[Milsurplus] R-390A L-601 swing choke question

[Ray Fantini]

Great that you are looking into real numbers and real world comparisons. I have maybe sold or repaired a flock of  R-390A receivers over the years and only worked on two R-390 non A versions. Don’t know where it would show up but cannot discount the role of heat and definitely think the R-390 produces more heat then the R-390A because of the B+ regulation. Imagine it’s a nice technical desire to have the regulator but look at the R-388, 51J or later designs like the 51S and all the Ham 75A and S family and none of them have regulated power supplies beyond the 150 volt bus for the PTO/BFO
Heat and its effects on component life is a big factor for the sets. I have worked on a couple R-390A sets where almost all the tubes were far below the minimum emission so going to assume that many of these receivers were installed in a rack, turned on and stayed in operation years because beyond that have no idea how you can wear tubes out beyond lot and lots of use?
Pure speculation but with receivers like the R-388 and 51J out there somehow always thought the R-390 program was more like a cost plus production contract for Collins to produce the best radio receiver? Maybe in the fifties Collins wanted to produce the top of the line box and drive Hallicrafters and Hammarlund out of the market? Will say Collins produce one hell of a box that until modern receivers with VHF up conversion, SSB  and synthesizers knocked the R-390 A from the top of the hill.

Ray F/KA3EKH


From: milsurplus-bounces at mailman.qth.net <milsurplus-bounces at mailman.qth.net> On Behalf Of Jim Whartenby via Milsurplus
Sent: Monday, June 23, 2025 1:14 PM
To: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] R-390A L-601 swing choke question


Ray
Thanks for the reply.
I read the final report on the R-390/URR program.  Collins mentioned designing the power supply to support a current load of some 120 mA but as the design was fleshed out, the current demand went to 200 mA.  My thought was that this would be a worst case current with all power hungry functions selected and at a 125 volt AC line voltage.  I was just curious to see if anyone had measured the typical B+ current at the mean line voltage at the time of the design, 115 VAC.

I have always wondered about the performance of the original design so I have collected a few carcasses so that I can cobble one or two working examples.  Of course most of the carcasses are missing the same modules and those that are there are defective.

In the great scheme of things, spec is spec.  I think that some of the complexity with respect to the regulated power supply and ballast tube were Signal Corps requirements.  It would be interesting to get a copy of the original RFQ to see what the SC had in mind.  In the cost reduction program, Collins was able to successfully argue that a regulated power supply was not needed.  Not so with the ballast current regulator.  Using the 12 volt versions of the oscillator tubes will eliminate the ballast and free up a tube socket for a possible product detector mod.

I am sure that the R-390/URR was not the only radio that was over specified.  <grin>
Jim

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