HL41

The HL41, or HL.41, is a Mazda indirectly heated triode for AC mains radio equipment. The manufacturer lists low-frequency amplification with resistance-capacitance or transformer coupling, and oscillator service requiring a high-amplification-factor triode. Our sources describe first audio-stage use in tuned-radio-frequency receivers and superheterodynes with external detection. The HL41DD instead adds two diodes: this entry covers the HL41 triode alone. Its metallised envelope provides external screening, visible on photographed specimens. The heater requires 4 V and 0.65 A; the Ediswan Radio sheet is dated 1 March 1939.

Listed maximum anode voltage is 250 V. At 100 V anode voltage and 0 V grid voltage, Mazda gives 3.5 mA/V transconductance, amplification factor 36 and 10.3 kΩ anode resistance. Two resistance-capacitance amplifier examples use 250 or 260 V supplies, a 50 kΩ anode load and gain 27. The first, without a decoupling resistor, gives 2.2 mA, 3.1 V bias magnitude, a 1,400 Ω self-bias resistor and 45 V RMS output at 2.5% total harmonic distortion. The second adds 20 kΩ decoupling and gives 1.9 mA, 2.75 V, 1,450 Ω and 40 V RMS at the same distortion. The 260 V is circuit supply voltage, not a new anode maximum. The secondary point at 250 V anode, −4.5 V grid and 7 mA, with 3.1 mA/V and 11.5 kΩ, is separate from Mazda’s characterisation.

Original capacitances are 4.5 pF anode-to-earth, 5.25 pF grid-to-earth and 5.25 pF anode-to-grid. The sheet defines earth measurements with the remaining electrodes at earth potential and metallisation connected to the cathode. Curves show anode current against grid voltage at different anode voltages. Viewed from its free end, the British Octal base has heater on pins 1 and 8, cathode on 2, anode on 3, grid on 5 and metallisation on 6; no connection is shown on 4 and 7 is omitted. Maximum dimensions are 94 mm overall and 32 mm diameter. Descriptions of the two specimens instead give 33 mm diameter and 79 mm height without pins, or 32 mm and 80 mm without pins, observations to retain as measurements of individual specimens.