HP 200A

The HP 200A, first built in 1938, was the first product[1] made by Hewlett-Packard and was manufactured in David Packard's garage in Palo Alto, California.

Earlier, Larned Meacham had used light bulbs in bridge circuits to stabilize and linearize oscillators in 1938.

[10] It covered from the subaudio 5 Hz to the low end (Long Wave) of the AM radio band at 600 kilohertz.

To simplify analysis, all the components other than R1, R2, C1 and C2 can be modeled as a non-inverting amplifier with a gain of 1+Rf/Rb and with a high input impedance.

This does not change the principles of the circuit's operation, but it does reduce the amplitude of the output at equilibrium because the bias current provides part of the heating of the lamp.

HP 200A front panel
Insides of the Hewlett-Packard HP 200A. The light bulb repurposed as a positive temperature coefficient resistor is to the right of the upper section of the variable capacitor, which is the large structure in the center.
Simplified schematic of a Wien bridge oscillator from Hewlett's US patent 2,268,872. Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors are considered to be appropriate values for biasing and loading the vacuum tubes. Node labels and reference designators in this figure are not the same as used in the patent. The vacuum tubes indicated in Hewlett's patent were pentodes rather than the triodes shown here.