[ARC5] [Milsurplus] Cold Filament Inrush Current

[DSP3]

Well, for what its worth.... a few weeks ago, now, I had two unrelated 
failures, both receiving tube filament failures.  In both cases, the 
receivers, an ARC-5 and and ARC-2, had the fils in a single 12SK7 open.  
The voltage applied was 27.5v with a fair amount of current headroom 
available on one receiver, the other was right at the design limit.  
Maybe these bullet-proof tubes are not so bullet proof after some 70 
years!!!  I fell off the turnip wagon in 1959, so have seen a lot, 
since.  Fil failures have been few and far between.

Jeep - K3HVG


On 3/15/2018 5:18 PM, Jack Antonio wrote:
> On 3/15/2018 4:28 PM, Michael Clarson wrote:
>> Wasn't the original post about command set rcvrs? These were used in
>
> I pulled a couple of 12SK7s off the shelf and measured them.
>
> Setting the supply at 12.6, the first tube initially drew .4A and
> took roughly 5 seconds to come down to .15A.
>
> The second tube hit .5A, at which point my little
> lab bench supply hits current limit, and came
> down to .15A in 10 seconds.
>
> I tried a 6SK7, but it severely went into current
> limit at .5A, so would need a beefier supply to
> check that. It came down to .3A in
> about 10 seconds. Probably would have come down
> faster had the supply not gone into current limit.
>
> I have been poking around BC-348s lately, and the
> current bench project involves in de-ham-modding
> a BC-348-N.
>
> One question that has come up, is that the designers
> of the -348 deemed it necessary to put in dropping
> resistors to bring the nominal 28 down to the
> specified 25.2V the 4 series 6.3V tubes require.
>
> Our beloved command sets, and many other receivers
> of the era do not bother with that, and simply
> accept the variations in the source voltage that
> come with a generator/battery system.
>
> I think the answer has been long lost to
> history, but it would be interesting to know
> why the designers of the -348 thought it was
> necessary. It would have provided some protection
> for inrush current, but don't know if that is
> the reason or not.
>
> Jack Antonio
> WA7DIA
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180315/7a7bada7/attachment-0001.html>