The C8 is a barretter, that is a series resistance to stabilise current flow. The barretter characteristic comes from the use of iron wire. The attached data sheet has a good explanation of the barretter and its function. The data sheet shows the C8 as having a Ct8 side contact base but clearly they also made them with the B4 base for the UK market. This exhibit carries the Mullard brand but is described as Foreign Made - probably made in The Netherlands by the parent company Philips.
The introduction of AC mains in the UK was a gradual affair with some areas opting for DC supplies and remote areas having no mains electricity. DC mains supplies could not be transformed to other voltages with transformers and so for radio receivers the 'Universal' sets were introduced. The HT came straight of the mains and the valve heaters were series connected with a dropping resistor also across the mains. Mains voltages can fluctuate and thus so will heater current and cathode temperature. The barretter would keep the current constant over a small range of changing voltages. When DC mains were replaced with AC the need for universal valves reduced. However the series heater chain came back into fashion with the post war television receiver.
The iron wire is coiled and held in the folds of the supports.
The wide glass tube envelope is 38 mm in diameter, and excluding the Ct8 base pins is 123 mm tall.
Reference: Datasheet Type C8 was first introduced in 1934.
Absolute Maximum Operating Conditions¶
Updated July 17, 2019. shape:wide.glass.tube construction:pinch type:barretter age:1930.1940 base:b4 pins:8 pin:1.- pin:2.- pin:3.f pin:4.f