Philco Mystery Control Receiver

[RJMiranda]

I think the two condensers on the primary are important. Look at the schematic in the link suggested by Phil. The power to the control receiver circuit comes from the primary of the main transformer (it is not isolated). It looks that Philco wanted the chassis at "half-way" potential between both AC wires. I would use an isolating transformer to test this receiver, to be safe.
Check whether your thyratron is defective: short it´s grid coil. The thyratron should NOT ignite. If it keeps dark, it looks OK. Remove the short and try lowering the sensitivity control on the 78´s cathode.
If you have a signal generator, put .01mF 600V condensers in series with BOTH the signal and earth connections, set the generator at the max. amplitude, supply the signal at the 6J7G´s plate and sweep the freq. between 350 and 400 kHz. Maybe the signal will not be strong enough to fire the 2A4G. Supply the signal at the 6J7G´s grid (maybe at a lower level). Lower the generator output as much as you can, and the correct frequency will be the one when the thyratron still fires.
Supply the found frequency (at much lower level) to the input by using a coil coupled to the "secondary inductor" and adjust the interstage and first transformers and the trimmer so the thyratron reacts at the lowest possible level.