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I have to thank you and other readers for the curves. The curves display a huge potential current draw 170 volts (0 grid voltage) of 1550 mA. At this point, the mu is about 7 and the rp is about 110 ohms and the Gm is about 65,000µS. Thus the output impedance of this tube when used a cathode follower would be about 15 ohms. In other words, this tube is a good choice for OTL fans.
The curves seem to have a problem area at the -25 volt grid line: it seems to be too close to the -20 line. My guess is that the grid voltage was off the mark. If anyone has seen an actual accurate curve tracer scan of this tube in the triode-connected mode, please let us know if I am right about what I see.
As for a 1 watt OTL amplifier, I promise to add it to the next issue. I want to model the curves mathematically first so that I can better evaluate what the EL509 needs to work best.
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But as the amplifier will undoubtedly run in Class-AB, you can refer to the 4 watt amplifier as a 1 watt Class-A amplifier. In other words, all Class-AB amplifiers are also Class-A amplifiers up to some wattage, which can be as high as 60% or as low as 0.1% of the total output wattage. Unfortunately, it is not the B+ voltage that sets the Class-A boundary, but the idle current.
The big problem tube OTL amplifier face is having to deliver the high current needed to drive the low impedance loudspeaker. One watt into 8 ohms requires a peak current delivery of 500 mA, as (0.5² x 8) / 2 = 1. Which means that a total of four EL509s is needed to meat the 250 mA idle current demand of the push-pull amplifier. If the load impedance were 285 ohms, then the same 500 mA peak current swing would yield 35 Class-A watts. What the world needs is a great 100 ohm loudspeaker!
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