We91,
The lack of SE IT's in general and pentode SE IT's in particular has to do with how difficult it is to get them to work. There is no such thing as a generic SE IT for pentodes. Each would have to be made for a specific driver tube, run under a specific set of conditions. Then would come all of the cut and try and measuring because the math just falls apart when you get to this level of complexity. If you are willing to spend the money, and it will run to several hundred dollars, you might interest a custom house in attempting it. But, there would be no guarantee you would get what you thought you paid for and that it would work as you wanted. Spend your money on a really good coupling cap.
Bud
The lack of SE IT's in general and pentode SE IT's in particular has to do with how difficult it is to get them to work. There is no such thing as a generic SE IT for pentodes. Each would have to be made for a specific driver tube, run under a specific set of conditions. Then would come all of the cut and try and measuring because the math just falls apart when you get to this level of complexity. If you are willing to spend the money, and it will run to several hundred dollars, you might interest a custom house in attempting it. But, there would be no guarantee you would get what you thought you paid for and that it would work as you wanted. Spend your money on a really good coupling cap.
Bud
WE91 said:
It's not done due to the enormous plate resistance of pentodes. That would require likewise enormous inductances. You also can't use high u triodes for the same reason. with xfmr coupling, you can't use any triode with a u in excess of 20, or so to keep the plate resistance down.
There are some topologies that add lots of local NFB in order to reduce the otherwise unacceptable pentode plate resistance. However, I haven't seen that done in anything other than AM modulators where fidelity isn't a concern.
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