On the Fisher schematic for the 50A, they specify an output tube plate voltage of 420 vdc. If that is in fact the voltage your unit produces, that backs the quiescent current down to about 40 mA per tube -- based on the excellent curves Who has provided. With today's higher voltages however, Who's graphical representation is probably more accurate, if you hold grid voltage as a constant. But then there is the ideal value based on the actual circuit conditions as well. This takes into account everything from the load impedance the tubes operate into, actual B+ voltage, manufacturing tolerances of the tubes, and even power supply regulation as well. Setting the quiescent current by way of schematic indicated voltages is surely a good start, as normally, Fisher has already done all of this homework. But adjusting them for the least amount of distortion based on the actual circuit they are operating in is always best.
I have a pair of 50A's here I'm in the process of finishing up for a client, and will be posting on them soon. Since Fisher did not provide the quiescent current draw value on the schematic, I'll be happy to provide that value based on the lowest distortion operating point. These units are operating with 6BG6 tubes installed, but again, these are just another variation of the same 6L6 family of tubes that the 1614, 807, etc., etc., are all based on. Stay tuned.
Dave