Never operated or worked on a TBX-8, only know what I have seen in the manual and that is that although they use the same plugs in all the series and assume somewhat the same voltages, 12.5 and 500 for the transmitter and 90, -6 and -3 at least for the TBX to the TBX-6 would suspect the older 6 may not have the same voltages and current draws as a TBX-8

The 8 is a completely different animal in many ways, more modern and different tubes in the receiver and three rubes in the transmitter and the ability to be able to operate in PTT mode with a normal microphone. The TBX pre 8 family all use just one tube in the transmitter, require manual switching between receive and transmit and just use two different tubes in the receiver with the same type 34 tube as the IF, Detector, Audio and BFO. I think the 34 is also a 2.0 volt tube. Somehow think one or two of the "Non-Eight" sets may have wanted 135 volts for the audio output stage in the receiver. I think there was an adapter that went on the power cable to correct the difference.

The older sets also required a special high output microphone to suppressor modulate the one tube transmitter. The eight got away from that by using a tube to modulate the PA tube along with a separate oscillator as opposed to the wild feedback path of the self-excited PA in the earlier radios.

The TBX-8 is like using any of the BC-1306/GRC-9 sets that were around at the same time. The TBX-6 and all its non-eight brethren are a more primitive design using cutting edge technology from the late thirties and although it lacks things like a working AVC system, requires a big fat carbon microphone and lacks any type of useful frequency display can be a fun radio to operate in the field that works surprisingly well for technology pushing ninety years old. 

I am thinking along the lines of building up a box for the receiver that has a 6-volt SLA battery to provide -6 for the bias, regulated -3 for the filament supply and drive an inverter assembly that will provide +135 if I need it and +90 for everything else.

The transmitter may be driven from a 12-volt SLA running the larger 40 watt inverter for the +500 for the plate of the 837 with the 12 volts being used directly for the transmitter filament.

Twenty years ago, when I did this just used the self-excited MO in the transmitter for 3885 but today, I am thinking of using a Progrock assembly to drive the 837. Don't know if it will have enough juice for driving that tube, but the idea of a Pre War design being excited by device that would have been beyond anyone's vision in 1939 appeals to me.

GRC-9, BC-1306 and SCR-284 sets are around in good numbers today. But who has a pre-war design QRP portable that they can run? 

Ray F/KA3EKH