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Re: Haven't tried it but I've a theory that ...

Hello,

with no feedback you are right with nonlinear pentodes, but comparing triodes and pentodes for gain stages needs similar conditions for output voltage and gain.

You can give a negative feedback with an voltage divider from plate to grid (Rk must be increased for making the correct Ug1) to reach the same gain. In this case pentodes are more linear than triodes.

For Output stages it is very simple. Power Triodes are more linear but needs higher grid swing than pentodes, and this gives more distortion in the driver stage. Thorstens experiments with cancelling 2nd harmonious are interesting, but it is very complex to find perfect matching driver tube.
Pentode with feedback can reach the linearity of a triode and need less voltage swing on the grid to drive, and the driver produces less distortion.
It is easier to use a pentode and feedback between both tubes.
The relationship 2nd and 3rd harmonious for pentodes can be changed through increased or made smaller the anode resistor. In this way it may be possible to cancelling 2nd harmonious.

But there is another fact for using pentodes. Fullrange speakers have a nonlinear impedance curve, impedance rises toward both sides of the frequency range. Triodes with low output impedance (damping factor >1) give less power to the speaker if the impedace is higher, pentodes with higher output impedance (damping factor < 1) give more power on higher impedances. It is very complex to fit Amp and speaker for best results.

Gerd

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