I'm in the process of ordering a MQ iron upgrade and some other parts to allow for the use of 6EM7/6EA7 tubes in the S.E.X. as an experiment. From what I understand, the main thing is to replace R1 (620 ohms) with a 1600 ohm, 5W resistor.
Now, other than needing to change one of the caps to a higher voltage rating, couldn't I just parallel a SPST switch and 5-10W, 1k resistor with the existing 2W 620 ohm resistors already in place for each 6DN7? Using the parallel resistance formula, I would need 1012 ohms paralleled with 620 ohms to get a total resistance of 1600 ohms (the value listed in the S.E.X. manual for 6EM7's). Throwing the switch would place a parallel resistor in the circuit, thus allowing me to go back and forth from 6DN7 to 6EM7 tubes. Is this a good idea, even though the stock resistor is only rated for 2W? Or should I get rid of the 2W resistors and just replace them with the same value, rated for 5W? (Or, should I not even waste my time with the 6EM7's in the first place? :-)
Are any other changes needed to use 6EM7's? (I already have C4S boards in place of the plate resistors, which is supposed to make a big difference in terms of making the 6EM7 tubes sound like the 6DN7s). Once I get the iron upgrade and switch out the tubes, I will be sure to post very detailed observations between the two. It seems like nobody has really quantified the difference between these triodes, even on the old forum, other than to say that they only do well with CCS's instead of plate resistors. I would think that the claimed 3W output and lower plate impedance (880 ohms?) of the 6EM7's would make things a little better for speakers needing good woofer control?