Both are 7 pin miniature: 6C4 6AH6 1 & 5 P 1 G1 (Grid) 2 IC 2 G3 (Suppressor) 3 & 4 Filament 3 & 4 Filament 5 & 1 P 5 P 6 G1 6 G2 (Screen) 7 K 7 K (Cathode) Looks like minimal rewiring at the socket. Heater current for 6C4 is 150 mA and for the 6AH6 is 450 mA. Ken W7EKB