HL1320

The Mazda HL1320 is an indirectly heated triode for DC or AC receivers and car radios. Available descriptions date introduction to 1934. Its heater is 13 V, 0.20 A, designed for series operation; documentation also allows a 12 V car battery. Applications are detection, audio amplification and oscillation. The bulb is metallised with screening connected to a separate pin; control grid is on the top cap.

Both Mazda sheets give maximum anode voltage 250 V and, at 100 V anode and zero grid, transconductance 3 mA/V, amplification factor 30 and AC anode resistance 10,000 Ω. The individual sheet adds maximum anode current 12 mA. Low-frequency coupling may be transformer, choke or resistance-capacitance, the latter with recommended anode resistance 50,000–100,000 Ω. To minimise hum, it specifies grid-cathode resistance at most 0.25 MΩ. Amplifier biases are −1.5, −2.5 and −3.5 V at anode 100, 150 and 200 V; recommended cathode resistance for self-bias is 600 Ω.

Low impedance also suits anode-bend detection. Biases for that function are −3 V at 100 V anode, −4.7 V at 150 V and −6.2 V at 200 V. Separate metallising connection helps reduce reradiation in a superheterodyne oscillator. An audio example is W. T. Cocking’s Wireless World article of 13 November 1936: two HL1320 precede push-pull Pen3520 valves with negative feedback. A switch inserts the first stage for less sensitive sources or feeds the second directly. The article reports about 3 W on 230 V DC mains, a result of that complete amplifier, not single-HL1320 output power.

British seven-pin base viewed from below: metallising pin 1, heaters 4 and 5, cathode 6, anode 7; 2 and 3 have dashes. The Ediswan sheet of 1 March 1939 gives maximum length 125 mm, diameter 39 mm and capacitances 5.25 pF anode-cathode, 5 pF grid-cathode, 2.5 pF anode-grid. The individual sheet gives 5.25, 6.5 and 3.25 pF respectively; values stay associated with their documents. Photos show a marked Mazda specimen with deteriorated metal coating. Its reported 97 mm height excludes pins and belongs to that 38 mm-diameter specimen.