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Philco 95

Philco introduced their model 95 "Screen Grid Plus" in October of 1929. Philco used the word "Plus" to indicate their best receivers through 1931.1  The 95 was the result of a great management decision; to hire the Hazeltine Corporation as their engineering firm.

Hazeltine's rise began when Freed-Eisemann used Hazeltine’s design to build their first Neutrodyne, the NR5, pushing Freed’s profits for 1923 one hundred times higher than those of 1922.2  For the next five years, the Neutrodyne offered the best performance at the best price of any design on the market.

The introduction of the screen-grid tube in 1929 made neutralizing circuits unnecessary and put an end to Hazeltine Corporation’s monopoly in that field. But another Hazeltine employee, Harold Alden Wheeler, created an endless parade of innovation that would endure for the next 40 years. In 1925 he invented Automatic Volume Control, or AVC. He designed the first broadcast receiver in which diode AVC was marketed, the Philco 95. Diode AVC “became the keystone of the Company’s patent portfolio in the 1930s, from which royalties carried the Company through the Depression.” 3
Philco 95
Philco 95

On September 27, 1932, Wheeler’s first three AVC patents were issued, “just days after the RCA labs had released a bulletin recommending diode AVC using their new tube type containing two diodes and a triode on one cathode. [type 55] ‘When they saw my patent, it spoiled their whole day.’ ” 4

Here's a simplified version of the 95's AVC circuit (below). V4 is drawn as a diode, even though it actually is a 27 with the grid and plate tied together and connected to the junction of #19, #20, and #25.
Philco 95 schematic

In spite of operating with sharp-cutoff type 24A tubes, our 95’s AVC functioned flawlessly; as long as the sensitivity control was in the “local” position. In the "local" position, the 20 ohm resistor #1 is shunted across the antenna coil, and the bias on V1 and V2 was –3.4 volts for weak signals and –5.5 volts for strong signals. Bias on V3 was –3 volts for weak signals and –4 volts for strong signals.
With the sensitivity control in the "distant" position, strong stations drove the bias to –11volts on V1 and V2 and –6.4 volts on V3.

With RCA still in control of the Superheterodyne, Philco chose to build a TRF. Not only does the ad on the right tout the thoughtful design features of the 95, but it highlights the superior engineering facility of Hazeltine Labs who designed, what is arguably, the finest TRF ever built.

Accomplishing automatic volume control with sharp-cutoff tubes in 1929 was nothing short of remarkable. The design was immediately popular. Wheeler's next implementation of his AVC circuit was the 1930 Fada model KA receiver. The arrival of the variable-mu type 35 tube in 1931 allowed widespread use of Wheeler's design.

Although Wheeler filed the first patent application for Diode AVC on July 7, 1927, RCA beat him to market with their Triode AVC in the Radiola 64 on October 23, 1928. The heart of the circuit (shown on left) was a type 27 tube used as the actual volume control. It was a bold design that worked quite well.

On September 27, 1932, Wheeler's first three patents on AVC were issued. "The AVC patent litigation came to a climax in 1936, after which RCA took a [Hazeltine] license and set an example for other companies not yet licensed. It came to an anticlimax in 1941, with the Supreme Court reversal of favorable decisions in the lower courts." 5

selectivity curves of Philco 95
Radio magazine (March 1930)


end notes:

1. Ron Ramirez, Philco Radio 1928-1942 (Schiffer Publishing, 1993), 18.

2. Alan Douglas, Radio Manufacturers of the 1920’s Volume 2 (Vestal, NY: Vestal Press, 1991), 2. “Freed-Eisemann did rather well with its NR5: 1923 sales were four times the previous year’s, and net earnings (profits) 100 times higher.  The company moved into a new factory of 15,000 square feet in December, 1923, in the Sperry Building in Brooklyn, eventually occupying 50,000 square feet and employing as many as 570 workers.  It also brought suit against PDME [the law firm representing the IRM] and refused to pay royalties to Hazeltine, claiming that PDME’s lawyers were financially interested in the Hazeltine Corporation at the same time they were advising their client Freed-Eisemann to sign the royalty agreement.  Hazeltine in turn sued Freed-Eisemann for breach of contract.  Freed-Eisemann was hoping to have its royalties reduced, from 6% of the wholesale receiver price, to 6% of the neutralizing condenser alone, and offered Hazeltine $30,000 a year, but eventually lost, and grumpily went back to paying the whole amount.”

3. Harold Alden Wheeler, The Early Days of Wheeler and Hazeltine Corporation, (Poughkeepsie, N. Y.:  Hamilton Reproductions, Inc., 1982), 198.

4. Ibid,  206.

5. Ibid, 206

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