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I would avoid CCS on the cathodes

As you can imagine, there's a spectrum of results that you get, depending on the signal fed to the common cathodes. On one end, there's a current source, which forces the circuit into full differential, and prevents any kind of Class AB mode of operation. On the other end, the circuit is running in parallel, with generous bypassing either to ground or to the common B+ point on the center-tap of the output transformer.

Since the error term we're trying to minimize is 3rd harmonic, and all odd harmonics, pure differential operation doesn't necessarily have the lowest distortion. John Atwood found to his surprise that an unbypassed current source actually had the greatest 3rd-harmonic distortion, in addition to the lowest power and sharpest overload characteristic.

In terms of reduction of 2nd harmonic, all of the balanced circuits were fairly similar - but it doesn't matter that much, since 2nd harmonic is much less audible than 3rd harmonic. Where the circuits differed was the proportion of 3rd and other other higher harmonics.

John also found that tubes with ultra-low distortion, or more accurately, tubes with the lowest amount of high-order terms, had the lowest 3rd-harmonic with the pot set all the way over to complete bypassing. Tubes with a bit more high-order content had the lowest amount of 3rd-harmonic had an optimum adjustment with a 1~3 K of resistance in the circuit.

The overall results of the test indicated the phase and amplitude of the common-mode term on the common cathode are what controls the 3rd-harmonic. With an unbypassed current source, the common term had one polarity, with the cathodes connected to the B+ terminal, the polarity was reversed, and with a judiciously chosen resistor, the term can be reduced to zero. Again, this is the COMMON-MODE term, which is all 3rd-harmonic and higher in character.

(By the way, this is why it is fallacious to think that a Class A PP power supply sees no deviation in current demand. This would only be true if the tubes were distortionless, which obviously isn't true. The distortion in the tubes appears as 3rd-harmonic on the power supply, and also appears in the common-cathode circuit.)

This series of common-mode tests, by the way, was only in private correspondence between me, John Atwood, and Mark Kelly, so you won't see it in the Clarisonus blog right away (maybe after the RMAF when we all have a bit more time).

As for software, any good FFT program will show you what you need. 48 kHz and 16 bits is plenty for measuring tube amps. Be sure to protect your sound card with back-to-back protection diodes, and possibly transformer coupling to chase out any ground loops, which can be hard to remove otherwise.

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