The Cossor 240B is a double triode AF output valve designed for class B push-pull use in battery receivers. The 240B was introduced in 1933 as was the Mullard QP21. Other double push-pull valves in the museum are the triode PM2B and the QP230 double pentode from Mazda plus the QP22B from Mullard.
This type of operation with very little standing anode current is known as Class B. Quiescent push-pull stages were biased to cut-off and never went into grid current operation, The B2 stages operated with little or zero bias and required a driver to supply the grid current. The 240B required up to 40 Volts peak to peak grid to grid input signal.
Cossor in the leaflet that came with this 'new' valve recommends the Cossor 215P as a driver. To drive the 240B
The envelope is mainly covered with the silvery deposit of the Magnesium getter material but from the rear the twin box electrode systems can be seen.
When seen close-up and from the top the grid can be seen through the top mica support.
The balloon envelope is 47 mm in diameter, and excluding the B7 base pins, is 90 mm tall.
References: Datasheet & 1043. Type 240B was first introduced in 1933.