[Milsurplus] Fw: Who is GHS?

Thu Oct 4 00:20:04 EDT 2018

BruceFirst off, we have worked on the same UHF equipment.  So you were a tab bender too?
I can attest that the GRA-53 / TRC-68 got quite hot.  When I was in the PI, all we had was the -53 / -68 and a few ARC-27s.  Don't remember any particular installed unit being a lot of trouble.  Some were in air conditioned areas others were not.  Don't remember any catastrophic failures, other then one ARC-27.  PMIs found low power output so that was the usual bench problem.  A little touch up on the tabs was usually enough to bring it back to spec.
One GRC-32 (ARC-27 with big heavy 28 volt power supply) was run outside of it's case and it wasn't much trouble but I don't know how often it was used.  I believe it was a backup radio in the control tower at Mactan AB.  At that time I was a 3 level and was only interested in learning how to troubleshoot and repair so paid little attention to reliability.
Ken and I have traded a bunch of emails almost exactly two years ago about the SRR and FRR receivers; they should be in the archives if we didn't go off list.  I cannot verify Ken's assertion that the SRR-13 or FRR-23 ran hot.  Especially after changing the power transformer tap to the 400 cycle position which lowered the power supply output voltages to as low as they can go.  
The radio internal temperature never exceeded 107 degrees F. after six strait hours of running on the bench.  Radio still exceeded sensitivity spec so I wasn't bothered by any lack of performance.  B+ was about 110 VDC and tube filaments were at just over 6 vac, so all were in spec with 118 vac line from a ferroresonant transformer.  Had to use this transformer as the line filters tripped the GFCI.
In looking at the schematic for the SRR-11, I only see one or two pentodes like the 5902 in the audio module that have no screen grid resistor.  Checking the tube manual, the plate and screen are both run at a typical 110 VDC.
I had bought perhaps a dozen or more of these radios at auction, spent less then $100 for all.  Some were in really bad cosmetic shape. some had even been through a barn fire.  In going through the receivers that I did not scrap outright, the biggest problem I found was open transformers in the RF modules.  
I suspect that this was a common problem aboard ship with antennas so close together, lots of high power RF floating all around.  In any case, cobbling together enough RF transformers to make good modules was the main problem, not tubes.  All of the subminiature tubes in these radios are either heat sunk to the chassis or tube module or, as in the case of the ARC-21, have subminiature IERC tube shields.
The ARC-21 and SRR-13 are contemporary since both entered the inventory in the early to mid 1950s.  By 1955 or so, the tube problem had been addressed and tube failures were no longer the leading cause of equipment failure.  By the time the ARC-58 / KWT-6 appeared, things were running smoothly, tube wise.  
I can't put a figure on how many modules from SRR, FRR and KWT-6 radios I have reworked but I can say that the few subminiature tubes I did bother to replace did not solve the problem in the module.  The one exception was in a KWT-6 TGC module, that had an open tube filament.  
The biggest PITA module in the KWT-6 is the Side Step Oscillator and the problem is almost always the narrow band pass mechanical filter.  Have yet to see a failure in the RCA designed mechanical filter.  Different design and it didn't use foam rubber that turned to liquid some 50 years later.  This is a bigger problem in the R-390A, IIRC.Regards,Jim I wonder why people argue over the 10% of their differences and ignore the 90% they agree on?

      From: Bruce Gentry <ka2ivy at verizon.net>
 To: milsurplus at mailman.qth.net 
 Sent: Wednesday, October 3, 2018 7:36 PM
 Subject: Re: [Milsurplus] Fw: Who is GHS?

There were many pieces of gear from that era that ran blazing hot.  In 
the case of the SRR_____  series, omitting the screen resistors may have 
been for economy and/or "reliability", eliminating the screen resistors 
and bypass condensers left fewer parts to fail. Tubes were expected to 
wear  out, so replacing them when weak  was probably not counted in 
reliabily figures unless they failed catastropically.  ARC-27s, and 
GRA-53s/TRC-68s were two I knew well that cooked themselves to death, 
and a lot of the radio relay gear of the same era did too. Many of the 
sub miniature tubes were run as hard as 7 or 9 pin miniatures, without 
the large envelope and structure to dissipate the heat, and a lot more 
of them were crammed into a very small space. I also think many 
manufacturers knew transistors were on the way,  tried to miniaturize 
tubes so their gear would not look so obsolete,  and stave off the costs 
and effort to change to solid state as long as they could.  Does anyone 
know of any military rigs that used Nuvistors or Compactrons?  Those 
were other efforts to hold off the inevitable.

        Bruce Gentry, KA2IVY

On 10/3/18 2:49 PM, Kenneth G. Gordon wrote:
> On 3 Oct 2018 at 14:19, Jim Whartenby wrote:
>
>> As with the ARC-21, subminiature tube reliability was dismal.
>
>
> In that rig, many of the tubes are run very hot and this contributed to short life in that series
> of receivers.
>
> Yet if those tubes are used at or below their maximum ratings, they are very reliable.
>
>
>
> At first, we had a lot of failures, but after I modified them so that the screen voltages to the
> pentode/tetrodes was reduced to "somewhat below" the plate voltage (!), temperature
> dropped markedly, operation appeared normal, and the receivers then became much more
> reliable.
>
> As I remember it, the screens were essentially connected directly to the plate supply. I
> added suitably bypassed 51K resistors to the screen feed at all those tubes which required
> it and that made a big difference.
>
> Ken W7EKB
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