Just for clarity's sake:
6SN7GT
Medium-Mu Twin Triode
Heater Voltage ................................ 6.3 V
Heater Current ................................ 0.6 A
Maximum Heater-Cathode Voltage
Heater Positive with Respect to Cathode
DC Component .............................. 90 V
Heater Negative with Respect to Cathode
DC Component .............................. 90 V
Maximum Ratings (Design Center Values)
Each Triode
Plate Voltage ................................. 300 V
Plate Dissipation ............................. 2.5 W
Cathode Current ............................... 20 mA
Characteristics and Typical Operation
Class A Amplifier
Plate Voltage ................................. 250 V
Grid No. 1 Voltage ............................ -8 V
Amplification Factor .......................... 20
Plate Resistance (approx) ..................... 7.7K Ω
Transconductance .............................. 2600 µ
Plate Current ................................. 9 mA
6F8G
Medium-Mu Twin Triode
Heater Voltage ................................ 6.3 V
Heater Current ................................ 0.3 A
Maximum Heater-Cathode Voltage
Heater Positive with Respect to Cathode
DC Component .............................. 90 V
Heater Negative with Respect to Cathode
DC Component .............................. 90 V
Maximum Ratings (Design Center Values)
Each Triode
Plate Voltage ................................. 300 V
Plate Dissipation ............................. 2.5 W
Cathode Current ............................... 20 mA
Characteristics and Typical Operation
Class A Amplifier
Plate Voltage ................................. 250 V
Grid No. 1 Voltage ............................ -8 V
Amplification Factor .......................... 20
Plate Resistance (approx) ..................... 7.7K Ω
Transconductance .............................. 2600 µ
Plate Current ................................. 9 mA
As you can see, not much difference
Just the pin-out (and the grid cap of course)...