Roger that.

I turned off the lights and the arc is from the tube socket foil right to chassis , which is almost a qtr inch gap. Looks like the plate only has 300Vdc on it, so am bewildered what can jump that
distance...unless its RF ? The send switch is off, heaters are off.

I had re-soldered those foil pads for the tube socket and while I was in there replaced the (2) 100k half watt resistors nearby the tube, plus R38. R38 (470k) was ok but it was carbon and I hear they are noisier than metal film types.