testing RCA 6l6GC tubes

[docmh]

I recently purchased 4 used RCA Blackplate 6l6GC tubes. I was told they were a matched set. I have a bias tool that I use to check milliamps.
on my 2 6L6 VibroKing with the bias trim pot turned all the way up, one tube measures 62mA, one 58mA, one 56.5mA and one 41mA. I assume I can run the 62mA and the 58mA together, or the 58mA and the 56.5mA. I don't think they can be a matched set because of the 41mA tube. Also all tubes light up with a healthy glow on the filaments, but the 58mA tube which has a much more subdued glow on its filaments, it doesn't glow nearly as bright as the others. Since I don't really know much about tubes other than changing them and using my tool to set the bias, I don't want to hurt my amp. The seller said he tested the tubes in microhmos and they tested 5300, 5400, 5600 and 5700. I don't know what that means, but was told 5000 is new. Anyway I think I have one good set out of the 4 which I would bias at 35-38 milliamps. Any advice or input with knowledgeable techs and or players would be greatly appreciated.

Learning as I go along.

[rockon1]

I might adjust the highest ma one you measured (62) to 32 ma and then plug the others into that spot and see what they draw. From what Ive read tubes 5 ma or even up to 10 ma apart wont hurt anything. Too far and some hum might be induced. Bob

[Faustine Amps]

You shouldn't measure their idle plate current with the bias turned all the way up. Measure them at their real operating current. Put in the pair of tubes that measured 58mA and 56.5mA and set the bias to about 35mA on the highest measuring tube. Now take your measurements and determine how well matched they are, then swap in the other two tubes one at a time without changing the bias and measure their idle bias current. This will give you a better idea of how well matched they are at your normal operating conditions.

It sounds to me like you may still have one tube (the lowest reading tube) that's a little far off from the others. But if it's off by no more than 10 or 15% from the others, I guess you could call it "matched".

It sounds to me like the seller only matched them by transconductance and didn't measure their actual plate current. If he did, and he still told you they were a matched quad, than he has a looser definition of "matched" than I do.

Regards,
Tim Gregoire

[RussB]

Was this an ebay tube purchase?

[docmh]

Yes it was.

[GearHeadFred]

:nono62 mA ! Yikes! You are going to burn up something doing "tests" like that. If not the tubes, it might take out one of your transformers. You are setting the "idle" current to about 100% of the tube's rating when you do that.