function/purpose of capacitor from anode to cathode

[Dave H]

I should have started with the voltage gain expression for a simple bypassed common cathode triode: Av = u. R/(R=r) , where R is effective anode load impedance and r is plate resistance.

Assuming R is 78k, and r=60k, then voltage gain falls -3dB when R is reduced from 78k to about 40kohm.

A capacitive reactance of about 83k in parallel with 78k resistance achieves 40k impedance. A 270pF parallel capacitor has 83k reactance at about 7kHz corner frequency.

In the above calculation I think you need to use the point where the voltage gain would be -6dB for resistors but is in fact only -3dB (@ R = Xc) for a resistor and capacitor because of the phase shift. This give a Zc of about 33k, -3dB @ 18kHz for a 270p cap.

[Gregg]

I'm not going to throw any formulas or numbers here but talk from experience.
I've built several of those and I can say that the 270pf caps affect the overdrive tone in the upper range so they can be considered a tone control thing as well.
The local FB (actually 2x22M+50nf) applied to the second triode stage also has an audible effect on the tone. I usually make it switchable adding a toggle switch there.

[trobbins]

Sorry, had my blinkers on a few days ago when I was out on the road. As you identify, the -3dB corner should use the effective parallel resistance seen at the plate node, so 60k//78k using the example values.

Indicating that the filtering would be targeting the just out-of-band harmonics generated by clipping/overdriven waveform, that each stage would generate after the overdrive trim pot would generate.