Basic Circuit Question

[jazjamr]

I have a basic understanding of amp circuits. How tubes function, how cathode bypass caps allow different frequencies enter the circuit, how the tone stack allows frequencies to be bled to ground. In a typical Fender AB763 circuit using a 250pf, .047, .1 tone stack, varying amounts of high,mid and low frequencies are allowed to pass on to the phase inverter. At the entrance to the phase inverter there is a .001 or 500pf cap. After the phase inverter there are usually .1mf caps to block DC from the power tube grids. Why is the cap entering the phase inverter a lower capacitance than the tone stack mid and low caps. How are those mid and low frequencies recovered on their way to the power tubes? Thanks to all the incredible contributors to The Amp Garage!

[martin manning]

The input impedance of the LTP PI is around 2 Meg, as compared to the 220k grid leaks after the PI coupling caps. The input cap can be proportionally smaller to achieve the low=pass frequency cutoff.

[sluckey]

Read this. Especially the grid resistor section...

http://www.aikenamps.com/index.php/the-long-tail-pair

[jazjamr]

Thanks ! Aiken's text is very well written. After I read it a few times, I think it will sink in. Following schematics is not difficult, however understanding the theory behind the circuits is a little more difficult for me. Here I am trying to understand circuits designed a hundred years ago.