The 200-A Hewlett-Packard's First Product
Reprinted form SMEC Vintage Electrics Vol #3, Issue
#1 1991 ( now SMECC)
|The following questions were sent to several electronics engineers that author here, along with the patent of the 200A.
1 - How did the 200A compare with existing products on the market at the time?
2 - What was unique about the design of the 200A?
2 - Did the 200A affect future designs of other audio oscillator?
Here are the replies we received.
|W. R. HEWLETT’S
STABILIZED WIEN BRIDGE OSCILLATOR (The 200A)
By Morgan McMahon
From all I can determine, Bill Hewlett's use of a ballast resistor (e.g. light bulb) for amplitude stabilization was considered ingeneous by Hewlett’s peers.
The wide frequency range, combined with excellent frequency and amplitude stability, made the H-P 200 oscillator family the standard audio frequency generator for a generation of engineers and scientists. I well recall spending many hours with these oscillators in grad school, in electronic research labs at Hughes Aircraft Company, and at TRW.
To my knowledge, the first public disclosure of this circuit was in Proceedings of the IRE, Volume 24 page 649, 1939. The article was by F. E. Terman, R. B. Buss, W. R. Hewlett, and F. C. Cahill. I can’t recall the title.
Samuel Seely, in his well-known book Electron Tube Circuits (McGraw-Hill, 1950, 1958), describes the Wien-bridge oscillator in some detail. He cites Hewlett’s amplitude stabilization as: "A very ingenious modification of this circuit which serves to stabilize the amplitude against range switching and against aging of tubes replaces R4 by a tungsten-filament lamp..."
Greenwood, Holdam and MacRae’s book Electronic Instruments Radiation Laboratories Series Vol. 21 (McGraw-Hill), 1948, cites the H-P model 200D for open-loop testing of servo-type systems (Page 465). In addition, the same book (pp 546-7) shows a war-time precision oscillator for use in airborne analog computers, obviously Bill Hewlett’s circuit with embellishments.
With regard to your questions on the impact of Bill Hewlett’s circuit on other audio test oscillators (e.g. beat-frequency and L-C types), the answer is simple; it rendered them obsolete.
By Ivan Saddler
I have just reviewed Hewlett patent #2268872. It appears to be the earliest one to come from the association of Hewlett and Packard. It is worth considering first what is a patent and the contract between the US Government and an inventor.
Many have tried to define what is an invention. It’s much easier to find what is not. In the usual case an inventor comes up with a novel idea. He files an application for a patent. It is then he may find what he thought of is not novel. But he must first take some steps to enter the system of patents and trademarks in this country.
An invention consists of two things; a conception and a reduction to practice. If a patented idea is challenged the inventor must demonstrate the reduction to practice. For this reason it is better to document the idea and then speedily make a model to prove the workability of the conception. In this country an inventor can publicize his idea as long as the application for a patent occurs within one year of the publication. In some countries publication of an idea prevents issuance of a patent.
Using the Hewlett patent as a model let’s go through the steps to obtaining a patent. When the light bulb goes on in the inventors brain he should immediately describe in a sewn bound notebook his conception. Next he should have someone who understands what he has done read the description, watch the inventor sign and date the entry then, witness that he read and understood what was logged.
These actions attest to the date of invention should another inventor claim he had done this earlier. Further log entries must show progress toward a reduction to practice. Lack of pursuit of proving the idea often equates to abandoning an idea.
The inventor through his attorney files a patent application when a working model has been built. There are so many rules and conventions that at this point it is wise to turn to a patent attorney. He will search existing patents to determine what is really novel. What no one before has claimed. The patent attorney can search all previously issued patents. Today patents are well codified and fit into categories published by the government. Since 1978 all patents can be searched with a computer equipped to read CD ROMS. It is only necessary to know the full name of the inventor or the assignee or the patent number or the field of invention. This really simplifies a searchers review for novelty. Such searchers are often law students or trained independent searchers.
Just what is the worth of a patent anyhow? The inventor’s ego is far down the list of reasons for pursuing a patent. Only time will tell whether it is a big winner a moderate one or is never used. A US Patent is a contract between an inventor and the US Government. An issued patent is the exclusive property of an inventor for 17 years. He may use it, license it use by others, sell the rights, assign it to another or not use it at all. In exchange for that exclusivity the government has the right to publish the improvement in technology to encourage others to make further inventions based on this one.
Searching for novelty is expensive. A thorough search can cost from $1000 to $5000. The filing fee paid to the Patent and Copyright organization is more than $3000.
With the patent application filed the Patent Office assigns a searcher employed by the government who is skilled in the technology field of the application. He searches both issued patents and applications he is reviewing in the same field. His intent is to grant only the inventors claims found to be really novel. Claims contained in applications are not available to attorneys. They are under strict security. The game most inventors and their attorneys play is to claim all possible novel things in hopes of having enough survive the government search process to give the patent some clout.
With that rather extensive background let’s examine the Hewlett patent. After one and one-half pages of background six claims of novelty appear. When reviewed critically they boil down to two. The use of both positive and negative feedback may have been novel. There were several technical journal articles written around that time involving that technique. They generally included negative feedback to linearize the frequency response of an audio amplifier. The positive feedback was from a small value resistor in series between the output transformer and the speaker. That feedback was to take care of non-uniform frequency response of the speaker and interaction between the speaker and the room acoustics. The second novel thing was the use of a non-inductive non-linear resistor to linearize frequency response of the Hewlett oscillator. Tungsten light bulbs act in just that way. That is indeed what was used. The trick was in selecting just the proper bulb.
All other claims were for variations of the original circuit to protect the novel ideas in similar but not exactly the same circuits. These include the use of one tube and two tubes. Use of both negative and positive feedback is common to all six claims.
The early audio oscillators from H-P did include use of this invention and were state of the art when introduced.
Winfield Salisbury Recounts First HP Experiences.
Let us introduce the reader to Winfield W. Salisbury, 87 years of age, who has lead a full and rich life in various phases of engineering in those early days when Bill and Dave were getting their start.
During the early days Win, as he prefers to be called, assisted Lawrence with work on the cyclotron, went on to work with Fred Terman in the Harvard Radio Research Laboratory, and was later involve himself in many great endeavors. In the future issues of VINTAGE ELECTRICS, we will have an in-depth accounting of Winfield Salisbury’s early days in detail. The Museum has on tape hours of oral history interviews conducted with him.
When Win was asked for commentary on the engineering of Bill Hewlett’s 200A audio oscillator he told us:
"It was a very fine idea, and a very timely idea, as it came when the movie sound industry needed it."
This refers to the widespread need for a stable and precise audio oscillator. Disney Studios was to become Hewlett-Packard's early customer and purchased a group of oscillators for use in the soundtrack of Fantasia.
Win continues: "The only competition that Hewlett and Packard’s oscillator had was the beat frequency oscillator that some people were making, such as General Radio. They were unstable oscillators (the beat frequency types), and you could not depend on their frequency or their amplitude. This trick that Hewlett used was a real hum-dinger! They sold those oscillators by the thousands!"
When we asked Win why the Hewlett’s oscillator design was superior to the General Radio Beat Frequency Oscillator he replied:
"Now the reason that is important, is because you can make resistors and capacitors that are thermally stable. That’s why they were way ahead of everybody else. It is almost impossible to make an inductor that is thermally stable, as the iron changes permeability with temperature and the wire changes its inductance with temperature because the coils expand and contract, "but if you make capacitors with mica, and the right type of resistors, you can make them so they do not change with temperature."
When Win was asked if he wanted a 200A, he replied he: "...wanted one so bad he didn’t know what to do!"
We asked Win if he had met Bill and Dave, and he remembers that down the street from the famous garage that manufactured audio oscillators there was an ice cream store. It was a chance meeting when "...they were all in there getting ice cream cones!"
Win went over to visit the manufacturing facility in the garage and remembers there being "two other employees" present soldering and building the equipment.
In later years Win recounts that Bill and he had lunch several times after he returned to Palo Alto after working back east.
By Edward A. Sharpe Archivist, SMEC (c)