Originally Posted by baker8
....a 100k grid to grid resistor that's parallel to the autoformer. ....explain the purpose of this resistor and how it's value is calculated?...
You have a capacitor (not mentioned). You have an inductor (transformer).
L and C together will always resonate!
Your Standard Texts will cover this, but not in this exact form, and not as specific worked examples.
The resonance is damped by resistance(s). The tube rp for one, series to the LC. Generally we need/want another resistance. Koonw's plot
shows that a simple LC can be optimally damped for a Butterworth (or other) corner at the band limit. The plot also shows how we do it these days: throw it in the simulator and ask The Idiot to sweep for an optimum.