A better choice might the EL509. This tube is available from Svetlana (yes, I know that the American portion of the company is kaput, but I understand that the Russian company continues). It is used in the Transcendent OTL amplifiers. Using eight EL509s yields 80 watts, which translates into a little over 1A peak current per tube. Therefore, two EL509s in a push-pull should easily give us our 4 watt output, if we retain the plate voltages used in the 80 watt version. Notice that quartered the output stage and reduced the output by sixteen-fold.
In the next issue, we go over a possible design of a 4 watt OTL that uses two of these tubes. Until then let's hope that someone can send in a set of triode plate curves for this tube.
Subject: Hybrid Amplifier
Thanks for the interesting article on Hybrid Amplifiers. I have owned many amps over the years, tube and solid state, and feel that the hybrid may be the best of all available designs. Attached is the Hybrid that I'm currently building.
The tube is a 7370 which is a 5687 with a 40 volt heater so it can be fed with the negative 40 volt rail. This keeps the heater to cathode voltage within limits and eliminates the need for a separate filament supply. The tube rails are provided by positive and negative voltage triplers for +/- 120 volts.