1. 12AX7A(V4A)
2. Epression Control
3. 12AX7A(V4B)
4. Volume Tab
5. Tone Control
6. Purpose and function of the 12BH7A(V3B).
The 12BH7 is a medium-mu twin triode, section #1 being
used for this application (V3b). The triode is operated as a single-ended
Class A amplifier stage. The output transformer (T3) matches the high
plate impedance of this stage to the lower input impedance of the TC/Leslie.
a. Value and function of 1200 ohm(R24).
This resistor is the cathode-bias resistor. Plate current
flowing through this resistor causes the cathode potential to rise
with respect to ground (earth). This causes the control grid voltage
to be negative with respect to the cathode, resulting in the "cathode
bias" or "self-bias" operation you've read about on
this and other lists. The value is chosen to create the proper bias
voltage at the control grid (pin 7) to allow Class A operation.
b. Value and function of 30/25(C26).
This capacitor is an AC signal bypass capacitor. In
performance of it's duty, R24 not only drops DC voltage, it also drops
the audio (AC) portion of the signal that is developed in the plate
circuit during amplification. This results in an undesirable loss
of gain for the triode stage. To compensate, a capacitor, whose value
is chosen to provide a low impedance path to audio frequencies, is
connected across R24 to prevent attenuation of the audio signal.
