The R-391 is the autotune version of the famous R-390. A separate power 28 VDC power supply is required for autotune operation. Much rarer than the R-390 and R-390A, with only 1,440 made.
Almost everyone has the provisional Collins manual rather than the official TM11-863. Dean Davidson managed to find the official version in the library of the Australian Army Signals Museum in Melbourne, and to obtain a copy of the real thing. The manual, that is, not the radio. Dean reports that the R-391 and accessories are also on display.
The electronic aspects of the R-391 are very similar to the R-390 (non-A). Dual 6082 tubes are used as active voltage regulators for all receiver B+ voltage (the R390A uses a split unregulated/regulated B+ scheme). There has been some recent traffic on the mail lists about a solid-state replacement that would eliminate at least the 30 watts of filament power the 6082s dump into the lower receiver chassis.
Some parts are also the same as the R-390A. The Carrier Meter adjust pot, power supply rectifiers and ballast tubes are the same, and can be repaired/replaced in the same fashion as one would a R390A.
Army TM 11-863, Radio Receiver R-391/URR, 21 March 1955.
"Communications Receivers, The Vacuum Tube Era 1932-1981", 3rd Edition, by Raymond S. Moore
"Shortwave Receivers Past & Present", 2nd Edition, by Fred Osterman
R-391 photo from www.dxing.com
R-391 Front Panel Back Panel R-391 Contracts
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N1TI YouTube audio/video demo of the related R-390A receiver
Ray, W2EC, has some great R-391 and other BA photos on his web site.
Additional R-391 photos from Duane, KE7BT - here, here, and here.
The RadioMuseum.org R-391 page.
More R389/390/390A information, including the R-390/391 Cost Reduction Report that led to the R-390A.
More R-391 images from Google Image Search