Yeah, but beam tetrodes. And ultra-linear!
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[Edit to add: This post was meant to be seen by the OP. For you experts who have graciously contributed, please disregard my comments that most likely look completely obvious to you, or possibly poorly stated. Thanks.]
The original D.T.N. Williamson amp used KT66 beam tetrode output tubes wired triode (short the screen grid to the plate and you get a triode).
Ultralinear ('distributed load' primary, first proposed by Alan Blumlein, who first proposed a whole lot of stuff!) is sort of midway between triode and pentode operation (requires a specially wired transformer primary) so it's not that far off the Williamson design idea.
If you're very interested in the original Williamson amplifier, you might want to check out the reprint of the Wireless World articles on the subject -- https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_06/235_WW_Williamson_Reprints.pdf
To answer a previous question about stability:
1. The original Williamson amplifier design *required* a very special, custom designed output transformer with 10k ohm primary to 4.8.16 ohm secondaries, with very low leakage inductance and stray capacitance. The original article went so far as to provide information on how to wind the special transformer needed.
2. The Williamson amplifier circuit creates a LOT of open loop gain, which is then used to apply a relatively large amount of negative feedback around the entire circuit. It's this high amount of NFB that creates the stability issues you'll see people talking about with this design.
Since it's difficult to find a contemporary transformer with the specifications of the originals, one needs to be very careful that the high levels of NFB don't cause ringing and oscillations on the high frequency end of things.
Also, since there are 3 inverting stages that are AC coupled (resistor-capacitor coupled) to the next stage, and the output transformer has limited low frequency bandwidth too, one needs to make sure the final amplifier doesn't oscillate at subsonic frequencies ('motorboating'). With three inverting amplifier stages, if each stage has a similar rolloff in the low frequencies, and if the power supply impedance is high at those low frequencies, those low frequencies can kind of 'bleed' into other stage's power supply feeds, in opposite polarity to each other, causing the different stages to amplify very low frequency perturbations. This can cause low frequency oscillations that you may not be able to hear (not audible) but sap power from the amplifier and/or muddy the audible frequency range.
The negative feedback is boosting both the very low frequencies that can't be passed by the output transformer (and possibly the individual amplifier stages) as well as boosting the very high frequencies that can't be passed by the output transformer. These large boosts can be reversed in phase and turn the negative feedback into *positive* feedback, which will oscillate.
One has to be very careful with a Williamson amplifier to carefully check the final amp for stability both below and above the audible frequency range.
The original Williamson design was meant to show the advantages to be gained by building a large amplifier with lots of gain and then applying a relatively large amount of negative feedback to make the final output very low in THD. That large amount of negative feedback means one needs to be very careful about measuring the amplifier after building it, and then modifying various filter networks to 'compensate' the amplifier, or damp down any subsonic or ultrasonic resonances or oscillations caused by the NFB 'ringing'.
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