The Philco Model 38-690 is a high fidelity receiver and worthy of special consideration, especially since it is designed with a 14 inch woofer (low-frequency speaker) and two 6 inch tweeters (high-frequency speakers), along with four 8 inch passive radiators which Philco called Acoustic Clarifiers. The 14 inch woofer is actually a full-range speaker, handling most of the audio load; yet it can easily shake the room with its thundering bass. The tweeters are there to provide added high frequency “presence.” Properly wired, they can do a good job at this. However, your 38-690 tweeters may not be properly wired. If you have to put your ear against one of the tweeters to hear it, then it suffers this factory wiring error.

Fortunately, this error is very easy to correct.

Partial schematic of the 38-690 output circuit. Click the image above to enlarge it.

The schematic diagram of the Model 38-690 shows that its two tweeters are wired in series, driven by the set’s driver tube using a special winding in the audio interstage transformer. (See the partial schematic above.)

Why aren’t the tweeters connected to the audio output circuit?

It is desirable to have less power going to the tweeters than to the woofer. Philco’s engineers had this idea correct in the design of the 38-690. The previous year’s 37-690 did have the tweeters driven from the audio output circuit; if you have ever heard a 37-690 in operation, you immediately notice how loud the tweeters are!

The original schematic shows that the two tweeters are wired out of phase. The closeup drawings below will help illustrate the point.

Original tweeter wiring in Model 38-690. The tweeters are connected out of phase.

Corrected tweeter wiring in Model 38-690, placing the tweeters in phase with each other.

See how the two white wires are connected together? This places the tweeters out of phase with each other. When speakers are out of phase, they cancel each other out, allowing very little sound to be actually heard.

This effect had been noticed before on Model 38-690 receivers, and many folks have wondered why the tweeters were so quiet when Philco went to the trouble to include them in their top-of-the-line 1938 receiver.

Examining the amplifier/power supply chassis, sure enough, the tweeters were wired exactly as the schematic shows – white wires connected together, both green ends connecting to the low-impedance secondary of the interstage transformer with a 1 uF electrolytic in series with one green lead. We shall learn more about that electrolytic later.

Changing the wiring as shown in the image above right resulted in improved performance – the tweeters made their presence known once this change was made.

Now about that electrolytic crossover…

The size of the electrolytic capacitor was increased to 2.2 uF, using a new 50 volt non-polarized unit. This first order filter reduced the crossover frequency to 9000 cycles, assuming a series impedance of 8 ohms as the two tweeters are connected in series. A 1 uF capacitor yields a crossover frequency of over 19000 cycles, near the upper limit of average human hearing and certainly above the design capability of the 38-690, as it contains a 10 kilocycle filter to reduce audio response above 10 kc!

A better crossover point would be around 4000 cycles, which would require a capacitor of 4.7 uF. Just for fun, I tried hooking the bottom white wire direct to the yellow-blue tracer lead of the interstage transformer, removing the ground and the crossover capacitor. This resulted in even more volume from the tweeters – not overpowering, but a great improvement in volume. However, as the capacitor blocks lower frequencies from the tweeters, the 2.2 uF unit was left in place with the ground per factory specs. However, a 4.7 or even 10 uF capacitor might be better instead. The circuit was left wired as in the “corrected” drawing above, however, with the tweeters wired green-white-green-white as shown, so that they would be in phase with each other.

For further reading on the subject of crossover capacitors for high-frequency speakers, read more at the Basic Car Audio Electronics site.