Quote:
Originally Posted by jjl
Perhaps, but it should also be borne in mind that the PX4, PX25 etc. were 1920s designs i.e. they were designed and released when oxide coated filaments were recent technology and indirectly heated cathodes were absolutely cutting edge. No doubt, valve manufacturers could have designed and marketed indirectly heated power triodes later on, but they chose not to, probably because indirectly heated power tetrodes and pentodes had a much larger market and these could be triode connected if desired.
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Linsley Hood provides a few interesting comments upon this aspect, attached, that point to difficulties with indirectly heated output triodes.
But as you say, even could it have been done, there was no real need. I understand though that the PX25 (and maybe the PX4) underwent an upgrade or two in their lives, the last for the PX25 possibly being after WWII. Osram evidently opted for improving its existing output triodes rather than designing new ones. Thus amplifier builders wanting 25 W (anode dissipation) triodes had a choice of the improved PX25 or the KT66, triode-connected.
Cheers,