Im still confused as to the role the grid resistor is playing in
the amplifier, if the bias is generated by a power-supply , then any
change to the psu output voltage would be resisted by the psu ...
assuming the feed is via a choke ?

Yes, the bias supply is a zener-stabilized -33V supply. I have verified that the
supply voltage stays constant independently of grid current.

where is the grid resistor .. from the grid to real earth ..or
in series with the power supply ??

The grid resistor (and a low-resistance RF choke) are both in series with the
bias supply. So, for zero grid current (no excitation), the grid bias is
just -33V (so keeping plate current at about 10 mA). When I apply RF drive, the
bias grows up (sum of -33V and the voltage that develops across the grid
resistor), so pushing the tube into class-C operation

Convention was to feed via a low value resistor / rfc or both
with a medium value bleed to earth in the 1 K range

To obtain a total of -50V @ 12 mA grid current (class-C operating conditions
recommended by the tube manufacturer), a grid resistance of about 1,400 ohm is
needed (33 + 0.012 * 1400).

Liner service then requiring the psu to resist voltage
fluctuation under drive variation and as such required a low Z path
to the valve and the ability to dissipate voltage .ie gas
discharge tube or dc feed back loop ... zener diode

I have the feeling this is a driver / pa grid matching problem ?

As I mentioned above the bias supply resists to grid current and its voltage is
perfectly stable.

73

Tony I0JX