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Design excellence: R-1051, the swan song of vacuum tube sets

Martin Renz Ernst Erb Vincent de Franco Martin Bösch Mark Hippenstiel Bernhard Nagel Dietmar Rudolph Otmar Jung Heribert Jung Eilert Menke 
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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » Design excellence: R-1051, the swan song of vacuum tube sets
Emilio Ciardiello
Emilio Ciardiello
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07.Dec.10 22:10

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The architecture defined by Collins in R-390 receivers and in their derivatives showed some limits, the most severe one derived from the use of independent oscillators, the PTO and the two separate crystal oscillators. Even if crystal sets were thoroughly selected and aged, each xtal was supplied with an initial deviation from its nominal value and, more, it was subjected to an unpredictable drift through the life of the receiver. Due to the above tolerances and drifts, operating procedures asked for the dial calibration against an internal reference after each band switching and frequent overall performance checks. The next step in top performance receivers was the introduction of the frequency synthesizer, locking all the beating oscillators to a single frequency reference. The early synthesized receivers in 1960 were the FRR-59 and the WRR-2, each using over than 60 tubes. But in the early ‘60s semiconductors were mature enough to replace vacuum tubes and the revolution went on rapidly.

The R-1051(x)/URR was the latest communication receiver still using vacuum tubes, even if just two tubes in the RF module. Designed by General Dynamics for US Navy, its production started in 1964 and continued with several variants and several suppliers well in the ‘90s. Probably it is still in service today. No other communication receiver can claim a nearly comparable service life. A simple survey of its extraordinary design well explains its incredible life span.

Known variants

  • R-1051/URR - 0.5 kc steps, metal chains, dual meter, discrete semiconductors + 2 tubes, "six-pack" frame, early deliveries in 1964 by General Dynamics at $25,250 each. Also license built in Italy by Elmer (Montedel).
  • R-1051A/URR - 0.5 kc steps, metal chains, dual meter, discrete semiconductors + 2 tubes, "six-pack" chassis frame. Elmer (Montedel) (?) only known manufacturer.
  • R-1051B/URR - 0.1 kc steps, metal chains,     dual meter, discrete transistors + 2 tubes, "six-pack" chassis frame. Manufactured by Bendix (Allied Signal)
  • R-1051C/URR - 0.1 kc steps, metal chains, dual meter, black face, discrete transistors + 2 tubes, "six-pack" frame. Probably from General Dynamics and Radionics.
  • R-1051D/URR - 0.1 kc steps, probably metal chains, single meter, SSI and MSI logic + discretes + 2 tubes, "six-pack" chassis. Low-cost version sold by General Dynamics at $16,830 apiece.
  • R-1051E/URR - 0.1 kc steps, plastic chains, single meter, SSI and MSI logic + discretes + 2 tubes, "six-pack" chassis frame. Made by Bendix (Allied Signal) and sold for $25,250 apiece.
  • R-1051F/URR - 0.1 kc steps, plastic chains, single meter, SSI and MSI logic + discretes + 2 tubes, card cage. Manufactured by Stewart-Warner and sold for $21,210 apiece.
  • R-1051G/URR - 0.1 kc steps, plastic chains, single meter, SSI and MSI logic + discretes + 2 tubes, card cage. Stewart-Warner.
  • R-1051H/URR - 0.1 kc steps, plastic chains, single meter, SSI and MSI logic + discretes + 2 tubes, card cage. Made by Stewart-Warner and sold for $50,490 apiece.

General description

R-1051(*)/URR is a digitally tuned receiver tuning from 2.0 to 30MHz and capable of receiving LSB, USB, ISB, AM or CW transmissions. Other types of transmission, FSK, MCW, compatible AM and FAX can be also received, provided the use of suitable ancillary equipment.

A triple conversion superheterodyne architecture is used. Tuning is digital with 5 frequency controls, megacycles and kilocycles. A further control selects the increments, 0.5 or 0.1KHz, depending upon the variants, and operates as a continuous vernier between calibrated steps. Accuracy in calibrated position is better than +/-0.05Hz at 5MHz; frequency stability is better than 0.01ppm (parts per million) per day over 0 to +50 degrees C. Sensitivity is better than 1 microvolt for 10dB signal to noise ratio in SSB, 2 microvolts in CW/FSK and 4 microvolts in AM mode.

IF frequencies: first 20 or 30MHz, depending upon the selected band; second IF 2.85MHz, third IF 500KHz. IF rejection better than –75dB, Image rejection better than –80dB.

Power requirements: 115VAC, 48 to 450Hz, single phase, 55W.

The design

Here is the simplified block diagram of the R-1051B/URR receiver, very similar to the Elmer R-1051A/URR shown in the pictures below:

Fig. 1 – Simplified block diagram.

In the early models, up to the ‘E’ suffix, six plug-in assemblies contain most of the circuits, with the exception of the power supply, the Mcs code generator and the antenna relay board. The chassis frame distributes the supply voltages and the signals to the six modules. The setting of Kcs knobs from the front panel is distributed through metal chains to the RF fine tuning turrets and to the proper synthesizer subassemblies.

Fig. 2 – The chassis complete with the plug-in assemblies. B bottom view showing the tuning chains.

Plug-in modules are:

  • Frequency Standard Assy, A2A5
  • RF Amplifier Assy, A2A4
  • Translator/Synthesizer Assy, A2A6
  • Receiver Mode Selector Assy, A2A1
  • IF – Audio Amplifier, 2 each, A2A2 and A2A3

Here are some internal views of the above modules.

Fig. 3 - The frequency standard unit operates from a stable 5 MHz crystal oscillator housed in a temperature controlled oven. The frequency reference can be switched to an external 5 MHz standard or even it can be compared with the external standard and the blinking of a built-in lamp indicates the zero beating adjustment. By multiplication and division of the reference signal, the module also generates signals at 500KHz, 1MHz and 10MHz, used in the receiver.

Fig. 4 - The RF assembly, the one that includes the two vacuum tubes, is very complex. A motorized rotary turret, driven according to the position of the two Mcs knobs through a code generator, selects one out the 28 tuning strips (4C), one for each MHz band. A multiple independent-plate rotating subassembly (4B) which contains the fine tuning capacitors is mounted inside the Mcs turret and driven by the 100 and 10 Kcs tuning knobs. Four tuned circuits, two of them in input, provide the required selectivity. The gain of both the vacuum tubes is controlled by the AGC circuit.

Fig. 5 - The translator/synthesizer assembly.

This unit includes six subassemblies on a common distribution subchassis, to generate the beating frequencies and the three mixers. A spectrum generator module feeds the three synthesizers related to Mcs, 1/10Kcs and 100Kcs selections. The conversion scheme and the related frequencies in the 28 bands are given in the following figure.


Fig. 6 – Conversion table.

PLL circuits are used through the synthesizer, even for the variable frequency oscillator that adds the fraction of Kcs in uncalibrated position. The sole exception is the 100KHz synthesizer, where the oscillator is locked on the reference signal by a very smart error canceling loop, involving two of the mixers one in additive and one in subtractive mode.


Fig. 7 - The mode selector. The mode selector, at the output of the third mixer, routes the IF signal to the appropriate filter(s), depending upon the selected mode, AM, LSB or USB. This unit also includes the BFO generator, variable from 496.5 to 503.5KHz, and a diode gate to feed the product detectors in the IF/audio assemblies with the required 500KHz carrier (B).

Fig. 8 – The IF/Audio amplifier module. Two identical modules are used, one for each channel in ISB mode. Each module contains a gain-controlled 500KHz IF amplifier, a product detector driven by the 500KHz gate of the mode selector module, an AM detector, a step AGC circuit and an audio amplifier.


Some notes

The R-1051 was designed to outperform any other receiver in use before. The operator had to select the right frequency and the wanted mode and the receiver performed the rest of the job, even selecting the proper bandwidth or the best AGC response, no uncertainty, no drift.

Unfortunately even this receiver has some drawbacks, other than the top price paid by US Navy and I believe by Italian Marina Militare. The service of plug-in modules, very simple if one has the replacement modules, can be very very difficult without the proper extenders. What is worst is that it is almost impossible to sweep even a limited band, since its digital tuning requires the setting of five or six knobs, some very heavy, for each new frequency.

Further experiences on this receiver are welcome.

This article was edited 06.May.12 11:50 by Emilio Ciardiello .