[Boatanchors] TZ-20 Amplifier Questions

[Niel - W0VLZ]

I've been working on a RF amp using a couple of TZ-20s in push-pull. 
Please see http://w0vlz.blogspot.com/search/label/TZ-20%20RF%20Amp Now 
that I have it up and running its not quite performing like I expected.

1) The Taylor tube manual indicates I should get over 100 watts input if 
B+ is 750V. I'm running B+ at 500V and getting only 25 watts input max. 
An over 75% hit in power for dropping B+ 33% feels like a big hit.

2) The manual indicates 75% efficiency. At 25 watts input I'm seeing 
about 10-12 watts out. That's only around 50%, not 75%.

Any combination of my three TZ-20s seems to produce the same results so 
I don't think it is the tubes.

Three things I can play with are bias, B+, and output coupling.

I'm using a 2K bias resistor as recommended in the manual for two TZ-20s 
in a RF amp (I assume Class C). Does this need to change when running at 
500V? Is there some way I can use the tube curves from the manual that 
are linked into my blog pointed to above to calculate a new/better bias 
resistor value?

Is the 75% drop in power to be expected given the 33% drop in B+? If so 
I'll need to come up with a 750V supply.

What is the best way to couple this amp to a 50 ohm antenna system? 
Again see the tube manual page I pointed to above. The plate resistance 
of a zero bias TZ20 is 26.7K while the plate resistance of a T20 is 8K. 
Typical triodes seem to be in the 8K and lower range. I believe the 
optimal size of the link is tied to the plate resistance and the size to 
the tank coil, correct? On 80 I ended up with a 40 turn tank coil. How 
many turns should be in the link? or is there some other way to couple 
to the antenna that I should consider?

Thanks for any help.

73,
Niel - W0VLZ