G'day Don,
As ED says the low screen voltage is probably reducing the power significantly. The Pentodes Bias volts required will increase when you increase the screen voltage.
I'm also worried about the 20K Ra (the anode load rsistor) on the 12AT7 stages. That will cause a lot of distortion.
For guidance on this: As the current in the tube changes then so does the internal rp. The change in rp value with signal current swing compared to load resitance IS the distortion level, so keep Ra value up for lower distortion.
A handy reference for recommended Resistor Values and what Gain you can expect in Resistance Coupled Amp Stages:
http://www.tubebooks.org/tubedata/hb-3/ ... ifiers.PDFThe 12AT7 data is about the 6th page. You will note it only covers Ra values of 100K (0.1M) and up. It assumes that no-one will want to use values lower than 100K
If I'm being OCD today then I would also say that EVERY triode should always have a grid stop resistor even though a lot of amps schematics don't show them, in fact there a re quite a few amps which do have them (gridstop resistors) but the schematic does'nt show them.
The minimum recommended value of a gridstop resistor from the long since safely deceased guru's of tube design is equal to 8 divided by the triodes gm. gm for 12AT7 varies with operating point but from the datasheet it will typically be greater than 4mA/V.
Calc: 8/0.004 = 2000.
So minimum of say 2K2 on 12AT7 (compared to minimum of 5K1 for 12AX7).
gm for 6BM8 triodes is approx 2.5mA/V so minimum 3k3 grid stops for those.
When you ditch the zener on the screen supply you will probably want to drop the value that 15K too. 15K will give you a lot of compression altghough you may like that.
EDIT: Also - next time you buy shielded tube sockets DON'T buy those shiny silver things. They concentrate the tube heat back into the tube. You want the ones which have the black inside and outside of the shield to first absorb the tube heat and then radiate it from the outside. The shiny silver on the shield inside reflects the tube heat instead of absorbing it and the silver outside is crap at radiating what little heat is absorbed. Using the shields with black inside and outside make a surprising difference to tube temperature. Your tubes will last longer. Check out some of the old Tektronix or Hewlett Packard Tube based Test equipment - full of shielded sockets on which the shields are black inside and outside.
Cheers,
Ian