This is the first time I test directly heated tube, so probably a stupid question. I just tested 6C4C, a Russian analog of 6B4G. The plate current was changing from 37mA to 70mA depending on the filament side (contact 2 or 7) I used as a cathode. Does it make any sense?
http://www.goldenmiddle.com/files/6S4S.pdf
http://www.goldenmiddle.com/files/6S4S.pdf