Of course, these last variations lose the thread of the SRPP flavor. But then the goal is the best sounding amplifier, not the slavish adherence to one topology over another topology. The economies made in the SRPP type circuit show how malleable circuits can be. The secret is to follow the flow of current.

Mixed-class SRPP
      Normally, an SRPP must be run in strict Class-A, but mixed output classes could be possible with added output tubes. In the circuit below, we see the first set of 6DJ8s continuously draw current throughout the output waveform, but the remaining 6DJ8s turn off during part of the waveform. This trick uses two different bias voltages for the tubes, with the four diodes allowing this discrepancy in bias voltages.

     So what insights can we take away from these circuits? First that the load counts. The load to be driven defines the starting point.
     If the load is a punishingly low 32 to 10k-ohms, then abandon any notions of constant current sources and single-ended operation. In this case, follow the design procedures outlined in "SRPP Decoded.
    On the other hand, if the load is 1-meg to infinity, then high-current push-pull operation make no sense and single-ended operation is closer to the actual functioning of the circuit. But as the load is so high why not move the connection to the external load to the bottom tube's plate rather than the top tube's cathode? This change has the psychological advantage that we clearly see the top tube trying to emulate a constant current source and we can easily see how to increase its impedance (greater resistance in the cathode circuit).     

              Mixed-class SRPP amplifier

     Using a different set of final output tubes might also allow the same trick to be realized without having to use the diodes with the top triodes. For example, using a 12AU7, with its greater cathode-to-grid voltages would allow eliminating the need for the diodes. 

     If the load impedance is not specified, as in the case in a line amplifier, do not use any type of SRPP circuit, as the SRPP should be designed around one specific load impedance. In other words, if the external load is a 50k volume control potentiometer in a preamplifier, then the


www.tubecad.com   Copyright © 2002 GlassWare   All Rights Reserved