This exhibit is prototype No. 25 of the Mullard transmitting double beam tetrode. At this point the anode pins are sealed into the envelope with additional glass beads. This technique can be seen in the American pattern double beam tetrodes such as the 832et al. A refinement that was used in the production valves was sintered glass on the top and the base that reduced the likelihood of the anode pins cracking the glass due to the extra strength given to the glass by the sintering process.
Prototypes No.s 24 and 25 belong to Derek Polley, his father worked for Mullard and was allowed to keep these prototypes after all testing was complete. Thanks to Derek for the images and information.
The anode pins sealed into the extended glass of the envelope.
The basic construction matches the QQV06-40A but the neutralising capacitors are located above a screen.
Serial Number 25 clearly visible.
The wide glass tube envelope is 44 mm in diameter, and excluding the B7A base pins is mm tall.