To continue the discussion of Ultra Linear vs. pentode vs. triode, ultra linear and triode connection of a pentode is a technique that applies local feedback in the output stage. Feedback lowers output impedance and lowers distortion. When the plate of a pentode is connected to the screen, that applies the most feedback and the output stage has its lowest gain (triode connection). In ultra linear, only a portion of the plate voltage is applied to the screen, and this is part way between pentode where no voltage is fed back and triode where 100 % is fed back to the screen. The output impedance of the triode connection is the lowest, and the gain is the lowest. For pentode the gain is the highest and the output impedance is the highest. Distortion is the highest also. Ultra linear lies in between in all of these characteristics. Power output is highest in the pentode mode because the screed is maintained at the highest voltage during tube turn-on and the tube can pass more current at higher screen voltages.
It should be noted that this is treating the output stage as an isolated entity. When an amplifier has global feedback, the overall gain of the amplifier may not change much when switching between ultra and triode. This is because the feedback network at the input of the amplifier is working to define the gain. But what happens is that the quantity of feedback shifts and is divided between the input error stage and the output stage, relatively more going to the output stage and less to the input stage for the triode connection. Generally, this is desirable as shorter feedback loops are better than long ones. But which one you prefer to listen to may not be as expected and is clearly up to subjective debate.
David
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