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Hello,
I checked the biasing on my power amplifier two nights ago using compubias. The amp has 4 EL34's and are recommended to be biased at 12W -18W which is 50%-70%.
Left tubes where at 13w and 14w and the right tubes 12w and 13.5w.
I decided to adjust them to 14.7w per tube which I managed to do. I noticed an improvement.
Last night I checked the bias again and whilst the left tubes seem to be quite steady at 14.5w the right tubes are reading at 13w and 13.5w.
My question:
Can anyone help me by explaining why the reading is now different. Should I try re-biasing the right tubes again? Why are there fluctuations?
Thx so much, Bob
"You have to leave something to your imagination"
Follow Ups:
Swap the tubes and see if the weak bias follows the tube. If so the tubes are going bad. If not check your socket pins, clean tighten and recheck. If that doesn't work you have an internal problem.
This is an amp set to Class AB, push-pull? Most EL34 max idle dissipation rate is 25 watts. 15 watts = 60% of max. Which is much better than 12-13 watts, IMHO.
As for the discrepancy between tubes, one set of tubes maybe weaker than the other.
Hello FenderLover,
Thx for that. Yes, it is Class AB Amp.
I'm measuring the tubes again and the right are still a lower watt. Do you think the tubes will be damaged if I raise the bias again to get to 14.7w? If they are weaker, should I expect the bias to drop to a lower level again?
Kind regards, Bob
"You have to leave something to your imagination"
or can not sustain a certain bias value it means that the tube is gassy and its failure is eminent. It's best to replace them the sooner you do it the better it is for your unit.
If a thing's worth doing, it's worth doing well
(Proverb)
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