[Henry] Strange problem in Henry 2K3

[John King]

I posted this once before but go no replies that pinpointed the problem.  If you have had such experience with a Henry 2K3 or other amplifier that has  pair of amplifier IN SERIES, I will appreciate your trouble shooting determinations.
The filament of one of the 2    3-500ZG  tubes never  lights up. I can switch sockets with the tubes and no change. I can change tubes to another pair of tubes and no change, Only ONE TUBE filiament lights. op.
This is my favorite and most powerful Amplifier !!!
John King, K5PGW