With a 6AQ5 driver I doubt Ig will get much beyond the RCA 1952 4 ma spec per tube was an issue, it wasnt until years later the DX-100, Viking II, and other owners with grid current metering found short 6146 life at those levels. B&W was off in another world I guess and "assumed" the "Tune" procedure/position for Phone would limit HV and screen current to low values and G1 would just follow along!
I didnt keep my 5100B long enough to take out the finals.
Carl