It is not too easy to identify the round valves. However the etched lettering on the glass reads CA2235 and as the CA looks like the N with a different base the CA it is. The letters PIF can be seen but the prefix is obscured by previous adhesive residue. The make is Marconi's Wireless Telegraph company.The CA Marconi triode dates to around 1912 and PIF indicates a Platinum Iridium filament. The optimum anode voltage of 35 applied to the N but as these were soft valves the anode voltage was low.
The Round valves were all soft vacuum types with the performance controlled by the amount of gas in the envelope. If performance was dropping off then the top bulb would be heated to release more gas. To prevent an excessive current flow through too much ionised gas a series resistance of about 2,000 Ohms would have been used in the anode circuit. These valves outperformed the early high vacuum types but were difficult to both make and use. The demands of WWI for large volume communications saw the development of the high vacuum valve and the end of the era of the Round valves. See alsoR Type.
The anode and grid connections are taken through the side of the envelope.
The four base pins are brass and screwed in place in the plastic base. The centre is a ceramic material like plaster.
The base. The four pins look like the presence of three filaments - a feature of the valves.
The grid is a wire mesh that encloses the filament. The gas reservoir can be seen on the right. The asbestos pellet is held in a 9 mm diameter tube and from the dome of the envelope this tube is 45 mm in length.
The grid external wire is wrapped round the glass bead seal.
A closer view.
Photo micrograph of the grid mesh. There appears to be a seam indicating that once woven the mesh was shaped into a cylinder.
The wide glass tube envelope is 50 mm in diameter and, including the base is 178 mm tall.
References: Data & 1003. Type CA was first introduced in 1912. See also1912 adverts.
