Increasing pentode gain

[Enzo]

Here is an interesting little article you may find enlightening. Make sure to read the several comments below it.

Obtain a gain of 450 from one vacuum tube | EDN

[J M Fahey]

Nice.
Transistor power amps do this all day long, by loading the predriver gain stage either with a bootstrapped load resistor (which is seen as a *huge* impedance) or as shown, with a constant current source.
SS amps avoid the mentioned (and very real) idle "plate" voltage instability by incorporating *huge* negative feedback down to DC, so their output sits only millivolts away from what's expected.

[philbo]

Here is an interesting little article you may find enlightening. Make sure to read the several comments below it.

Obtain a gain of 450 from one vacuum tube | EDN

<edit> my bad, didn't finish the article before posting...

[Austin]

Isn't this just using a constant current source as a plate load?

[philbo]

Isn't this just using a constant current source as a plate load?

yes, exactly so.