using "Y" cables on turntable out jacks to sum the signal.
IMHO there is some sort of looking for problems. Output signal of a turntable is so tiny that adding more contacts is likely the way of deteriorating it. If we agree that our cartridge is good than why we should bother about choosing from the alternatives: playing each channel through the corresponding channel of stereo amplifier-corrector system or summing the signal and play it again through the same stereo system?
1) If I install the Mono/Stereo switch cartridge loading drops to 23.5K. I have a Grado Ref Sonata hi output. How does this affect sound? I recall reading that Grado is less fussy about loading than other carts.
It's normal. Tying the load resistors together you will parallel the sources too, so the resulting currents will be the same (well, not entirely, of course - there will be some "equalizing" currents sourcing from the differences in the output signal from each coil of the cartridge, but they are very small).
2) in one posting here it is suggested that R200 (47K resistors) be changed to 100K values for mono play. It isn't very practical to switch resistors every time I want to play a mono record. A switch arrangement raises the spectre of hum and noise injection.... has anyone figured out a way to do this in a practical manner?
In my opinion anything that is put on the way of the tiny signal is bad. So the only "good" way of doing things in that regard is switching the outputs of the first stages of phono-amplifier, each tuned to its own cartridge.
3) Is it advisable to add a "Y" adapter at the output of the Cornet2? I read Jim's comments that tube sections need to be exactly matched. I have specified tightly matched sections in buying my NOS tubes (as well as lo noise, lo microphonics). And then there is the matter of adding "crummy wire" into the chain.
If you still insist on summing the channels - the Y-adapter at the output of the Cornet 2 is less harmful than at the input.
4) Or should I just not worry about it?
IMHO, the only good thing I can think of.
Really, you have different cutting in the mono and stereo records, sometimes you have different correction parameters, even in the Cornet 2 channels differ from each other... And you still want to hear the tiny difference in the cartridge channels?
Any thoughts on best approach? I don't mind opening up the box and adding the switch if hat is the best way. Any excuse to fire up the soldering iron.
I think the best way is leaving everything "as is".
You can also try to "sum in cartridge" - make an interconnect that disconnects the ground wires from the input of the Cornet 2 (in a balanced sort of interconnecting).
Y-adapter at the output of the Cornet or even further in the amplifying chain is also possible way.