If we still wanted 50L6 tubes, they are fairly plentiful--lots of radios used these tubes. Same with 12AX7's, they're still being manufactured today and are plentiful (although not cheap.) I already had three for my Ampeg...
But choosing the 6DG6GT for a power tube was truly a pleasant surprise. The tube was standard in many TV sets, and they're cheap and easy to find. I bought 4 from ebay, at a cost of only $3.50 each! (shipping included!) Contrast that with 6V6's--good ones run $20+ per tube, minimum...
And availability is always a concern. No point in building an amp if you can't buy any replacement tubes.
The 6DG6GT tubes are RCA NOS. The 12AX7 is a NOS Raytheon, which I "borrowed" from my Ampeg Gemini II amp (it needs work, anyway--a future project.)
Heater Voltage and Current Requirements
The big concern with all 50L6 variants is the large amount of current required for the tube filaments. A bit of background: most (US) tube names begin with the voltage requirements for the heaters:
Tube name : filament voltage
50L6 : 50V
35L6 : 35V
25L6 : 25V
12AX7 : 12V (they have a split filament, and also run @ 6V)
6V6GT : 6V
6DG6GT : 6V
(Pardon the weird formatting--Instructables yanked the ability to use PRE tags, and screwed up the conversion when they did so. I've tried to fix it the best I can...)
But if the 6DG6GT is to have the same electrical characteristics as the 50L6, the heaters must perform in a nearly identical fashion. The filament wire itself must be designed to compensate -- using more current, at the lower voltage:
Volts X Amps == Power consumption
50L6 : 50V * .15A = 7.5 watts
6DG6GT : 6.3V * 1.2A = 7.56 watts
Obviously, the heater power requirements are practically identical; of course that follows since the electrical characteristics of the tube also match. But 2.4 amps (two 1.2A tubes, and not counting the preamp tube) is a fairly high amount of heater current @ 6V for a small amp... (the total heater current is 2.7A @ 6.3V)
Datasheets for 12AX7, 6DG6GT: