Biasing and tube life

[Structo]

No it was the 6L6GC-STR which I believe is their tall bottle.

[krash]

I have biased JJ 6V6's as high as 35mA and seen them run for months of regular gigging under those conditions. I would think a 6L6 that flames out at 33mA must be a flaky tube. Or maybe it was screen current that was causing it to fry.

I get many years of use out of 6V6s in my Princeton Reverb with it biased at about 25+ mA and 410+ V plates.

[Structo]

I guess the bias current value would depend heavily on the plate voltage at the power tubes.
One bias number cannot be global.
Thanks for that chart, that will come in handy.
My power tube plates are around 430v so when bias at 34ma that's pretty cold according to that.

[Lonely Raven]

One bias number cannot be global.
.

I think you got it now. That one line nails it right there, and even a newb like me....I get upset when I hear guitarists state XXma is the golden spot for such and such tubes. That's not right, it's circuit dependant!

Side story:

I couldn't find my bias rite, and I had loaned my Fluke to a friend, so I biased my TW by ear, and did the same for my Peters Dual Channel which I happened to be loading up with JJ KT-77 the same day the TW build was finished.

I admit, I did redplate the TW at first, but that was due to the bias pot being turned the wrong way first time fire up. I dialed TW and the Peters by ear, then later checked with my Fluke once I got it back...the Peters was a hair over 70% and TW a bit under 70% and right about where others have found these two amps dialed in right.

In the past I had heard of techs who could bias by ear and be bang on or close enough to be safe...I didn't believe it till I did it for myself.

I'm going to try this again in the near future with some other amps, and see if I can find those sweet spots on other amps. It does help if you know what the amp is supposed to sound like, and if you know when it sounds too cold (since you should be starting cold and working up anyways).

Sorry to get off on a tangent, this thread just made me think about that.

[Phil_S]

OK, so to get my terms right, a hot bias is increasing the plate current and a cold bias is lowering the plate current.

Yes, but you might find it easier to think in terms of bias voltage, with the more negative number as colder and it gets hotter as you move towards zero. For example -25v is hotter than -33v. Indeed, the -25v will produce more mA on the plate.