I think I'm confused, and probably have a few things mixed into something that's not quite right.
I've been rolling a pile of tubes through my newly built 5G9-ish amp. I figure, I've got fixed bias, so let's see what's what with the tubes I've got on the pile. Two of these tubes I'm almost 100.1% certain are NOS. They are pristine looking, the boxes look like they came from the factory yesterday, and the seller (some time ago) was unloading some unneeded inventory from a high school. One is Sylvania, and the other GE rebranded RCA and both have coin bases. The Sylvania pulls around 18mA (I didn't check the bias voltage....not sure it matters if you follow my point), but the GE pulls around 60mA and doesn't cool off much, even at the coolest bias, about -35V, with plate voltage around 375.
First question, is the GE an outlyer? None of the other tubes on the pile do that, but none of them appear to be new, except for one that I think must be a reject. Is it safe to put this tube in an amp? Since it seems unlikely to match anything, I'm thinking it could be OK in a SE amp, but I'm going to have to increase Rk?
Leaving a good, not new, and one of the stronger tubes in one socket, I rolled the other tubes through the other socket looking for a reasonable match. I figure, for relative judgment, one tube needs to be constant. I left the bias voltage alone, so the static tube is at around 20mA. This seems like a reasonable spot. 20mA @ ~375V ~= 7.5W static plate dissipation. I recognize plate voltage will vary with each tube a bit.
The other tubes range from a low of about 8mA to a high of almost 30mA. Is it reasonable to conclude:
A) 8-12mA range is weak
B) 13-16mA is pretty good
C) over 16mA is strong
D) the one at 29mA is better than new?
There is one other tube in the pile that looks new, but only pulls about 11mA. It is branded Channel Master, the base is stamped Japan, the type is etched in a stop sign on the side of the tube. I'm thinking it's dud?
Am I headed in the right direction here or am I on a fool's errand?
Pssst....anyone wanna buy some 6v6's. D'oh, they're tested!
Bias confusion
Moderators: pompeiisneaks, Colossal
Re: Bias confusion
Groove Tubes made a fortune exploiting precisely this phenomenon. If you look at their old rating charts, you'll see a 9V bias voltage difference between a "#1" rated tube and a "#10." Considering that tubes are not only analog devices, but made (in part) by hand to tolerances that (by modern standards) you could drive a truck through, it's not surprising. As long as emission is consistent (the current doesn't wander all over the place) and doesn't fall off too sharply with reduced heater voltage, I wouldn't automatically assume anything about each tube's health.
As you say, tubes that seem to be outliers might be best used SE (and cathode biased, too), but you can also use pretty disparate tubes in PP fixed bias if you make provisions to bias each tube individually.
One caveat: a tube that requires very little (fixed) bias voltage might have problems with large input signals: it will be too easy to drive the grid positive and the grid will want to conduct. Those, you might want to use with cathode bias, which pretty much makes Class AB2 operation impossible.
As you say, tubes that seem to be outliers might be best used SE (and cathode biased, too), but you can also use pretty disparate tubes in PP fixed bias if you make provisions to bias each tube individually.
One caveat: a tube that requires very little (fixed) bias voltage might have problems with large input signals: it will be too easy to drive the grid positive and the grid will want to conduct. Those, you might want to use with cathode bias, which pretty much makes Class AB2 operation impossible.
Re: Bias confusion
Firestorm,
Thanks for the comments. I'm just trying to wrap my mind around what I saw while looking for a reasonably matched pair. I'm not driven by some compulsion here, but I did get curious. I think I'll let this be. I've got a couple of pairs now that I'll try out. If I can bias them to around 7.5W each, I'll assume they are operable.
Phil
Thanks for the comments. I'm just trying to wrap my mind around what I saw while looking for a reasonably matched pair. I'm not driven by some compulsion here, but I did get curious. I think I'll let this be. I've got a couple of pairs now that I'll try out. If I can bias them to around 7.5W each, I'll assume they are operable.
Phil