The 41MDG mixer from Cossor was designed in 1932. The mixer is an essential component in the superhet receiver. The signal is applied to one grid and a local oscillator voltage applied to the outer grid. The two frequencies multiply in the non-linear transfer characteristic of the valve and both sum and differences will be present in the anode circuit. The wanted frequency will be where the anode circuit is tuned and then further amplification at the Intermediate Frequency can take place before the signal is demodulated in the detector.
It is indirectly heated and in this variant the bulb is coated with a metallic paint to act as an extra electrostatic screen. Our other 41MDG has a clear bulb and the internal construction is visible. It is not clear why the two variants were made, but without the external paint the valve will run cooler.
In addition to the B5 base there is an extra terminal on the side of the base.
Bernhard Nagel (of radiomuseum.org3002) found this valve in a german superheterodyne receiver Imperial Junior from Stassfurt factories. Originally a Telefunken REN704d has been used.
The balloon envelope is 48 mm in diameter and, excluding the B5 base pins is 104 mm tall.
References: 3002 & 1043. Type 41MDG was first introduced in 1932.