Where does AC signal go that is blocked by coupling capacitor value?

[Chuck H]

In simple RC circuit, the AC signals are attenuated across the XC-reactance and R (in voltage divider ratio), scaled by their frequency content.

He said for those that have been schooled on the matter (I haven't ) Though I get what you're saying. I was trying to put it in layman's terms for other illiterates like me. Having any understanding of some principals while lacking math language and electronics classes has been difficult for me. I try to make it easier for others in my position when I think I can.

[Dave H]

In other words, the effective input impedance of a LTP is about twice the input grid resistance. For the example circuit shown, this means a resistive input impedance of about 1M and a (second) corner frequency of 1.7Hz.

That's without output stage negative feedback isn't it? I seem to remember the input impedance being about 5 times the input resistor value with the usual amount of negative feedback applied.

[Helmholtz]

That's without output stage negative feedback isn't it?

Right.
Input signal current only flows in the grid leak resistor. Input impedance is given by input signal voltage divided by input signal current. So if the current through the resistor is reduced by bootstrapping, input impedance is increased proportionally. The closer the cathode signal voltage gets to the input signal voltage, the higher the input impedance.

A positive voltage applied to the PI input produces a positive feedback voltage from the output of the transformer, thereby raising the cathode signal voltage. This in turn increases input impedance with closed NFB loop. The input impedance without global NFB thus is a lower bound.
Instead of doing a complete circuit analysis I suggest to find the actual input impedance by measuring the differential signal voltage across the grid leak resistor.

To those interested in circuit analysis and calculus I highly recommend the books by Richard Kuehnel, especially "The Fender Bassman 5F6-A".

[mikepukmel]

Thanks for the book tip Helmholtz