[Radioroslyn]

Ok so this is where the rubber meets the road. I would make an adapter that would only connect the filament pins. In other words a socket and a tube base with jumpers connecting pin 2 from the socket to the base and pin 7 from socket to base.For the 50L6. This will remove any HV current that the tube would draw. With the loctals it's a little trickier but with a few clip leads and some heavy bare wire you should be able make it work. Go stage by stage till you find one that the HV doesn't drop after the tube heat. It sounds like one of the stages is hauling down a bunch of current once the filament gets hot rather than a short somewhere that would cause the volt to be low as soon as you turn it on.
Terry