Same 9HF pinout but Section 1 has much lower mu (17.5 vs 64). Section 2 is similar (mu=6, gm=6500). Not a drop-in replacement due to very different Section 1 characteristics.
Same 9HF pinout and dissimilar double triode structure, but different electrical characteristics (Section 1: mu=68 vs 64, Pa_max=1W vs 1.5W; Section 2: gm=6500 vs 7500, Pa_max=7W vs 10W, Ih=0.9A vs 0.925A).
Brans cross-lists in same group — Octal (8BD) vs Noval (9HF) base, different pin topology. Similar Section 1 (mu~64-68) but different Section 2 (ra=750 vs 800 ohms)
Same 9HF pinout but Section 1 has much lower mu (17.5 vs 64). Uses T-9 bulb like 6FD7. Not a drop-in replacement due to very different Section 1 characteristics.
6FR7
Same 9HF pinout and dissimilar double triode structure, but different electrical characteristics. Sams Tube Substitution Handbook lists 6FD7 as substitute for 6FR7.
Brans cross-lists — similar dissimilar twin triode with different characteristics
Electrical Specifications
Absolute Maximum
Anode Voltage (max)Max DC Plate Voltage330V
Anode Dissipation (max)Max Plate Dissipation1.5W
Cathode Current (max)Max Average Cathode Current20mA
Heater-to-Cathode Voltage (max)Max Heater-Cathode Voltage200V
Max Grid Resistor
Notes
10FD7 is identical to 6FD7 except for heater ratings (9.7V, 0.600A) and heater warm-up time. 13FD7 is identical except for heater ratings (13.0V, 0.450A).
Pin 2 and Pin 3 are both connected to the control grid of Section 2 (dual grid pins).
Maximum heater voltage range for the 6FD7 is 5.7-6.9V.
Originally designed for vertical deflection circuits in television receivers, but widely adopted by DIY audio designers (notably in Woo Audio WA6/WA6SE headphone amplifiers) as a combined driver/output tube in low-power amplifiers.
In stages operating with grid leak bias, an adequate cathode bias resistor or other suitable means is required to protect the tube in the absence of excitation.
Pin Layout — Noval
1Plate (Section 2)
2Grid (Section 2)
3Grid (Section 2)
Socket-Compatible Tubes ⚠ Not electrically compatible