The PL500 (27GB5) was an odd TV sweep tube, 27 volt filament, 9-pin Magnoval,
12 watts, and could handle a peak anode voltage of 7,000 Volts.
The PL500/PL504 seems to have been the most popular sweep tube for
Western European black & white TVs.Tesla coil builders use it, due availability and price.http://www.sciencezero.org/vasil/teslina/stdf.html
German mfg. (Braun) used the PL500 tube for audio amplifiers.
Then you get the current audiophiles, look at this photo (how many tubes?)http://www.audioasylum.com/forums/otl/messages/3/34846.html
Side Band Engineers was ahead of the time with the SBE model 33:
Light weight (20 lbs), solid-state receiver (germanium transistors, as you noted),
and hybrid transmitter.