Sensibly equivalent¶ to:
The Pen4DD is a double diode output pentode, it was to perform the function of signal detection, AGC detection, and audio output. The 4.3 Watt output was significant. The second diode suggests use in superhet receivers rather than TRF sets. To achieve power output from the detected signal required significant gain, and the mutual conductance of 9.5 mA/Volt was high performance for the times. To achieve such levels requires the control grid to be finely wound and to be very close to the cathode, this in turn required that the cathode was ultra smooth. Quite a mature product designed for local station wireless sets where signal strength was good and prices had to be kept low.
Aware of the potential problems of providing insufficient gain to offset the loss of the normal triode stage, Mullard really went to town with this one. One consequence is a pretty huge heater current in relation to power output, indicating that high gain has been achieved by providing an over-sized cathode.
Other consequences included increased susceptibility to mains hum (larger magnetic hum field x extra gain) and to spurious oscillations (very high gm). These problems could be reduced to acceptable levels by careful design on the part of the set manufacturers but somehow the simple cheap solution aspect has been lost. One further problem was that trying to get too much into one valve meant that even minor degradation or ageing could have unacceptable results, whereas with the old double-diode-triode plus pentode solution very considerable valve degradation could occur before the average radio user noticed anything wrong. The double-diode-pentode fad was a feature of the late 1930s, and little used in post-war sets.
Looking into the envelope, the bright shape of the double diode anodes can be seen below the lower mica. This accounts for the tall envelope. One diode is the rectangular box and the second diode is formed from the vertical rods inside the box.
At the top the rectangular top mica can be seen. The anode is oval and blackened. It is a true pentode with a wire suppressor grid.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 128 mm tall.
References: Datasheet, 4048, private communication & 1043. Type Pen4DD was first introduced in 1937.
Absolute Maximum Operating Conditions¶
PDF scanned from an original document held by the museum
Updated January 09, 2015.