Cossor Types P1 and P2 were tungsten-filament 'brights', like the R Type and were not gettered. Early samples were fitted with shiny metal base caps; the red (bakelite?) type dates from around 1924.
Cossor did later make thoriated versions of these types but these were designated the P3 and P4, had larger and more bulbous envelopes, and were gettered.
The unique feature of the P1 and P2 (which looked identical except that one had its top dipped in red paint) was that the filament was self-sprung in the form of a semi-circular bow, following the 'hooded' form of the anode. The grid also followed the same hooded shape. This arrangement not only avoided the need of a filament tensioning spring but also side-stepped patents covering the 'French' construction arrangement used in the Type R and many other early 'brights'.
The anode is a flattened parabolic hood with a bright finish. The control grid is a series of wire loops attached to a flattened metal ring, seen under the anode hood. The B4 base is 31 mm in diameter.
We have acquired another P1 with the bakelite base. The shape of the moulding with the opening between the pins is characteristic of Cossor manufacture.
The etched lettering has the patent details on one side of the envelope and the type designation on the other side. In addition to P1 lettering gives filament voltage 3.5 - 4.0 Volts and anode voltage 20 - 80 Volts. Looking into the anode bell the grid wires on the support can be seen to form a cage. The thicker supports in the centre are for the arched filament.
This image shows the form of the grid wires and the welds to the support.
The filament is broken but the remains of the wire can be seen attached to the right hand support.
The wide glass tube envelope is 27 mm in diameter and, excluding the base pins, is 78 mm tall.
References: 1003 & 1004 Type P1 was first introduced in 1922.