At first sight this exhibit looks to be a BTH Type B made in the mid 1920's as the letter B can be seen on the glass. However, looking at another BTH pip-top from the same period (Type B4) shows that the envelope style, before the lettering migrated to the dome after the evacuation tube moved to the base, was to place the Type designation in a line under the BTH logo. The main image shows the outline of a circle at the top above the B and a curved hook attached to the circle. Comparison with the B4 and others strongly suggests that what we can see is the BTH logo and not the Type designation. The latter is not visible. One guess is that this is probably an R type made before the flat dome version seen in R Type. However, the grid of that valve is wider than the grid of this valve and so the possibility of this being a Type B remains.
This exhibit has a tapered tubular envelope that narrows from 37 mm to 30.5 mm and a plated base shell. It has been gettered with phosphorus and this imparts the red colour to the inside of the glass.
The Type B was introduced in 1918 and the design closely resembles the R Type. It was initially made with a spherical envelope and would have been un-gettered. With the pumps of the day it would have taken two to three hours to achieve a hard vacuum.
The B Type was used for both transmission and reception and like other valves of the time the filament was over-run for transmitter use and the lifetime was short. As a receiving valve the filament was run at four volts and about 0.7 Amp but for transmission the filament could be pushed to six Volts.
All of the WWI valves had simple split pins fitted to the base, this valve has the later petal pin fitted. This is a solid core fitted with a split outer as can be seen from the photographs. The enveloped is etched with the BBC logo and that places the manufacture date to between Autumn 1924 and late 1927 when the use of the BBC logo ended.
Thanks to Steve Jordan for sending the images and dimensions.
This image shows the outline of the vertical electrode system. The filament is a single tungsten strand and the helical grid can be seen below the anode. The latter was probably made of nickel and can be seen to have a single support strut. The base shell has the letters A and G to denote the electrodes.
The base shell infill carries the BTH logo and the connecting wires can be seen to go to the outside of the pin tops where they are soldered into position.
In this view the pin construction is clear, the A letter on the base shell can be seen. The letter to the right under the B could be an H.
This image shows the deep red of the phosphorus. Smaller amounts would give a more golden orange colour as can be seen in this R Type.
There is no doubt that this is the BBC logo.
BBC logo showing the font a little better.
Another glimpse of the envelope where the Type designation should be seen below the BTH logo.
The best view possible from the available images.
The balloon envelope is 37 mm in diameter, and excluding the B4 base pins is 92 mm tall.