A pentode is a tetrode with a third, suppressor grid between screen and anode. It is tied to the cathode and throws the secondary electrons back to the anode. The dynatron dip disappears and the anode characteristic becomes flat: the current barely depends on the anode voltage. The pentode behaves as a grid-controlled current source — exactly what an output stage needs.
The circuit is a single-ended output stage. The load is coupled through an output transformer that matches the valve's high impedance (about 5 k) to a low-impedance loudspeaker (8 ohm). The screen grid is fed straight from +300 V, the suppressor sits at ground with the cathode, and the generator supplies the −6 V grid bias.
This page is a utility for simulating pentode: power amplifier online with specified initial values.
Using the model. Once warm the stage delivers about 1.5 W into 8 ohm. The three scopes show the grid voltage, the load voltage and the instantaneous power in the load — the power pulsates at twice the signal frequency, as it should.
Use the "signal amplitude" slider to increase the drive: the output first grows linearly, then the peaks clip — that is overload. The readout beside the valve also shows the screen current: in a pentode it stays nearly constant, about 4.5 mA against 60 mA of anode current. Try changing the transformer ratio: with the wrong match the delivered power drops noticeably even though the valve itself works in the same regime.
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