As an aside, I would mention a possible use for a simple non-linear element, the vari-Mu triode, in the feedback loop of an Op amp. under semi-small signal conditions, as a means of generating "anti-tube characteristic curve" distortion (by inverting the curved transfer curve) at a variable correction level (by selecting the tubes operating point by potentiometer). This could be used in an amplifier's signal chain to correct its open loop distortion before overall feedback is applied. Imagine the possibilities with a beam deflection tube in place of the vari-Mu triode.
 
Don

  Thank you for the compliments. And thank you for the tube suggestions. The Japanese tube audio community leads the world in their willingness to try different tubes in audio circuits. Here, in the USA, few are willing to be so bold. During the 70s and 80s, octals were shunned by many audiophiles and the reason given was that Conrad Johnson and Audio Research used only 9-pin miniatures in their preamps and the front ends of their amplifiers; therefore, octals must be inferior.
   Today, I have friends who tell me that only 5692s can be used in input stages or that only 300Bs can be used in output stages. I am using a triode connected 6AC7 for the first tube in my single-ended amplifiers and I like the sound much better than that of the 5692 I used to use. (Quality 6AC7s can be had for $2 each; 5692s for $40.) 15 years ago, John Atwood and I argued that the 5687 and the 6072 were great tubes, but few believed us. Now that a famous $27,000 amplifier uses these two tubes, I am told by the same non-believers that only they can be used in an amplifier.

Subject: Beam deflection tubes

    I have enjoyed reading your journal and appreciate your willingness to explore new circuit ideas. I would like to point out a class of tube that has probably never been considered in the audio design arena before, but which fits well with common cathode coupled amplifier design and has many other possibilities.  These are the beam deflection tubes (7360, 6ME8, 6JH8, 6HW8) used mainly in color TV demodulators and Amateur Radio single sideband transmitters. Although the 7360 is expensive nowadays, the others are quite reasonably priced and actually perform better anyway (at least the 6ME8 and 6JH8). These tubes are the equivalent of a common cathode coupled triode pair with a current source feeding the cathodes, all in one tube envelope.  This class of tubes was specifically designed for extreme linearity in balanced modulator applications (distortion products at -60 dB, see "Single Sideband for the Radio Amateur," ARRL, c1970, 4th or 5th ed.), they are very linear in their transfer characteristics between deflectors and dual plate outputs except at high signal levels where they become "soft" limiting.  The main limitation with these tubes is the relatively low voltage gain from deflectors to plates (about 6). A 6JH8 could make an interesting phase splitter for push-pull output stages.  The soft limiting under large signal conditions could be used to soften the saturation characteristics of the output stage.  Since these tubes are capable of signal multiplication too (grid #1 input times deflector inputs) they are interesting for such uses as automatic volume control or as intentional low order distortion generation (by squaring a signal).  Combining an operational amplifier with them allows division or square rooting too.  I'm sure a few more uses can be conjured up. 

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pg. 14