Output Stage & calculating halfboost freq. can you doublecheck my results?

[Helmholtz]

That is why my plots above are on a linear scale, with the half boost point solved for and marked on the plot.

f1 and f2 appear to be definitions. For example, the time constant at the cathode depends on R2, C, and the impedance looking into the cathode. Since the ratio G1/G2 is usually not very large, f2 cannot be interpreted as a 3db point either.

I noticed and appreciated your linear scale.

The book (not a typical educational textbook but rather a collection of articles) does not give any analysis nor a definition of f1 and f2. But the G1/G2 ratio can be easily calculated from the formulae in this table contained:


It also gives the internal cathode impedance as rk=(ra+Ra)/(µ+1). This makes G1/G2 = 1+ Rk/rk and f2= f1 (1+Rk/rk) = 1/(2piCkR*) with R*= Rk//rk. (I skipped one or two trivial conversions). The fully bypassed gain is G2= Ra/rk give or take. The cathode impedance acts in parallel with Rk and both together influence the -3dB (upper corner) frequency.
I don't think it is of much practical (audible) relevance to discuss the exact meaning of f2 for low G1/G2 values, as this means that f1 and f2 won't differ much while the boost change is small.
The mean value of f1 and f2 should be a decent approximation for the half-boost frequency.