Ralph-thank you for your very polite and informative response. I apologize for my strong words directed at you.No worries- and no offense taken. But I appreciate your words.
What is odd, and I am not quite understanding this part, is why with my older CAT phono stage, which did not use a SUT at all, that a load of 750 ohms was better sounding than straight in at 47Kohms, yet with my new model, this is definitely not the case. Presumably due to the SUT in my new CAT??A lot to unpack here....
Its a lot easier to build a tube input circuit that has much higher overload characteristics! In a tube circuit meant for LOMC, if you overload it with too much input voltage, the overload isn't occurring in the input stage- it happens further downstream. With solid state, the overload often occurs at or very near the input, often because the circuit uses feedback, and the input section might be outside the feedback loop- even if its only the base of a transistor. But semiconductors are far more likely to rectify RF energy too, since they are diodes at some point or another. Its this latter characteristic that makes them more pesky in this regard.
SUTs are another matter altogether! I suspect Ken got tired of people calling about noisy tubes and SUTs are a way to get around that. For tubes to be really quiet in the front end of a phono section, they have to be at the top of their game. As they lose transconductance with age, the noise goes up. You have to keep the tubes active even when the preamp is being used playing CDs, so the tubes are going downhill all the time. By installing an SUT, you can easily quadruple the usable life of the input tubes.
But that comes at a price! SUTs have to be properly loaded to prevent ringing (distortion) and the proper load varies from cartridge to cartridge, since transformers **transform** impedance. So if you have a 10 ohm cartridge, the output of the SUT will be an impedance much higher than if you have a 5 ohm cartridge. So the load it needs will be different too. If the load is insufficient (too high impedance) the transformer will ring, which is to say some very high amplitude harmonics will appear at its output. This makes them very tricky to use! I find that even with them set up right, you lose a bit of detail (bandwidth at this signal level shouldn't be an issue)- that's why I've really stayed away from transformers in the audio path.
It does make sense-once one understands the higher resistor value being to ground-that the lower resistor value approaches a short circuit and that the cantilever becomes stiffer and less damped, not less stiff and more damped. Damping is the absorption/dissipation of energy.Just to be clear- if you load the cartridge more, the cantilever will be more damped in addition to being stiffer. This can affect tracking if you get the resulting mechanical resonance outside of the 7-12Hz window.
I still maintain-righty or wrongly-that two things are at play with loading. One can argue that they are interrelated. One is preventing ringing/overload and another is changing the magnetically induced behavior of the cantilever.
We need to be really specific about what is happening here. Many years ago I had this idea about building a little box that would sort out what the ideal loading value was for a LOMC cartridge. This might have been about 30-35 years ago... At any rate, what I found was that **the cartridge itself does not ring at audio frequencies**. You can pass a 10KHz square wave through it and it will look exactly like a 10KHz square wave at either end of the cartridge. Quite simply the inductance is so low that its inconsequential at audio frequencies. It can't ring (and on this point, MM cartridge most definitely **can**, so loading with them as affecting things at or very near audio frequencies). BUT- it can have effects at much higher frequencies as I described earlier. (The result of my research in this regard was that I would not be able to make such a box, since ringing wasn't the issue.)
BTW, if there is some question about what the load should be, @lewm 's rule of thumb of being 10X higher than that of the cartridge itself is a pretty good one. Such a value will detune the radio frequency issues and won't affect the output level of the cartridge.
There are more than just two things going on with loading- and they are very much interrelated as you say. Two are caused by the cantilever getting stiffer- it can affect how the cartridge tracks and its arguable that being less supple, is less able to trace higher frequencies. So that's two things. But the loading affects the preamp too; by eliminating the RFI at the phono input, it can make the preamp less bright (a common result of RFI in audio circuits) and possibly less ticks and pops if the phono section has poor high frequency overload margins.