The PenDD4020 is a Mazda second-generation double-diode-pentode. The PenDD4020 had rather better gain than the Pen453DD and was correspondingly more successful, being used in a good number of AC/DC Murphy sets from around 1936 onwards. A good valve but, interestingly, its eventual end of life was often due to failure of the diode cathode emission rather than weakness in the pentode section.
above the pinch is a lower screen plate and then a mica sheet. The open box screen attached to the mica contains the small plate anode of one of the signal diodes.
The box anode and circular top mica. The side mica sheets provide the spring force to hold the electrodes firmly in place in the envelope. The control grid has significant heat sinking attached to the top supports.
A better view of one signal diode. The other looks the same on the other side of the envelope.
The classic envelope is 54 mm in diameter and, excluding the B7 base pins, is 127 mm tall.
References: Data-sheet & 1043. Type PenDD4020 was first introduced in 1935. See also1935 adverts.
