PEN453DD

The PEN453DD combines two diodes and an indirectly heated audio output section for receivers powered from either AC or DC mains. The Mazda sheet presents it as a beam power amplifier, while the general description also calls it an output pentode. The sections share the cathode sleeve, but the manufacturer states that the diodes and output section can be considered separately for circuit purposes. The 45 V, 0.2 A heater is intended for a series chain, whose resistance should provide that current at average mains voltage. One valve thus provides detection, automatic volume control and output amplification, with separate connections for their different tasks.

The specified limits are 200 V at both anode and screen and 10 W anode dissipation; the latter is power dissipated internally, distinct from audio output. Transconductance of 12 mA/V is specified at 100 V anode and screen voltage and zero control-grid bias. One typical operating point uses 150 V anode and screen voltage and approximately −8.75 V bias: currents are 50 mA anode and 10 mA screen, with a 145 Ω self-bias resistor and 2.9 kΩ anode load. Rated output is 2.95 W with 4.8 V RMS input. At 160 V anode and 175 V screen voltage, −10 V bias, currents of 64 and 13 mA, a 130 Ω cathode resistor and 2.6 kΩ load, the sheet gives 3.75 W with 5.5 V RMS input. These outputs correspond to 5% third harmonic and second harmonic no greater than 5%; they are not a claim of 5% total harmonic distortion.

The application notes preserve a feature of contemporary receivers: the field-coil loudspeaker supply. With a series field consuming about 6 W, the document expects 145–150 V screen voltage and about 2.7 W audio, with second and third harmonics each within 5%. The 250 Ω output-transformer resistance mentioned is its winding resistance, not the anode load in kilohms. Mazda always requires self-bias and limits grid-to-cathode circuit resistance to 1 MΩ at the 10 W dissipation limit. It also suggests a 50–75 µF capacitor across the bias resistor for decoupling and a parasitic-suppression resistor mounted close to the terminals; in the anode circuit, 50 Ω is suggested. Normally the diode on pin 5 detects audio, while the diode on pin 7 supplies automatic volume control voltage. The first diode’s load returns to the cathode; the second can return to a negative voltage to provide the circuit’s required delay.

The base is Mazda octal, with the control grid at the top cap and a metallised band around the lower bulb. Viewed from below the base, pins 1 and 8 are heater, 2 cathode, 3 anode, 4 screen, 5 diode 2, 6 metallisation and 7 diode 1. The sheet specifies maximum overall length of 131 mm and diameter of 54 mm. Specimen photographs show the lower band and the structure connected to the top contact; recorded specimen measurements without pins remain distinct from the complete maximum dimensions. The manufacturer states that characteristics are identical to the PEN DD4021, a documented relationship to be read alongside actual connections. The printed sheet date is 1 May 1939, documenting this edition of the specifications rather than establishing the valve’s introduction year.