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Rough Draft of
THE A0-28

A. Power Supply

1. 6X4(V8)
2. 40/450(C56)

3. 900(R91)
4. 40/450(C57)
5. 4.7K(R94)1W
6. 4.7K(R95)1W
7. 30/350(R96)
8. 4.7K(R96)
9. 10/350(C56)
10. 4500(R97)
11. 50/300(C60)
12. 7500(R98)
13. 180(R99)1W
14. 2.7K(R100)1W
15. 2.7K(R101)1W

B. Non-vibrato Channel

1. 6AU6A(V1)
2. 270K(R3)
3. 47K(R6)
4. 10M(R5)
5. 1.8M(R4)
6. 39PF(C2)
7. 100PF(C3)
8. .33(C4)
9. 2.2M(R7)
10. 470K(R8)
11. .01(C5)
12. 820K(R9)
13. 3.9M(R10)
14. 30PF(C6)

C. Vibrato Channel

1. 6AU6A(V2)
2. 270K(R12)
3. 47K(R16)
4. 10M(R14)
5. 4.7M(R13)
6. 24PF(C7)
2. 12BH7A(V3A)

D. Percussion Channel

1. 6C4(V5)
2. 6C4(V6)
3. 12AU7A(V7A)
4. 12AU7B(V7B)

E. Final

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.


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