I have re-calculated the parameters to operate a 6EM7 in the SEX kit. This is not a supported, documented option, it's just some preliminary calculations for those who want to experiment.
The main change is the cathode bias of the power section. Replace the 680 ohm, 3 watt bias resistor with 1800 ohms, 5 watts. And replace the bypass cap 1000uF 35v with 330uF 63v or 100v. The resistor will be physically larger, and I have specified a lower capacitance to prevent the cap from also getting too large, though a greater capacitance would be fine if there is room for it. This combination goes from A3 to 12U, and B3 to 22U. The instructions are on page 34 of the manual. I would make sure there is at least 1/4 inch of space between the capacitor and the resistor (unlike the photograph) because this resistor will get pretty hot.
That will work quite well, but for those who feel the need to further optimize, replace the 1.27K ohm resistors (A6 to 12U, B6 to 22U; page 33) with an HLMP-6000 LED, cathode to ground. This will reduce the effective plate resistance of the driver, extending the high frequencies slightly. Once the C4S is available, I would run it at 1.5mA, using R1=576 ohms (562 is close enough and might be more readily available). You should in any case see about 190v at the driver plate (A5, B5); this value is subject to substantial variation (+/-30%) if the LED is used.
The above numbers are calculated, not determined experimentally - they are for a 6EM7 that meets the specification, which may or may not be a typical tube, much less a particular one. If anyone actually tries these changes, please post tube voltages so that I can adjust the resistors if necessary.
