The Mullard datasheet specifies a maximum supply voltage (Va(b1)) of 550 V, but a maximum anode voltage of 250 V. This means the tube can operate in circuits with HT rails up to 550 V provided the actual anode voltage does not exceed 250 V.
KT61 is functionally similar but is a beam tetrode rather than a true pentode; pin-compatible on octal base.
Grid resistor max is 1.0 MOhm (cathode bias). The datasheet also specifies Rh-k max = 5.0 kOhm.
Sams Substitution Handbook lists 6AG6G as a substitute for 6M6G in parallel-filament circuits only.
Pin Layout — Octal
1No Connection (metallisation/shell)
2Heater
3Anode
4
Socket-Compatible Tubes ⚠ Not electrically compatible