|PHILCO'S CROWNING ACHIEVEMENT|
By Bruce McCalley
With the release of the superheterodyne patents by RCA in 1931, manufacturers began using this system for all but their cheapest receivers. It wasn't long before additional features were added to home radios such as tuning meters, short wave bands, and "high fidelity."
The limitations in high-frequency response were apparently evident to Philco engineers in the early 1930s. Receivers in the early super-heterodyne era used intermediate frequencies in the 200 kHz range which had a severely limited bandwidth. In 1934 Philco addressed this problem in their model 200 and 200X receivers by using potentiometers in series with the secondary windings of the first and second IF transformers which broadened the response as the resistance was increased. The IF frequency of these sets was 260 kHz. Both of these models used triode-connected push-pull 42's in the output, driven by a triode-connected single 42. (The major difference between the 200 and 200X was that the 200X had one short wave band while the 200 was broadcast band only.)
Philco Model 37-690
Philco entered the so-called high fidelity market with a model 200X in 1934 and then a 116X around 1935. The 116X was essentially a model 116B with 6A3 output tubes in push-pull instead of the triode-connected 42's of the 116B. While the change in output tubes may have made an improvement in the tone quality, the design of the IF amplifier, even with its triple-tuned trans-formers, limited the high frequency response.
The 1936 model, was the 680. This set had three short wave bands in addition to the broadcast band, and also had two IF stages with variable coupling in the 460 kHz IF transformers which enabled greater band-pass and the reception of higher audio frequencies. The audio output was similar to the 116X, with two 6A3's driven by a triode-connected 42. This was probably the top of the Philco line in 1936. The model 680 also used an automatic bass control system which increased the bass response at lower volume levels.
During 1936 Philco began the year-model with the 37-# system. The better sets this year used an "automatic" tuning system which worked somewhat like a telephone dial. A crank arrangement, attached to the tuning dial, was spun around to a marked station tab, pressed in and then cranked until it stopped on that station. Slight errors in just where the crank stopped were corrected by an electronic frequency control (AFC) of the local oscillator. Not as nice as the later push-buttons but effective never-the-less.
The tuner chassis of the Philco 37-690
Short wave bands were still the rage and the high-end sets that used the above tuning system now had four short wave bands in addition to the standard broadcast band. The tuner section was a separate sub-chassis which was mounted on rubber inserts and wired into the main chassis. This sub-assembly contained the RF, mixer, oscillator and AFC circuits.
Pbilco 37-690. Note the two "tweeters" next to the tuner chassis, and the four resonators around the main speaker.
At the top of the 1937 line were two receivers, the 37-116 and the 37-690. Both used the above tuning assembly and an audio output system similar to the earlier 680 except that the tubes were 6B4G's driven by a 6F6G, octal versions of the 6A3 and 42.
The 37-116 had two IF stages which used the same variable coupling IF transformer system as on the 680. It had a "treble" control which was ganged to the shaft that varied the coupling of two of the IF transformers, and a bass control which was a switch with three positions (plus the power switch which gave it four stops). Bass adjustment was accomplished by switching different capacitors in the compensating network at a tap on the volume control. The speaker was a large electro-dynamic unit and was accompanied with two smaller speaker-like resonators. (They looked like six-inch speakers but had no voice coils or field magnets.)
The top of the line was the 37-690. This model used the same tuner as the 37-116 (except for a switch on one of the wafers in the antenna section which disable the automatic bass control on the short wave bands). Three of the IF transformers had the variable coupling, instead of the two in the 37-116. The audio output section was about the same as the 37-116 except that it was now a separate chassis.
One feature of the 37-690 is the automatic bass compensation circuit, similar to the one used on the earlier 680. Rather than using a tap on the volume control to which a compensating network was connected which increased the bass at low volume, this circuit used a separate amplifier for the bass frequencies. The audio signal fed a rectifier which in turn developed a bias voltage which was applied to this bass amplifier in such a way that its gain was increased as the audio level decreased. The output of this automatic bass control system was then mixed in with the regular audio, giving a bass boost. The amount of boost was controlled by the bass control know on the front panel. Talk about over-kill! The volume control potentiometer had no tap as was used in the 37-116. The bass-boost circuit was disabled on the short wave bands by means of a switch on a wafer in the antenna section of the turner.
The separate audio chassis, located at the bottom of the cabinet, had two power supplies which used 5X4G rectifiers mounted on the top shell of the power transformers. One of these supplies powered the audio output and 6F6G driver and some of the amplifier stages on the main chassis. The second power supply supplied the bias voltages for the 6B4G's and other stages in addition to the plate supplies of other sections which were not supplied by the first supply.
Balancing the plate currents of the 6B4G output tubes was done by using a separate filament supply for each tube and by using a dual potentiometer across these windings which served as a variable "center tap" for the windings. This control was adjusted for minimum hum. (Unfortunately, as the tubes heated up they did not do so exactly alike. The hum adjustment had to be made when the tubes were well-heated. When first turned on, though, unless you had a well-matched set of output tubes, there was always some hum for a short period.)
The power supply and power amplifier chassis of the Philco 37-690
Like the 37-116, the 37-690 had a large main speaker. In addition to this, though, there were two smaller electro-dynamic speakers which served as "tweeters." (Using that term rather loosely.) These were fed via a separate output transformer connected across the plates of the 6B4G's but isolated by a .015 mf capacitor which passed only the higher frequencies. (The main speaker was fed all of the audio, not just the lower notes.) In addition, the 37-690 also used four of the small resonators like those in the 37-116. In spite of the lack of much good music on the AM band these days, this is a mighty fine sounding radio! It makes today's AM radios pale in comparison.
The next version of the 37-690 was the 38-690. It was quite similar to the '37 model except for the use of 6L6G's in the output stage. The high-frequency speakers were now driven by the 6F6G driver stage via a winding on the input transformer to the 6L6G's. The use of 6L6G's did eliminate the hum problem during warm-up, and no doubt gave greater power output..... but for we triode lovers, it just aint the same!
Philco now used a less-complicated RF tuner. (The earlier tuners had twenty-six adjustable mica capacitors in the antenna, mixer and oscillator sections while the later ones had none in the antenna and mixer sections.) One might presume this less-complicated tuner was less sensitive on the short wave bands than the earlier design since the individual coils could not be tuned but this writer has never made a side-by-side comparison.
And so ended an era. 1939 models featured pushbuttons and remote control but no more variable bandwidth Ifs, no more high-power audio systems, no more complicated RF sections, and no more "woofers and tweeters." In all fairness, the other major manufacturers also produced elaborate "high-fi" receivers during these years. Zenith had its Stratospheres, RCA had automatic volume expansion, and of course Scott had the Philharmonic and McMurdo Silver had his Master-pieces. Scott soldiered on until the war, Silver went bankrupt, and the others went back to the basics for the most part. Apparently the sales never made these elaborate sets profitable.