wrote:

"It all about raising up the plate supply voltage while keeping the
screen supply under 400V - plus matching the load correctly. "

Seems to me to would have to load is pretty heavily, not just to get the power, but also to try to keep the plate volage down.

** The load impedance is doubled, since the max tube current cannot increase.

Double voltage into double load = double the power.


If you go to 2X 700 volts, it might not be a matter of the tubes,
it might be the tubes sockets.


** Most sockets can handle the extra voltage.

What does limiting the G2 do ?


** Keeps the screen current within limits.

Having the screens glowing bright is the biggest killer of power tubes.


..... Phil