Audioman58, I believe that some of your statements are incorrect. First, you probably meant "6106" when you said "6101," the 6101 being a twin triode. More significantly, while many of the 5Y3-GT specs are identical to those of the 5Z4, according to an older GE tube manual I have the voltage drops of the two tubes at 125 ma are very different (50 volts and 20 volts respectively). Also, although I'm not sure it would make any difference in itself, the 5Z4 has an indirectly heated cathode, while the 5Y3-GT is directly heated.
Also, regarding:
Going up to s 5ar4 gz-30 requires much more power
You could put a 5z4 in a 5ar4 slot. It may work putting a 5ar4 in a 5z4 Slot But it may gave very short life.
The 5AR4 draws slightly less filament current than the 5Z4 (1.9 amps vs. 2 amps), and the 5AR4 is rated to be ABLE to handle considerably higher plate voltages and output currents than the 5Z4. Therefore I believe that your sentences I quoted have it backwards.
Regards,
-- Al