[AMPS] SB-220 Conversion Confusion

[a.p.]

Amp aficionados, et al;

I have a 6 M SB-220 conversion that works quite well with
one tube. Input SWR is 1.1:1 and output plate tune is at
about 1/3 mesh. Efficiency is as expected. However, when I
put two tubes in the beast I have to go to min plate C for
max output. There's plenty of plate current and input SWR is
just as good as for the one tube case. The output coil has
been modified  from 1.5T to 3T with out much change in the
two tube performance. Any one got any ideas on "optimizing"
the output pi for two tubes? Note I have done a few of these
successfully but this particular amp is weird.
Any ideas on what might be causing this?

TIA
Art


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