that's a great suggestion Peter
(I worked with a radio architecture that obtained higher and higher harmonics from a base crystal frequency by a somewhat similar process, very simple and robust, but most importantly, the harmonics maintain a set phase relationship relative to each other, which we can modify of course and most likely need to.
I agree with most of what you say, which is what SM said about SS, but let's think about this. Are solid state circuits really more noisy then tubes? I don't think so. I worked on a RADAR in the past that had a front end receiver amplifier built with tubes, and the noise factor of that front end was around 20 or so, which is very high. Compare that with a modern solid state satellite LNBA which has a noise figure of around 0.4 dB, which translates to a noise factor of close to 1.09, (FYI, a noise factor of 1.0 is the best, it means that the signal to noise ratio at the output is the same as the signal to noise ratio at the input, so the device did not add any extra noise which would unbalance the ratio at the output and degrade it)
When SM talks about "controlling" a frequency, the most obvious meaning is controlling the frequency (of a signal), as opposed to it's amplitude, if we interpret his use of the word "frequency" in the general sense, meaning a "signal". However, I think SM does mean controlling the frequency, of the generated tones, because most likely the operating parameters drift, or the external frequency that he taps into drifts, or both, so he needs to stay in tune.
Anyway, no matter what parameter we need to control, we have to derive an error signal that is fed back to the voltage controlled oscillator (VCO), and that may or may not be obvious how to accomplish, but in the phased locked loop, the mixer produces the difference of the phases, and that's what is fed back to the VCO to adjust it's frequency. The problem with my schematic is that when the signals are in phase, and we obtain a DC voltage, we can be anywhere in value, at 0 volts, 3 volts, or -1.2 volts, depending on the phase of the signal when we locked on. To avoid this common problem, radio architectures employ a separate I and Q channel, and we might have to do that as well, if that's what it takes. ("I" channel means In-phase, and "Q" channel means Quadrature, or 90 deg out of phase, so when one channel is 0 the other is maximum)
Or we can take a ferrite and bring a magnet next to it and hope for the best!
PS, I've been staying up late the past few evenings to re-read Stevens letters to Lindsay, and I'll tell you, this time around they make so much more sense and they are so interesting I can't put them down. I must also say that Lindsay did a fantastic job interacting with Steven, and I am just so blown away by how diplomatic and courteous he was, not to mention sympathetic to Steven's needs, a true friend which is so commendable. He also tried everything that Steven suggested and progressed along the way, showing Steven how serious he was, and Steven in turn produced more valuable information, but just as commendable, he maintained his firm resolve not to disclose proprietary information which he was obligated to protect, and stood firm to his commitments.
All I can say this was a superb interaction between these two individuals, and if this device is ever fully replicated we need to thank Lindsay just as much as Steven. My hat is off to both of you. Lindsay, thanks for persisting in asking about the collector wire, he dodged the bullet a few times and finally gave in and replied about the "3 wires on top of each other, not interleaved" and how the vertical control wires are wound etc., and obviously he is describing the large 17" diameter tabletop TPU, which is something we should keep in mind, because obviously the OTPU is different.