A 1939 Audio Oscillator Caught Disney’s Eye—And Helped Launch HP

January 01, 2024 by Duane Benson

75 years ago, one of the biggest electronics firms of the century got its start in a Palo Alto, California, garage—and it did so with the help of Mickey Mouse.

In the 1930s and 40s, David Packard and fellow Stanford University graduate William Hewlett planted a seed that grew into one of the world’s leading electronics companies. Beyond creating a company, the business duo set the stage for the tech mecca that would come to be called Silicon Valley.


HP Offices, Santa Clara, CA

HP offices, Santa Clara, CA. Image used courtesy of LPS.1/Wikimedia Commons (CC0 1.0)

In 1938, Hewlett-Packard’s (HP) first commercial product, the Model 200A audio oscillator, caught the eye of an engineer from Walt Disney Studios. Disney was trying to revitalize Mickey Mouse in the public’s eye with the groundbreaking movie Fantasia. Fantasia would use eight-channel sound for production and was the first commercial film to be accompanied by stereo sound. Disney needed a reliable audio oscillator to test what they called “Fantasound.” With a few modifications, the Hewlett-Packard design fit the requirements.


The Lecture That Started It All

The inspiration for the Model 200A came from a lecture by Hewlett and Packard’s professor and mentor. Professor Frederick E. Terman read his class a 1934 paper written by Bell Laboratories' H.S. Black about reducing distortion with negative feedback. As a graduate student, Hewlett dedicated his master’s thesis to this concept. Meanwhile, Packard had left with his EE degree to work in vacuum tube electronics for General Electric in New York.

Hewlett wanted to better the two primary types of oscillators at the time, the coil-condenser and beat frequency oscillator. Each had advantages and limitations, but they were both complex and unreliable. He decided to create something new that would deliver the advantages of both but in a simpler and inexpensive form.


A Wien Bridge Oscillator and a Clever Negative Feedback Element

He chose a simple Wien bridge as the primary oscillator. The signal from a Wienn bridge, however, had too much distortion to be used as an effective tone generator. Drawing on his studies about negative feedback, he devised a clever compensation circuit. He placed a 3 W lightbulb in series with the cathode of the Pentode (vacuum tube), the active component of the oscillator.

The bulb acted as a temperature and current-varying resistor. With a low current flowing to the cathode, the tungsten filament wire was cool and demonstrated low resistance. With more current flowing, the filament temperature increased and, with it, resistance. In this arrangement, the device delivered negative feedback based on the current flow through the pentode. With negative feedback, the bridge produced sine waves with less than 1% distortion.


Partial schematic of the Model 200A

Partial schematic of the Model 200A. Lamp R7 operated as a negative feedback element by thermally regulating the current to the cathode in V1. Image used courtesy of HP

Terman saw great potential in this design and the team; Packard’s vacuum tube knowledge complemented Hewlett’s innovative design skills. He convinced Hewlett to call Packard back to California and secured a small grant as a stipend for Packard to resume graduate studies at Stanford while starting a company with Hewlett. A coin toss put Hewlett’s name first on the business name registration, and the company turned the Model 200A into a commercial product. The first units were built in Packard’s rented garage in Palo Alto, California.


A Fortunate Public Showing

Hewlett brought his new oscillator to a regional meeting of the Institute of Radio Engineers (pg 5, HP annual report, 1964) held in Portland, Oregon, where an engineer from Walt Disney Studios also attended. The Disney engineer recognized the product was just what they needed for their advanced audio soundtrack for Fantasia. Disney asked for a few modifications, including a frequency range from 20 Hz to 20 KHz. The 200A originally had a range between 35 Hz and 35 KHz. With a few component changes, the 200B was born, and Disney purchased eight of the new oscillators for $54.40 each. Competitive equipment in 1939 typically cost between $200 to $600.


HP’s first ad for the Model 200B

HP’s first ad for the Model 200B, which was sold to Walt Disney Studios in 1939. Image used courtesy of Hewlett-Packard History

The movie Fantasia broke ground in many cinematic arenas, not the least of which was audio. The Model 200B oscillators were used to test the sound channels, speaker systems, and recording electronics. The sale of eight units to Disney justified Hewlett and Packard's confidence in the product and fueled their initiative to start a company.


The Extended Legacy of the Model 200A

The Model 200A and 200B were produced until 1952, when they were succeeded by the Model 200CD. The 200CD was the last vacuum tube instrument produced by HP and the longest-lived. It was listed in the Hewlett-Packard Test and Measurement catalog until 1985.


Front view of the Model 200A

Front view of the Model 200A. Image used courtesy of HP

The HP garage at 367 Addison Avenue in Palo Alto, California, is now the stuff of legend and is preserved as a private museum. It is listed in the National Register of Historic Places as the birthplace of Silicon Valley.

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    Larry Langrill January 08, 2024

    Loved it!

    Like. Reply
  • C
    chazzab January 15, 2024

    The lamp in the cathode wasn’t negative feedback as is commonly understood. The lamp stabilised the signal amplitude as the thermal inertia was longer than a single cycle of the lowest frequency, so the gain didn’t change quickly enough to be called negative feedback. The low distortion was as a result of this ‘soft’ amplitude stabilization, the alternative being the much harder clipping produced by the valve running out of headroom..

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