Well, that was odd. Why?

[iknowjohnny]

I was playing my amp at work and i had checked the bias and it was well lower than i saw last time i checked at home. Home=cheap ($40 digi meter) work=fluke meter. Now you'd figure the fluke was the one that was right. But considering my cheapo at home reads caps very well and accurately and the fluke at work is worthless at it, maybe it's also no as good at voltages? Could it be the fluke is fooked?

anyways, heres where it got puzzling. The fluke showed 7 or 8 MA lower than my home meter so i figured it was right and i set it for what is a normal bias for my EL34's at the plate voltage. It comes to around 37Ma at 70%. So i set it there and begin playing. After a minute the amp starts fading like someone jit the standby as i'm playing. I stop and then the volume comes back when i start playing again 10 seconds later and it begins to fade again. I'm thinking it's redplated and a tube is dying, but they look fine. So while it's fading i reach over while sustaining a chord and turn the bias down and the volume zooms right back up. So it was the bias being too high that caused it to fade.

Now, i never heard of that and don't understand how that would happen aside from the tubes going south due to to hot a bias. but like i said, no red plates. So i assuming the fluke is off, but how can that be? Could a worn battery cause that?> Then again, it's not showing a battery warning. Or could it be the home meter is actually more accurate because the fluke has an issue? It reads resistance just fine if that matters.

So i'm just confused here....if the amp COULD cause what happened by being biased too hot, then the fluke must be faulty and reading low. If a hot bias would NOT cause that as long as it's not red plating, then what did and why did turning the bias down bring the volume back?

[Bob-I]

difference in wall voltage?

[xtian]

Dirty or faulty bias pot?

[Phil_S]

Hot bias = serious clipping?

[iknowjohnny]

there is a few volts difference in the wall, but not that much. Bias pots (i have dual bias) are quality and i clean them with deoxit on occasion. Clipping....naaa....this was total volume fade from about 1/2 of stage volume to almost none. The odd thing is why did it fade like that. after all, it shouldn't even do anything out of the ordinary till it starts red plating, and even then it would just hum till death but not fade till they start to die i imagine. But none of that matters because anyways because they DIDN'T. I thought about a bad connection, but then why would turning the bias down a hair stop it and bring the volume back up quickly. oh well, not doing it now so...