you MAY be having the same problem that I had with my B & K 409B.
As in not enough amps to light the heaters of the tube and it is being loaded down.
http://manuals.repeater-builder.com/...0Operation.pdf
If what I see in that is right, it hardly has 1 amp of heater power, and the roundy needs almost 2A.
my 409B is foolishly the same way, in order to test a 21fjp22, or something like it, i have to use a separate filament transformer.