As SCD mentioned, the 38HE7 tube is an unexpectedly interesting tube. Probably only found in the US though. Vacuumtubes.net still sells them for $3. Hopefully for a long while yet. Not much demand for them. It has a 10 Watt rated pentode (horizontal deflection) and a big damper diode. On about 80% of the 38HE7 tubes, the two heaters are in series, with 21V for the pentode htr alone, brought out to pins 10 and 12. About 10 Watts of tube heat budget are allocated for the damper htr + diss. The pentode appears to be a trimmed down (plate) version of the 21HB5A tube (18 W pentode). Without the damper section running, the pentode should be good for around 15 Watt Pdiss. If you look at the 38HE7 datasheet, the top 3 plate curves are nearly equally spaced, which is a good indication for linear class AB1 operation. Ie, audio. The triode mode curves for the pentode are pretty decent too, but the screen is only rated for 150V in pentode mode. Should be good for more V in triode mode, but keep the B+ moderate. 38HE7 pics below:
See the 38HE7 DATASHEET for the pentode curves
On the other hand, 6JC5/6JB5 for $5, gives you a 6V6 like tube at 19 Watt Pdiss. Probably more suited to you output xfmrs. And like 1/10 the price of 6V6 tubes now.
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