Link-Miles DC Amplifier

[djrm]

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.

I have seen HP amplifiers with multiple LDRs used for chopper signal path selection, this design uses a single LDR and a pulse feed from the neon circuit. I'm trying to understand how this could be configured in the circuit, any help much appreciated, David.

Amplifier board and neon multibrator with photo-resistor:


Neon-LDR multivibrator  module:

[MathWizard]

So why do they use the bulbs? Do they have have very consistent turn-on voltages or something ? Or are they more stable or drift less than caps ? And then I guess they use the light for feedback or signal path, without loading the circuit ?

At 1st I thought the circuit had BJT's too, like a BJT multi-vibrator, but wow it doesn't.

I have a bag of little Neon bulbs, I know they barely use any current and ~100V, and I just made an adjustable, low current 200V supply. So I should try some experiments with them. How do they do at higher frequencies I wonder.

LTSpice has these bulbs too, I must try them too.

[TimFox]

Before LEDs, the only small light sources were incandescent and neon bulbs.
For photochoppers, the incandescent bulbs were far too slow to pulse at the required rate.
I have an old -hp- 425A sensitive voltmeter that used incandescent bulbs with a motor-driven chopping wheel (steady excitation of the bulbs) and photoresistors.
Photoresistors (at the other end of the chopper) come with different chemistry, some of which are fast enough for chopping:  compared with photodiodes, they are symmetric for current flow.
I did see a photograph in an electronics magazine of a magic-eye tube (with linear display) used to control a photoresistor for variable attenuation.