>The first glowing plate amplifier I made was using a pair of
>250th's. I drove them with a pair of 6146B and had about
>2Kv on the plate. Naturally I had to provide bias to the tube
>even though it was cathode driven. When I tuned up the plates
>not only glowed red you could see the dark shadow of the
>grid on them.
> I was only a young ham then and did not know that
>you "can't run 250th in grounded grid".
Any triode will run in g-g. The higher the Mu, the less cathode driving
V needed. A 250TH is easier to drive in g-g than a 250TL. The "you
can't" is running a high or medium Mu triode in grid-driven Class AB1.
For example, a grid-driven, Class AB1 8877 - Mu=200 - looks like it
might produce 20w in that configuration. Actually, the "you can't"
should be a "... but you won't get much output". For a grid-driven
triode in Class AB1, a Mu of under 5 is desirable.
>But it worked just
>fine and I did get about 1KW plate input power to the
>amp. ...
- Rich..., 805.386.3734, www.vcnet.com/measures.
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