Power Tube Biasing - Fixed

[Pressed_Rat]

Okay, this is related to fixed bias push-pull amplifiers. My son's amp (JCM800 clone) needed new tubes. One was redplating pretty bad - drawing a lot of current. To make a long story short I got him some matched EH 6CA7s - big bottle. Well after about an hour another tube started redplating - sounded pretty bad. By the time I got it back from him to look at it, the HT fuse had blown. Heater voltage was really skewed. I found the artifical center tap (two 100 ohm resistors) were shot. I replaced them and rebiased. The one tube is still redplating - shot. I have an extra E34L and an extra EL34 tube laying around (JJs). I replace the bad tube with a good JJ E34L. Well, when I go to bias the EH 6CA7 tube to around 38 ma (450 volts B+), the JJ E34L reads like 20 - 22 ma. Swapped them around, the bias voltages follow the tubes. So I swapped out the JJ E34L with the JJ EL34. With the EH 6CA7 biased at the 38 mA reading this JJ EL34 reads 9 mA. Same deal the biasing follows a tube swap.

So my real question is, is there a correlation between bias drift between tubes that is indicative of tube aging? Even with a dual bias setup I doubt I could bring these close with the limited bias adjustments available (22 Kohm trimmer). I am thinking the other EH 6CA7 is bad. Maybe all three are bad.

Any ideas here? I am going to just get another matched set - maybe KT77s. But, the bias drift has me wondering.....

[zaphod_phil]

I think there probably is some correlation, although I'm not sure what it is. While this isn't an 18watt related question, I guess the same issue could also be relevant to EL84s in an 18W. You also find that some brands of the same kind of tube bias hotter (or colder) than others.

[Alexo]

I think the bias point does usually drift a little once tubes are burned in a bit, combine that with EH not always producing the most reliable tubes for running at high voltages, and after an hour of burn-in it's conceivable that 1 out of 4 may have drifted significantly enough to redplate ..?

[Pressed_Rat]

While this isn't an 18watt related question,

Sorry, I forgot . Posted over @ AX84.