[Patrick Turner]

Its a little known fact that one may take advantage of the UL
connection even when you don't like to.

Most ppl think the UL connection for EL34 always means 43% screen
taps.
This % was always regarded as optimal because triode like THD is
achieved and Rout reduced to give Rout = RL. But this was mainly for
class AB PP amps.

With pure class A1 PP amps or SE class A1 amps there is no reason why
ppl should not use UL taps at 75% of the P turns.
One might say well the triode connection is the best but you'll only
get about 6W from an EL34 in triode. The power output is rarely more
than 27% of power input because the Ea minimum swing is restricted by
the onset of grid current. But when you look at the load lines you can
see that one could swing a bit further if you had the tube in pentode
and get 9W from 24W of idle power.
Well, To get the swing you need, try screen taps up to 75%.
The Ra will then be about 2k which is not much above triode Ra of
1,250 ohms.
And RL can be say about 5k, so the DF is still OK, and linearity
virtually equal to triodes.

I have tried this trick in conjunction with CFB windings where the CFB
= 12.5% and UL taps are at 62.5% of the total P turns. If the OPT has
5K:5 ohm ratio, you get Rout = 1.1 ohm.
Overall gain is then like a 300B, about 4.5, Va-k divided by Vg-0V.
About 48Vrms to the grid is needed for clipping.
I get 8.5W with Ea = 350V, Ia + Ig2 = Ik = 66mA. Distortion is
virtually all 2H until near clipping.


Patrick Turner.