The Brimistor (Brimar Resistor)
"Brimistor and Voltages added 28-6-65"
The part number was CZ4, the "Z" denoting that it had wired ends rather than metal caps (in the manner of a fuse). The electrical characteristics of the C4 / CZ4 are described in the Brimar specification booklet at the bottom of this page.
Brimistors in AC100 numbers 1584 and 1600 - mounted just behind the choke on two standoffs.
The brimistor was added by Vox to the AC100 Mk 2 (and a little later to AC50s) as a means of delaying the inrush current when the amp was first switched on - an attempt to mimic the "slow start" provided by a GZ34 valve rectifier - the idea being that the heaters of the output valves had time to warm up before the rest of the circuit rose to operating levels.
For early AC100 Mark 2s, ie. the first AC100s with brimistors, see this page.
Note of the addition of the brimistor on the AC50 Mk III schematic - 14-10-65
Whether the brimistor really worked as intended is a moot point - but the idea at any rate was a good one.
Advertisement prior to the introduction of the CZ4.
Brimistors - Current Surge Resistors by Brimar
Above, a Brimistor specification booklet from the 1960s (probably the second iteration). No date is given, but it is clear from a note on the last page that Brimar was at this time owned by Thorn. Click on the images for larger versions - these will download at around 2400 pixels in the longest dimension.