The Pen453DD double diode output pentode type of valve was a feature of the late 1930s, and the Pen453DD dates from 1938. The diodes were for signal and AGC rectification, and the high gain output pentode drove the speaker directly without the need for an audio pre-amplifier stage.
This exhibit is essentially a Type PEN383 with additional diodes. The PEN383 has a 38 V heater but in the PEN453DD this was increased to 45 V in order to provide additional heating power for the diode cathodes. The pentode sections of the two types are identical.
The idea was to save cost in a cheap radio by eliminating the normal stage of audio amplification between the detector and the output stage. However, the pentode section of a PEN453DD requires around 20 V pk-pk grid signal for full audio output. This much signal can be delivered by a good detector stage driven by a strong IF signal from a strong station received on a decent aerial but if anything in the receiving chain is weak there is no leeway to turn the volume up so the audio output level drops. Some Murphy sets used PEN383s preceded by a double-diode triode. The extra gain provided by the triode is certainly needed in some cases.
The double-diode-pentode arrangement is a valid technique and was used successfully (by Murphy and other radio manufacturers) during the later 1930s when output pentodes with sufficiently high gain, typically twice that of the PEN453DD, became available. PEN453DDs are rare valves, perhaps for this reason.
By re-introducing these 1930s valves in the post war era, rather than new designs, Mazda lost ground to competitors. Mazda then introduced modern miniature designs.
The anode load impedance needs to be 2,600 Ohms.
The metal paint covers only the lower part of the bulb. Above that point there is an internal coating on the glass.
The top mica holding the electrodes rigid. The anode below is blackened. Note the wire that connects to the metal paint and in this case has become detached.
The top components. Little can be seen. The control grid has a heat sink fitted and the top cap connection is welded to this heat sink.
The wide classic envelope is 54 mm in diameter and, excluding the MO base pins, is 113 mm tall.
References: Datasheet, 1040 & 1043. Type Pen453DD was first introduced in 1938.
Absolute Maximum Operating Conditions¶
Updated March 07, 2019. shape:classic construction:pinch type:double.diode.pentode age:1930.1940 base:mo heater:200ma pins:8 pin:1.h pin:2.k pin:3.a pin:4.g2 pin:5.a(d2) pin:6.m pin:7.a(d1) pin:8.h pin:tc.g1