Tenzor

Continuing a vaguely defined theme of obscure and peculiarly clumsy secret equipment.

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Soviet spies were working in Oklahoma and Kansas in the ’50s, as evident in the correct local pronunciations on their maps. Every city and province has its secret shibboleths, its own tests for You’re Not From Here. A spy who wants to survive needs to be From Here immediately.

Let’s imagine Резидентка Полистра ensconced in an unspecified Okla location:

She’s loyally receiving and sending data to the nearest Center with her Тензор spy rig. (The nearest Center is remarkably near!)

Тензор was designed and built by the Russian division of Sylvania. [Sources disagree on this.] Russia, of course, continued making radios and using tubes long after we stopped using tubes and stopped making anything at all.

Interior view:

The excellent writeup at CryptoMuseum mentions the unexpected English labels. They cite two reasons: Russian labels would give away the game immediately, and spies were usually locals who knew English as first or second language.

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Two aspects of this setup are unusually primitive for the ’50s, and seem wrong for a spy rig. But I can’t possibly outthink KGB, so I know they had excellent reasons for each choice. (It’s possible that the seeming wrong itself was part of their brilliance.)

(1) Big metal octal tubes were outdated in portable equipment. Transistors weren’t ready for serious use yet, but Russia made lots of mini and submini tubes. (I own some). Subminis were also available in most target countries. I’m guessing that big COMMON tubes avoided conspicuous consumption. 6J7 and 6L6 were ‘default tubes’, found in most house and car radios. You could buy them in a hardware store or drug store in every town. Subminis were only available through electronics stores in big cities, or through mail-order houses like Allied, which would leave a paper trail.

Good advice for dissidents of all sorts: Try to be a small fish in a big pond. Avoid specialized stores and specialized channels. Even if they’re not provided for you as false flags, small groups are easier to watch than big ‘default’ groups.

(2) Modularity. Radio developed like cars and houses and business machines. All started as a collection of separable modules, then merged inside a streamlined envelope in the ’30s. Exposed tubes were the norm before streamlining, and still a ‘classic symbol’ for tube-loving audiophiles just as free-standing headlights are a ‘classic symbol’ for cars.

A single self-contained transceiver would seem obligatory for a spy who needs to pack up his equipment quickly. These modules fitted into a custom-made travel pack with pouches for each device and antenna and tool, but removing the tubes and taking down the antenna and fitting the modules into the pouches would take a long time.

One possibility: An operating schedule linked at CryptoMuseum instructed the spy to operate in ‘duplex’, transmitting and receiving on separate frequencies. Most transceivers are designed to transmit and receive on the same frequency, with one dial controlling both. Perhaps the separate modules and separate antennas made duplex easier to manage?

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Now let’s look at the modules, assisted by HappyRedStar.

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The filter box is just a box, no knobs or lights or moving parts. It goes between the power plug and the rig’s own power supply. No fun for me to model and animate, but it shows plenty of design savvy. It contains a series of hefty RC filters to block transients in both directions.

Line filters are normal on ham equipment to spare your hearing from the buzz of the neighbor’s washing machine and to spare the neighbor’s TV from your keying.

In this case there’s a more important purpose for the backwash filter. Mains power systems are communication systems. Smart meters are NOT NEW. Power companies have been modulating the AC with their own Morse and data signals for 100 years. If they heard Morse in a neighborhood with no known and licensed hams, they might start investigating.

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The power supply is uniquely flexible. It contains a multi-tap transformer. A rotary switch on the front selects taps for a wide range of input voltages. Presumably the blank tap is the OFF choice, though not labeled. The output provides filament and plate voltages for both receiver and transmitter. The transmit/receive switch, a standard and necessary part of every station, is on the power supply. It trades only the plate voltages, leaving the filaments on all the time. The power supply also has a high/low voltage switch for the transmitter, so you can send at high or low power when needed. The available schematic is for an earlier version with only two tubes. Each is connected as a halfwave, and the second looks like a voltage doubler for the transmitter.

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The receiver has three identical 6J7 pentodes, inside the case unlike the other modules. The first is an RF amp with untuned input. The second is a regenerative detector, and the third is the audio amp. Regen seems an odd choice for a spy receiver, since a regen transmits on its tuned frequency. Regens were famous for interfering with neighbors. You can turn down the regen control so it’s effectively a TRF, but then you would lose sensitivity and lose the BFO. There’s no separate volume control on the audio stage.

The tuning dial is unique. It moves through a full circle and has two separate bands, which are automatically switched when the dial passes the halfway point. (The full circle might imply a butterfly capacitor? Not visible in the pictures.) A fine-tuning knob is on the left, but it’s not a separate capacitor, just a reduction gear for the main dial. Earphones are connected through separate pins for each wire, another ancient feature. Tip-ring phone plugs were standard by 1930.

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The transmitter is straightforward and simple, with one odd exception. A crystal oscillator drives a single stage of amplification, with LC tuning on both stages. The key interrupts the cathode of the amplifier stage. Instead of meters, each stage has a neon lamp to tune for maximum brightness. The oddity is a switch on the front that puts the second lamp in the primary or secondary of the plate tank. In the secondary the lamp would be viewing antenna resonance. Why the switch? Three lamps would be cheaper and less scratchy than two lamps and a switch.

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The antenna matcher / reel is another ancient leftover. It’s a wooden frame with a slide inductor inside. Slide inductors were typical of early crystal radios. The spy would wrap the transmitting antenna around the wooden frame for storage or travel, and would use the slide inductor to tune when the lamp switch was set to Antenna.

The rig covers the usual SW ham and broadcast range, 80 to 20 meters. The linked operating schedule used all the bands at appropriate times and seasons. Max power was about 25 watts, which could span the world on the right band with the right skip conditions. More reliably, 25 watts would travel about 100 miles. So the Centers would need to be fairly dense. If Резидентка Полистра was in Enid, her Center might have been in OKC or Wichita. This dense distribution strikes me as implausible. I’m inclined to think that the rig was meant for more urban areas like Europe or NYC… but again I shouldn’t underestimate KGB.

The receiving antenna came on another wooden frame without an inductor.

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The key is the only ordinary part of the rig. It’s a common Soviet key, and thoroughly familiar to me. I own one and use it daily for a few minutes of code practice along with my kit-built regen receiver. Because it’s instantly recognizable as a Soviet model, it would spoil the ‘native effect’ of the English labels.

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** Footnote: The available power supply schematic is for the Mark I version with only two tubes. This version would handle AC but not DC input. Many parts of the world, including most farms and even one section of NYC, were still on DC power in the 1950s. Many home radios were built to handle both AC and DC. Possibly the third tube in the Mark II version served as a vibrator or inverter for DC locations? I can’t find any better schematics or descriptions, so this is an open question.