Adding a SRPP flavor to this circuit is easy enough: just add an extra triode in series with the plate resistor.

    Of course, we can break the connection with push-pull altogether, as shown below. This circuit uses a classic common-cathode amplifier cascading into a constant current source loaded cathode follower. Obviously, a feedback loop can be added to any of these variations.

     What we end up with is a circuit made famous by Bruce Rozenblit in his Transcendent Grounded-Grid line stage amplifier. What Bruce added to this variation is a feedback loop from the output to the grid of the bottom triode. This feedback loop redeems this circuit for line stage use, as the SRPP usually is not suitable as a line stage because it relies on seeing a constant, fixed external load impedance, which line stages do not see, as power amplifiers greatly differ in input impedance. By bringing the output back into the negative input of the common-cathode amplifier the circuit regains control over the balanced  drive for the top and bottom triodes. (My only complaint with Bruce's design is its name, "Grounded Grid," and the trivial amount of current flowing through it, 4-mA.)
     What can be done to this circuit to move it closer to constant current source loading and cathode-follower based single-ended output? Adding two resistors and using a different top triode moves us closer. The 12B4 requires a large cathode-to-grid voltage, which allows using a larger valued intermediate resistor.

< PREVIOUS   Copyright © 2002 GlassWare   All Rights Reserved