[codefox1]

You can find a schematic easily enough (nostalgia air for a no frills Riders, free This is a Canadian Philco, otherwise you;ss never find it, or Chuck Schwark who is a moderator of this forum can get you a very much better one at a very modest cost,) either of which will show you the voltages you should be seeing. I think you have a bad output transformer, arcing tube socket, or the absolute worst #41 tube I have ever come across. Let's see what others have to say, but I would vote for an intermittent or corroded connection between the output transformer and either the B+ or plate winding end. Ohm iur out and let us know

Anyway it's a nice set, so take your time, be careful, use a variac and continue to measure everything.