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Because it is flawed thinking.

Manipulation of screen voltages is a bad design choice, IMHO. The pentode was designed to be operated with a fixed screen voltage to provide more precise control over plate currents and to increase efficiency. When you start to jack around with the screen voltages it becomes very difficult to track what tube behaviors you are changing. Cathode Feedback, or the distributed load connection provides a superior design solution to pentode operation. It is far more effective at reducing plate resistance than UL connection. The drive requirements are easy to calculate, and the dampening factors are usually better than even triode connection. In addition, plate efficiency remains quite high, usually around 40%. CFB is also better at reducing THD than UL connection. From a design point, it is also easier to pick a load line with CFB than UL because you can still use the pentode plate characteristics to plot your load line. With UL connection, you need the UL plate curves, which can be hard to find for tubes other than the most popular ones. Transformers for CFB are quite easy to come by due to the many competent transformer makers we have today. All in all, I really like CFB pentode mode. It is very flexible and you can tailor your operating point to emphasize the harmonic profile that best suites your system.

Rgs, JLH



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