This is somewhat off topic. To directly answer it however... The additional components of the power supply (vibrator or transistor driven) are overhead and high drain even for auto use as efficientcy is at best 80%. 12V plate voltage car radio tubes were a considerable savings in drain by deleting the HV supply. The heater power required for the 12V vs the more usual high voltage cousins is the same or only slightly higher, some cases it was lower depends on the specific tube. the real difference was generating the HV had a standing 10W load that the tubes maybe used less than 70% of. Correct. The transconductance is lower and the lower max plate current means lower power and overload thresholds. For computers that also translates to slower switching speeds. For AM car radio use it's acceptable but for higher performing radios it really is problematic. The serious limitation for tubes has always been the heat and power circuits. Computers suffered using them as heat removal and protecting other components from that heat were the issues. Never minding even with 135A 6.3V filiments used in the subminis say two hundred of them is a mere 27.A @ 6.3V or 162W of power and no small issue with bussing around that kind of power. when you consider they may run at 100V on the plate and maybe 2ma (.2W) and that same 200 (assuming all conducting the same) is only 40W power dissapated. It's the heaters that are at issue. Transistor systems removed the heater power and lowered the per device dissapated power by a typical factor of 5 so 200 transistors were running in the 8-10W range. The latter is important as now heat is less an issue and power distribution simplified. With heat being less an issue more compact and modular systems are easier to design and assemble. After transistors the second evolution in computers was packaging. Allison