What is the purpose of this cap?

[Steve Morley]

See attached simplified schematic. Is it to provide a bit of NFB to reduce gain at high frequencies?

Thanks, Steve

 

[Mark Tillotson]

Could be for compensation. Its a Miller capacitor if the anode is the output signal.

 

[Steve Morley]

Yes, the anode is the output.

 

[Mark Tillotson]

With a high gain triode, mu=100, then it would look like 1.2nF to ground from the input circuits point of view, reducing gain above 130kHz or so (with that 1k resistor)