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Re: Biasing multiple EL84s

Posted by Scholl on August 27, 2002 at 14:22:40:
The difference between 10 and 12 volts bias between 4 sensitive tubes like the 6BQ5 can cause enough of a current difference in the output transformer to impact the bass roll off of the amplifier. If a single bias control or cathode bias is used only one bias voltage will presented to the 4 tubes. One tube might produce 40ma at 10 volts another maybe 35 ma another maybe 42ma the forth maybe 38ma. Now if fixed bias where used with an adjustment for each tube and each one adjusted for 40ma itâ€™s not so important if one tube is biased at 10v while another is at 12 etc. dc balance is present in the output transformer for best low frequency performance. These values are used for example only.
My last adventure into PPP was with a pair of Frazier 102 monoblocks using 4 6GT5s per channel. I had 18 tubes to work with 10 of them NOS. It took 1 Â½ hours to find close sets. The bass response started at 40hz and improved to 24 hz â€“1db. Who knows what this will look like in two years without revisiting them every several months.
Getting tubes to work well together isnâ€™t a fine art, it just takes time and care, and has to be revisited two- three times a year thatâ€™s all. New matched tubes with separated bias controls should make this job easier and faster. Keep in mind if Beard engineers didnâ€™t think it mattered they wouldnâ€™t of added some form of regulation to keep tube bias in check. They would have used a single cathode resistor and saved a few bucks and some time.
Search the Asylums and youâ€™ll find more postings on this subject.
And noone talked about AC balance in this thread. I think Iâ€™ll go over to SET Asylum for a while.
Good luck.