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

Mon Jun 23 21:22:28 EDT 2025

There was a lot of equipment from the late 1940s and through the 50s 
that cooked itself to death. Miniature and other compact tubes allowed 
more to be packed into a smaller space, all while producing the same 
heat as equal predecessors. As for plain R-390s, seeing they have space 
below them and the bottom cover removed will help immensely. In the Air 
Force in Southeast Asia, we rigged easily removable fans to blow into 
the bottoms of R-390s. Reliability was greatly improved.

   B. Gentry, KA2IVY

On 6/23/25 8:20 PM, Jim Whartenby via Milsurplus wrote:
> I agree with you Ray, heat is the enemy of reliability.  Too much 
> attention is paid to high line and B+ voltages but high temperature is 
> ignored. If the tube envelope is kept under 200C then a tube's life 
> will be greatly extended.  If the R-390/URR power supply and audio 
> units were in a separate box from the rest of the radio, there may not 
> have ever been a cost reduction program.
>
> There is a chicken and egg question about the receivers that you have 
> mentioned.  I believe that the early beginnings of the R-390 program 
> has had much more influence then we think.  The 51J has it's beginning 
> before the end of WW2.  The earliest advertisement, that I know of, is 
> in the 1946 ARRL Handbook on page 51 which was printed in late 1945.  
> Somewhere there is a copy of the Signal Corps correspondence with 
> Collins Radio during WW2 that would shed more light on this subject.
> Regards,
> Jim
> Logic: Method used to arrive at the wrong conclusion, with confidence. 
> Murphy
>
>
> On Monday, June 23, 2025 at 01:38:55 PM CDT, Ray Fantini 
> <rafantini at salisbury.edu> wrote:
>
>
> 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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