Is the issue that you can't get, or don't want to use 7591s as the power tubes? I have rebuilt a Eico ST40 amp designed for 7591s to use 6L6s, but these were connected in triode mode. It may not be what you want, but it may give you some ideas. http://www.audiokarma.org/forums/index.php?threads/eico-st-40-redo-modo.211563/page-2
I'd like to stay with cheaper , more readily available tubes , I have quite a couple of 6P3S's ( ~6L6GT ) , some EL34's , some KT88 ( these appear to be quite expensive though ) , some GU-50 ( ~LS50 , I have sockets for these too , if I do then use voltage multiplier , output loads of power easily enough , but at very high voltage ) .
Thanks for the link , I'll check it out , perhaps I'll learn something .
Likely 6600 ohm - but you can determine that easily by feeding a some 100 volt (50 Hz) into the primary and calculating the primary impedance from the transformer ratio.
That's how you do it ! I remember watching a youtube video by " Uncle Doug " and his funny dog now that you mention it , just needed a reminder , thank you Ampdog !
Unlikely, as the OPT ratio does not permit a low enough primary impedance (high enough current) to reach 30W. Preferable to go the voltage-doubler power supply route. Any reason why not?
I wanted to keep it simpler , if it were possible , but if i have to go the multiplier route - so be it . Did you look at the multiplier in the original circuit Ampdog ? HT linked to bias supply . Is that an " OK " way to go ? Or rather keep them separate ?
Best triode operation is with higher load impedance (around 8K - 10K), thus high power supply voltage - in the 450V - 500V region.
So the voltage multiplier can definitely NOT be avoided then .
Something along Williamson line? A very good approach, with the right component values.
Yes , exactly , I think 30W from a pair of KT88's or LS50's in triode/pentode mode can be had with relative ease ( with higher HT then ) .
Note: The 7591 is somewhat of a 'supped-up' EL84; higher dissipation, output power at higher permitted voltages. In that sense different from a 6L6. The 6L6GC is rather more freely available but requires twice the grid signal voltage - not a problem with the pre-circuit at your determination. (However hardly driveable by the popular 'cathodyne' phase splitter - don't go that route as the higher power Dynacos did.)
Pity about no screen taps. Still, many pentode amplifiers existed with pentode feed, if somewhat more sensitive to output load (loudspeaker) and NFB stability. As said, triode performance would be compromised due to low primary impedance.
If the 30W is do-able with 6P3S ( with no screen taps ) , I'd like to use them just because I do have loads of them . If not , I'd prefer to go either EL34 or GU50 route , for economic reasons . For the money I reckon the KT88's can be put to better use elsewhere .
What I would do?
Use voltage doubler power supply etc. plus pentode operation and Williamson or Leak type topology, and hope the OPT plus associated circuitry will allow enough NFB to give good fidelity. Has been done. (You should have enough h.t. to reach 30+W output with cathode-bias.)
It would be foolish of me to ignore your wisdom and experience , so this is then what I'll do . Do you not think the fixed bias is possible ? Would be good for unmatched valves ?
Caveat: The E182CC is a good driving twin triode but the spread can be daunting because they were intended for computor use - low 'on' voltage and long no-current life - rather than analogue consistency. I use them but have to select.
I have a stash of 16x 6N6P's that have been measured and marked at least , I'll use the balanced/matched triodes pairs for drivers , and unbalanced/matched pairs for input/phase-splitter .