The M8137 is a special quality version of the definitive audio double triode the ECC83. This high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100.
Keith Snook has pointed out that this exhibit is not actually an M8137 but an ECC83. The anodes are orientated with the base as per the M8137 design but it lacks other construction characteristics such as the box anodes and double micas. The Mullard M8137 is the SQ valve. This exhibit was probably made by RTF or similar.
This valve is still found in today's designs of hi-fi audio amplifiers. In the classic Mullard designs of the 1950s and 1960s the valve's two halves were used as second stage amplification and cathode coupled phase splitter. The phase splitter being required so that the push-pull output stage was driven correctly with the output valves driven with a phase difference of 180 degrees.
The construction is between top and bottom micas that are an interference fit to the glass tube envelope. The two triode tower sections are separated by a screen.
The triode sections seen end on to the grid support through the holes in the anode. Electrons do not need to be collected from the ends of the grid and the holes reduce the anode area in this region as well as allowing the hot grid to radiate heat away through the holes.
This exhibit carries the Mullard logo and identification. Also it is marked CV4004 thus showing the direct equivalence between the two designations.
The anodes are opened on one side only.
The anode flat on. The ridges add rigidity and the carbonising improves radiation efficiency as black is the most efficient radiator.
The central cathode and close spaced grid in one triode.
Another view of the grid. The wires pass very close to the central cathode tube.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 48 mm tall.