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Referencng the Decware Zen.
What would happen if I use one resistor to bias a dual triode driver tube ( 2 channel operation) or
use one resistor to bias stereo power tubes?
Follow Ups:
Steve Deckert discussed something similar in describing his Tori Mk IV amp. His amp was configured to allow him to switch in an ammeter to measure total current flow from his output tubes. When the ammeter was switched "on" it acted like a small resister downstream of the two separate cathode resistors. Is made a dramatic positive difference in soundstage. I simulated it by putting 5 ohm resistor downstream of my cathode resistors. It didn't make much difference that I can tell. However, my temporary listening room is not ideal. Read his article. https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1626054622/77
I use a 6n7 as the front end for my SE 2a3 amp. It has a shared cathode.
I also have the 2a3 cathodes shared and use a 10k transformer. Jim Dowdy's design. No cathode bypass resistor.
I think Dave Slagle posted on here once about the concept and the summary was it mono's the bass.
It works fine. Great center image. :)
I don't do fixed bias so I can speak to its superiority. I like simple.
If bypassed, it's fine, though as mentioned, matching is important. If it's not bypassed, there is inverted crossover between channels (out-of-phase left in the right channel, and vice versa).
Sharing a cathode resistor between channels always seems to cause greater distortion. It might also limit power a bit. The resistor, in any case, must be properly bypassed at audio frequencies.
A good example if you want to see this in action is the Dynaco SCA-35. There are mods to that amp that eliminate this 'problem'. You can see one example of what this mod does at the link.
The problem you are up against is cathode bias. It might be OK for one power tube or a pair of matched tubes, but its never going to be prefect. IMO you might be better off if instead the cathodes were simply grounded and fixed grid bias used instead.
In a driver circuit the extra resistor needed for a stereo situation is trivial so I see no point in going further since it is a compromise.
I have the same question about the 6sc7. A high gain dual triode like a 6sl7 but with one common cathode. How might that work with the left signal in one triode and the right signal in the other?
Tre'
Have Fun and Enjoy the Music
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