How do we replace the constant current source with a tube? Well, since pentodes are known for their high output impedance, a pentode would be a logical place to start. In the circuit above, we see the third EL34 working as a constant current source. This third tube will draw a fairly constant current, which the bottom EL34 will have to work against at idle. As the signal level increases, the amplifier leaves single-ended operation once the top EL34 begins to conduct, but at no time does the third EL34 stop conducting. In other words,  we can expect that the bottom EL34 will wear out first, followed by the third EL34. And as the top EL34 is normally turned off at idle and at low levels, it should last the longest.
    Besides offering single-ended sound and push-pull wallop, this amplifier would turn many heads with its use of three output tubes. (H.G. Wells' Martians listen to such amplifiers.) The balanced  idle current through the output transformer means that we do not need an airgapped output transformer, which allows experimentation on old Dynaco gear.

    (A Stereo-70 would work nicely as a mono-block amplifier with the empty octal socket holding a 6SL7 or an octal dissimilar-triode tube, such as the 6DN7 or 6EM7; with the old circuit board replaced by a new board that hold only support components; and with the two output transformers wired in parallel.)
   The drive requirements for this amplifier differ from a standard push-pull amplifier in that the top output tube needs to see a much larger grid swing than the bottom tube. This could be accomplished in the phase splitter by juggling the resistor values or by letting a feedback loop do the work for us, or what would probably work best:  a combination of both.
     Note, this arrangement does not provide lots of power, just more than single-ended alone. And as most tube push-pull amplifier already run in a rich Class-AB, this modification would only marginally increase the demands on the power supply, but where a the power supply is already over tasked, this modification would require a power supply upgrade.

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