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.