The SRR-13A uses subminiature tubes and a few solid state diodes. The diodes were not part of the original design, but were included in receivers above SN 1431. A late modification changed the power supply rectifiers to solid state diodes in all SRR-series receivers.
Each receiver has a antenna and power inputs, a 200 KHz IF output, plus line and headphone audio outputs, on the rear panel. The SRR-12 and 13/13A also have a 1600 KHz panadapter output and detector AGC input/output connectors. All receivers have 2 local headphone jacks on the front panel. The audio output is relatively anemic -- only 200 millwatts into a 600 ohm line -- but the receiver was designed to be used with military audio distribution amplifiers. The antenna input can be configured for either low (70 ohm) or high impedance via movable links. AGC can be similarly configured on the SRR-12 and 13 for dual-diversity receiving. Separate metering is provided for RF carrier and audio output levels.
IF selectivity is provided by LC (8 KHz) and mechanical (3.2 KHz) filters. The SRR-13 has an even narrower 1 KHz filter for CW use. An adjustable squelch (labelled silencer on the panel) is available on the SRR-12 and 13. The frequency of operation is projected onto a window above the tuning dial. Frequency calibration is provided by tuning in the built-in calibrator or a standards station and then twiddling the projections controls to make the dial read that frequency (This is the opposite of the R-392, where the frequency must be set and locked, and then the receiver tuned blind to find the cal signal). A 0-1000 dual rate logging scale is also provided; the outer scale has markings every 100 units and revolves slowly, the inner scale is marked 0-100 in increments of 1 and revolves quickly.
The receivers have modular construction -- the complete receiver chassis slides out of the cabinet and can be tilted and locked at different angles for servicing. Each of the major functional units is a removable module. The subminiature tubes are soldered (with leads to the pins) rather than plugged into sockets. You can't easily substitute tubes to troubleshoot this receiver! Complete module replacement and stage gain charts are used to diagnose possible tube problems. The mil-spec construction method is impressive (and heavy!). The manual, which is about 4 inches thick, is comprehensive. It contains complete theory of operation and circuit descriptions plus the physical layout (several views) and schematics of each module as well as the overall receiver. There is even a table which describes each inductor used in the receiver, its wire size, number of turns, dc resistance, inductance value and method of winding!