American DHT power triode with similar power class but higher mu (3.85), 5V filament, UX4 base — different socket and filament voltage
AD1/350
Higher-voltage variant rated for 350V anode operation — same base and pinout but different maximum ratings, not safe as drop-in in 250V circuits expecting standard AD1 bias
Electrical Specifications
Absolute Maximum
Anode Voltage (max)Anode Voltage (max)250V
Peak Anode Voltage (max)Peak Anode Voltage (max)550V
Directly heated by AC at 4V/0.95A. Separate heater winding recommended for the AD1 to avoid excessive heater-cathode voltage on preceding stages.
For cathode (filament) bias in Class A: Rk = 750Ω (3W minimum) between filament centre-tap and ground. Bypass with at least 25–50 µF.
Maximum grid resistor: 0.7 MΩ (self-bias with separate heater winding), 0.3 MΩ (fixed bias).
Philips Miniwatt 1938 index lists Wa_max = 30W, but the detailed Philips datasheet (No. 30) specifies 15.5W — the datasheet value is authoritative.
Also manufactured by Philips (Miniwatt), Valvo, Osram, and Tungsram (as TAD1). First produced by Telefunken Berlin in 1935; approximately 264,000 units manufactured through 1953.
Push-pull Class A/B (2 tubes, auto-bias): Va=250V, Rk=375Ω (6W), Iao=2×60mA, Ra(a-a)=4000Ω, Pout=9.2W at 1.3% distortion.
Grid-to-anode capacitance is 15 pF — typical for an unscreened power triode. This limits bandwidth at higher frequencies.