[crestavega]

I am beginning to understand perhaps. please can you advise me if I am correct that C805 (highlighted in red on attached snippet) is the Grid Coupling cap. in question here?
this is apparently a 400V 4.7nF ceramic.
am I right in thinking that the purpose of this cap is to pass RF from the previous driver stage onto the control grid of the PA tube, but to galvanically isolate this grid from the (presumably high?) dc voltage associated with the anode of the 12HG7 drive tube.
if high dc bias was to get onto the grid, I can see that excessive current could flow in the PA tube.

I have to confess that I cannot see where the 12HG7 driver valve gets anode volts from. presumably there is a good reason for specifying a 400V cap in that position though.