I traced out the schematic and got this amp running. I measured a gain of 500000 up to the grids of the output stage. See shcematic below for detailed gain measurements. Full scale range at the input is around +/-100uV. The offset voltage measured less than 10uV and is limited only by thermoelectric effects. This very good offset performance is due to the chopper architecture and to the good quality of the Brown chopper. The small signal Brown chopper that handles signals in the microvolt range manages to be larger than the power supply vibrator. I only had to replace one bad capacitor, a paper 0.25uF 600V cap that smooths transitions at the power transformer high voltage output.

The output stage is very unique. It consists of a linearly controlled full wave rectifier. Four medium mu triodes in two 7N7 loctals are normally on with zero signal at the amplifier input. Applied input signal turns off all four triodes incrementally during one half cycle of the 120Hz chopper frequency. The sign of the input voltage determines which half cycle is to be turned off.

I have no documentation, other then what I traced and measured. It appears that the output will only provide a reasonable signal when heavily loaded with less than 10k. This is to be expected, considering the internal impedance of the quad triodes and the 150 Ohm cathode resistor needs bias current to keep the grids from clipping the drive signal.

I would guess that the output could drive one winding of a two phase induction motor. Perhaps the other winding is driven by the +/-160V output from the power transformer. This would make for a fairly conventional servo motor drive in mid 20th century. I own a Digitec DVM that uses a chopper amp and servo motor in this fashion. The servo motor drives a 10 turn precision pot that balances the input voltage applied to a bridge. A turns counter on the pot shaft provides the digital output.

If combined with a precision pot and two phase induction motor, this amplifier could make a nice thermocouple based thermometer, or  DVM with microvolt precision.

Comments invited to K2W_at_PhilbrickArchive_dot_org

Former analog computer engineer Ed Lyon had some very interesting comments about the Brown chopper amp.

SwitcherCAD schematic for Brown_D-7179_chopper_amp.asc