The Brimistor (Brimar Resistor)
14-10-65 Brimistor added.
Above, serial no. 6023 - the brimistor can be seen at the front of the chassis on two standoffs to the right of the mains transformer. For further pictures, see this page.
Serial no. 4282 - no brimistor - and no holes for the standoffs or the feed-through for the underchassis wiring. Pot codes for this amp are "GM" = July 1965. For further pictures see this page.
The Brimistor was added by Vox to solid state rectified AC50s (and AC100s) as a means of delaying the inrush current when the amp was first switched on - an attempt to mimick the "slow start" provided by the 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.
A further hope may have been that the brimistor would protect the main filter capacitors from higher currents when AC50s were used in America. The American market was a large one.
The type of brimistor adopted was the "CZ4", the highest rated that Brimar produced.
Above, an extract from a Brimar brochure of the early 1960s. The full text is available here.
Implentation evidently ran from around the date registered on the schematic until the end of the JMI in early 1968.
Whether the brimistor really worked as intended is a moot point - but the idea at any rate was an ingenious one.