent, you'd have to start by looking at the most likely suspects. If it starts humming before it craps out, I'd suspect a borderline filter cap or rectifier diode in the power supply.

Something I will often do to "smoke out" a problem (don't try this at home folks; I have over 30 years of experience and monitor current draw CLOSELY) is to use a much higher-rated fuse and/or overvoltage a unit to force the problem to the top in order to find it. Yes, it's guerilla-style tactics, but sometimes it is highly necessary, and not only in tube amps.

[Steve Conner]

Yup, the old trick of fixing an intermittent fault by turning it into a permanent one!

I also vote for a runaway power tube, get your son to look in the back and see if he can spot any of the tubes glowing abnormally red before it blows. Or goose the amp into a dummy load and see it for yourself.

[ricach]

Aside from AC power issue, to wit, high voltage out of the wall (an unlikely cause), it might be a runaway power tube or a bias issue, i.e power tubes biased too hot.

As a fact of matter, I noticed it would start getting noisey as it got warm - nothing plugged in, pounding on the top of the amp increased the noise. I started by replacing the power tubes and as I rebiased the new ones noticed the bias on one side was waaay off by almost 30 percent spec. Had him play at practice today, he hasn't blown a fuse after a couple hours of playing. I think that was it.