Is red plating on power tubes a sign of too much voltage or too much current?

[Steve B]

Not that it's happening, but I wanted to know.

Thanks,
Steve

[Enzo]

Well, really it is a matter of the tube trying to dissipate more power than it is capable of. So in that sense it is both. But in a practical sense, the voltage is always going to be about what it is, so too much current flowing causes the meltdown.

If you design an amp that runs the tubes maybe for example 200v higher than they are made to handle, red plating is not the likely failure. The tubes are more likely to arc or otherwise fail catastrophically.

ANy existing circuit - Marshall something or Fender something etc. - is not going to give the tubes voltage problems. If the tubes are red hot, too much current is flowing. Maybe loss of bias, maybe HF oscillation, maybe just a bad tube, or whatever.

[Steve B]

Thanks Enzo,

I guess I remember trying to bias an amp by measuring the voltage, & then setting the bias accordingly. When I increased the current on the tubes, I seem to remember the voltage dropping a bit, & having to adjust the current again. Maybe I'm thinking about it all wrong? Raising or lowering current should not affect the voltage?

Steve

[Enzo]

The power supply is not regulated, so the more current flowing the more the high voltage will drop, so you do have to pay attention to both. But the point I was making is that the problem is too much curent flowing. You have to set the current with the voltage in mind because power in the tube is voltage across the tube multiplied by the current through it Yes they are interactive. Once the bias is in the ballpark, the voltage will settle down to about what it always will be with biased tubes in the ballpark.

[Steve B]

You DID answer my initial question, I was just making sure my feeble mind was understanding the concept.

Steve

[Amp Kat]

Hey Steve, this was kinda funny but I was hollering at Enzo the other day and he's got a Mesa Mark II on his bench and I have a Mesa Mark III. I was telling him about only one tube going cherry and the bias on shunt test was at about -45 and dropping lower every couple seconds then would start flucuating and then shoot up to 500ma's and of course the tube would just turn cherry. So he tells me yeh, it's most likely the coupling cap so I unhook the shunt and just monitor the bias voltage and of course after a few minutes it dropped to zero which is only reasonable and glowed again. I mistakenly changed the wrong coupling cap on the other side of the transformer and it actually half solved the problem. The tube stopped glowing cherry but still had a tad of fluctuation so I changed the other coupling cap and it was steady as a rock. So if you ever see any fluctuation in the bias voltage consider the coupling cap leaking especially when only one tube is glowing. Overall it's basically ohms law to determine red glowing. Any way you look at it there has to be voltage,current and resistance and the relationships are always the same.