Edit limit reached...
Addition to the above: actually, your (@Rod Coleman ) EF80 Ia, Rk recommendation works slightly better (minimally more linear when reaching max. power) than what I had. So to me it looks the EF80 (and the EL802) are both good options.
Addition to the above: actually, your (@Rod Coleman ) EF80 Ia, Rk recommendation works slightly better (minimally more linear when reaching max. power) than what I had. So to me it looks the EF80 (and the EL802) are both good options.
If we assume that 8w can be obtained on a primary impendance of 5.2K we can calculate the voltage excursion in the primary of the transformer and knowing the amplification factor of 300b of 3.9 results in a voltage delivered by the driver of approx. 60v, this is the voltage required for attack on the end tube grid
Power depends not only load, but operating point (Ua, Ia) too, which is assings grid swing to A1 limit.
Obviously, a functional point must be chosen to ensure a symmetrical excursion, but for entering A1 the driver must have a capability to drive the final tube, without this breaks the harmonic distribution and increases THD
The biasing in #79 with R1 adjusted to 400 looks really good to me. The EF802 operates in A1, I'd say. Highly linear to max. power of about 9W. Harmonics nicely falling from H2 onwards.
I think what I will do is the following: I will create a base with 425V PS, 300B biased like above. I have now a couple of driver stages (EL802, E88C SRPP, 2x 6SN7), looking interesting and I will design for it to be replaceable. That way I need to invest into iron and tubes only once and can build 3 amps to play around with, using only a little more money.
Some driver stages need diff voltages (e.g. the 6sn7, but they can be easily derived from the 425V).
Of course the driver stages have different gain, but none has so little that my preamp will have way too less output voltage.
I think what I will do is the following: I will create a base with 425V PS, 300B biased like above. I have now a couple of driver stages (EL802, E88C SRPP, 2x 6SN7), looking interesting and I will design for it to be replaceable. That way I need to invest into iron and tubes only once and can build 3 amps to play around with, using only a little more money.
Some driver stages need diff voltages (e.g. the 6sn7, but they can be easily derived from the 425V).
Of course the driver stages have different gain, but none has so little that my preamp will have way too less output voltage.