Silver-Marshall (First Generation)

     Front view of a Silver-Marshall first-generation superheterodyne. Construction plans for a 7-tube Silver-Marshall superheterodyne using the Type 401 IF transformer block were published in the Fall, 1924, Citizens Radio Call Book. The Type 401 IF transformer block was tuned to 50 KC. My set uses a Silver-Marshall Type 401B IF transformer block (tuned to 60 KC), so it dates some time after the fall of 1924, perhaps to around January of 1925. The wiring of this set is identical to the schematic for the Fall 1924 article.

     Here are a few particulars of the set.

     1. It works on the standard 1920s superhet circuit.
     2. The front panel measures 24” long by 7” high by 5/32” thick.
     3. Silver-Marshall Type 401B IF transformer block and Silver-Marshall oscillator coupler.
     4. W. B. Sales Co. “Trans Atlantic” tuning capacitors with Apex dials.
     5. General Radio rheostat and potentiometer.
     6. Songbird audio transformers.
     7. Benjamin tube sockets.
     8. Seven tubes.

     Top chassis view. The original plans for this set were aimed towards a portable superheterodyne, and that’s probably why this set is relatively small. Five of the tube sockets are mounted right next to each other. There is enough room inside the cabinet to house the “C batteries, but the “A” and “B” batteries will have to be outside the cabinet.

     Close-up view of the Silver-Marshall Type 401B IF transformer unit. I knew there was a 50 KC transformer block, but I did not know there was a 60 KC transformer block until I obtained this set. Luckily all the transformer windings were good!

     Close-up view of the RF front end and the Silver-Marshall oscillator coupler. This set uses a standard 1920s superhet circuit with a grid leak first detector. A tapped loop is used in conjunction with a small trimmer capacitor to provide regeneration in the first detector.

     Close-up view of the IF amplifier.

     The second detector and audio amplifier. This set was in need of extensive restoration when I obtained it. It had been modified in the mid 1920s with a couple of choke coils and a 3-tube resistance coupled audio amplifier. The battery terminal strip had to be removed because there was not enough room with the resistance coupled amplifier. I returned the set to its original configuration. The Songbird transformers are not the original type used when the set was first manufactured.

     Rear chassis view.

     Schematic diagram of the first generation Silver-Marshall superheterodyne, as copied from the Fall, 1924, Citizens Radio Call Book. The different terminals of the Type 401 IF transformer block are identified on the schematic. My first generation set is wired identical to this schematic diagram. This diagram is also very close to my Silver-Marshall set using the aluminum square cased IF transformers.

     The table below records the pass band characteristics of the IF transformers. The first two transformers were measured with the secondary coils referenced to A-. The last transformer was measured with the secondary referenced to A+ and with the grid leak components in place.

     Silver-Marshall advertisement from the Fall, 1924, Citizens Radio Call Book.

Page added November 16, 2018
Updated February 10, 2019

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