Introduction and Background When I first saw the Philco 200-X radio in 1984 in an antique store, it was easy to see that it was an unusual set. The dealer's price was right, so I bought it. The cabinet was a disaster and still is, but I have fully restored the electronics. (My interest in radio does not include fixing or restoring cabinets. I prefer researching the history and design of 1930's era radios and restoring the electronics.)

Before we go into the details of the 200-X, I wish to give some background on Philco and requirements for high fidelity design for the AM broadcast band. The Model 200-X was introduced in June 1934, the premier set in a lineup of forty-nine new models. Philco hired opera star Lucrezia Bori to advertise the model. The dealers employed a deliberate "sell-up" strategy, i.e., attract customers in to see the 200-X, which most customers could not afford, and then sell them a radio as far up the sequence of models as they could afford. Figure 1 is a photo taken from a Model 200-X advertisement in a magazine.

During the late 1920s and 1930s, Philco was dependant on Hazeltine Labs for technology integral to the design of all early Philco radio sets. Specifically, Hazeltine developed the technology that enabled an increase in IF bandwidth, thus achieving high fidelity sound in the Model 200-X.

The double side-band system of radio signal transmission has been in use since the beginning of broadcast radio. Achieving fidelity of 5,000 audio cycles requires a 10 KC bandwidth. Boosting reproduction to 7,500 audio cycles requires a 15KC bandwidth. This point is illustrated in Figures 1 and 2, excerpted from the service manual entitled, Philco Model 200-X Circuit Description and Adjusting Instructions. With most radios, the situation is as shown in Figure 1. However, in many cases, there is much more attenuation of the high frequencies taking place than is shown. Fortunately, most of the cheaper sets use pentode or beam-type tubes in the output stages. These tubes use the rising impedance of the speakers (at high frequency) to accentuate the high frequencies. This partially compensates for losses in the IF amplifier.

Figure 1. The IF curve with the selectivity control in the narrow position.

Figure 2. The selectivity curve of the 200-X in the wide bandwidth of the selectivity control. Note the loss in gain, indicated by the dotted line which represented the narrow bandwidth setting.

In certain sets, high frequency losses in a selective IF amplifier are extreme and must be compensated for in the design of the audio amplifiers. The 1933 Scott Deluxe Receiver with a single dial control is an example of this design. In this set, the high frequencies are boosted to make up for the loss in the IF amplifier.

In the case of the Philco 200-X, the IF amplifier is adjusted and widened so that, when the signals are strong and well above the noise level, you can have what used to be known as high fidelity reception. At this point, I should mention that with the coming of FM, better records, better audio amplifiers and loud speakers, and more recently CDs, our concept of high fidelity has been vastly altered. However, the 200-X is extremely well designed, and among AM receivers, it really has no peers. I consider myself fortunate to have one of them.

Technical Details

RF Features. The front end has a bandpass tuner unit, which has a flat-top tuning section with steeper sides than the regular tuning section used in most radios. The tuned RF coil (see item 12, on the schematic shown in Figure 3) has a 10-ohm resistor (see item 15, Figure 3) to increase the bandwidth to 15 KC. The 6A7 mixer oscillator tube and its circuits are conventional.

Figure 3. The RF and IF sections of the Philco 200-X. A three-gang potentiometer varies the IF bandwidth and compensates for the variation in gain as the bandwidth is varied. (1)

IF Features.In order to get the wide IF bandwidth required for Hi-Fi reception, it is necessary to use staggered tuning. The adjustment and tuning of the IF amplifier is complex, and I will not go into that subject in this article. It is well covered by the Philco service notes for the 200-X.

Another problem in the IF amplifier is that during wide-band operation, the gain is reduced. In order to keep the IF gain constant, the gain is varied as the bandwidth is changed from broad to sharp. This is done by a variable resistor (see item 24, Figure 3) in the cathode of the 6A7. The dotted lines show that this control is ganged to the band width resistors labeled item 24 in Figure 3.

A set of this type requires very precise timing. This is done by a trap circuit in the last IF amplifier stage. The last tuned circuit in the IF amplifier is tuned to 175 KC, and feeds a DC amplifier controlling the shadow meter. The diode detector is of a low distortion design that is fairly immune to distortion from AC shunting of the diode load.

Figure 4, The detector and first audio section of the Philco 200-X. (2)

As seen in Figure 4, in the type 75 detector amplifier plate, there is a sharp cut-off 10KC filter. This is sometimes called a "whistle filter." This filter replaces or eliminates the 10 KC beats with the carrier of adjacent stations.

Figure 5. The audio section of the Philco 200-X.

Figure 5 shows a typical Philco output stage. The output section is a triode-connected 42, driving a pair of triode-connected 42s in class AB. Philco used this type of audio amplifier in many of their better sets. The loudspeaker used is a special hi-fi design mounted on an inclined sounding board to direct the sound upward to the listener's ears. A sound diffuser installed in the cabinet aids the lateral dispersion of frequencies. Note that the loud speaker radiates the higher frequencies in an angle. Both the inclined sounding board and the diffuser tend to make the sound field more uniform to listeners in different parts of the room.

Concluding Thoughts The Philco 200-X was priced at $200, which was a lot of money in 1934, so not many were produced. You could have bought a Scott radio for less money, but it would not have been as good a radio. In my view, the best thing about Scott radios were the cabinets, which were outstanding. There is also a Philco Model 201 that is similar to the 200-X, but has a short-wave band as well as the broadcast band. I have had no experience with the Model 201 other than a perusal of the wiring diagram. I expect that it came out during the 1935 model year. A good radio no doubt, but I personally prefer the 200-X model. One of these days, I must get the cabinet fixed on my 200-X. However, if any of my readers sees a Philco Model 200-X at a reasonable price, buy it! You will have an outstanding radio.(3)

NOTES

1. Bandwidth is changed by changing the resistance between the capacitor in the third winding of the first and second IF transformers and ground. As the resistance is reduced the circuit is detuned a bit, resulting in a wider band-pass. One section of the three-section control varies the bias on the 6A7 mixer, compensating for the reduction in gain as the band width is increased.

3. The Philco 200 and 201 were the first of a series of Philco "HiFi" receivers. In 1936 they were superseded by the model 680. The 680 used two 6A3's in the output, variable-coupling IF transformers, and was an all-band receiver. The 1937 model was the 37-690, using two 6B4Gs in the output, separate "tweeter" speakers, and many other new features. The 1938 line had the 38-690, similar to the '37 model except for 6L6s in the output and other minor revisions. There were no later HiFi versions in the Philco line. The "big deal" in 1939 was remote control and push-button tuning.