They are matched for transconductance and current draw. At 325 plate volts they draw 29.4, 27.3, 29.8 and 30.3. That's a nice match! Where 7500 is nominal new on the Hickok 600A, all four of these test at 9,750!! This is a really nice quad of great sounding tubes. Great quad for that vintage HiFi!! All 4 have matching etched codes of rx3 X8K.
Price $175
Thanks,
Mike
