Metropoulos Forum

I just purchased a fresh set of tubes because I have a show Saturday and just wanted security of having a fresh set in the amp. Like an idiot I did not test the matched quad in my tester because I was in a rush to get to practice. I was going to set the bias, flipped the standby, noticed a large hum and saw v4 getting bright from the top of the amp. I immediately turned the amp off. No blown fuses and no smoke or burning. I put two of the old tubes in to avoid the V4 position in case something happened, checked bias which was fine and played the amp and everything is fine. I put the other two in and noticed I could not get a bias reading on V4 although everything is fine when you turn to stand by. Today I checked the 1 ohm bias resistor for continuity and there is no continuity. I'm going to pull the resistor out tonight and check it out of the amp. I also put a new screen resistor in for good measure, however I do not have a bias resistor on hand and won't get one by Saturday either. All the resistor does is let you read bias so as long as I know the position of the hottest tube and ensure its not in V4 I can bias without the resistor. Anything I am missing and any idea why the tube would have taken out the bias resistor? I checked the tube on my tester, it's definitely a bad tube.

Why the resistor is gone? It depends on the wattage of the resistor and the current passing through it. The last one is the same current that goes trough the cathode of the tube (plate + screen current). If the tube was bad or the current was too much than this cold kill the resistor. I actually use 2-3 watt resistors in that place. There is a chance that the tubes were unmatched and when you tried to set the bias (or just to use the "Old" bias setting) the current was too high to begin with... You said that there are 4 tubes in the power amp section, so they work in pairs and in the "weakest" tube suffers the most.
Some thing like this I think