Golborne Vintage Radio

I recently bought a piece of vintage test equipment which contains a Link-Miles DC amplifier of which I can find no information. I'm starting to reverse engineer it to get an idea of how it works and I see that it may be a chopper amplifier.
There are a pair of neons arranged as a multivibrator lighting a light dependant resistor. With power applied it oscillates at 151 Hz and generates an approximately square wave. I hadn't seen this configuration previously but a search shows similar configurations being used in chopper amplifiers.

Amplifier board and neon multibrator with photo-resistor:
[attachment=22901]

[attachment=22903]

Complete test set:
 [attachment=22902]

The other amplifier module is easier to understand being a relatively simple AC power amplifier. I have made a schematic of this and managed to use it to drive a 400Hz servo motor. The complete system should be able to implement a complete closed loop control system, I'd like to use it to help test and repair old aircraft instruments.

I like the big dial and control knobs.
Has anybody seen any schematics for a similar DC amplifier?

Best regards, David.

Good DC amplifiers were difficult in the days of valves. Philbrick made some good early valve op-amps, including the famous K2-W: https://sound-au.com/articles/opamp-history.htm Even into the 1960s, if you wanted really good DC performance you needed to use a chopper stabilised design. You didn't get much bandwidth!

The Marconi TF1313A has a chopper amplifier in it.
A circuit diagram for it might be easier to find.

The HP412 meter used a chopper amp IIRC.

Thanks for the suggestions, much appreciated, David.