[Arran]

I have two of these, and a post war RCA equivalent. The can of the filter condenser is common negative, it goes directly to chassis. I seem to remember that the original filter caps were 20 and 10 microfarad uf, at 350 Vdc, the third cap is a cathode bypass cap connected to the cathode pin of the 6K6 tube, it's also 10 uf but something like 25 volts. Realistically the original filter cap can is probably too small to restuff, so I would use a terminal strip to install new ones under the chassis, probably under the original can, except for the cathode bypass cap. that can go between the tube socket terminal, and the nearest convenient connection to ground.
Regards
Arran