WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Where is the best place to ground a HF decoupling cap for small signal stages? At that stages local ground, or at the main star point? Its Class A, SRPP stage with mono-polar power supply.
For best HF decoupling, I’d place the cap to the local stage ground. Placing it to a distant star ground could introduce path inductance and resistance that might not shunt HF as well. Think of the closed loop at HF.