[ARC5] [Milsurplus] Cold Filament Inrush Current

[Tim]

In a similar vein I once modified an old "right angle" military flashlight
with a key jack in series with the D Cells.  (the built in push button
"key" switch would kill your finger very quickly)

As a "flashing light" training device in the Navy.  Didn't work very well
as you could imagine.  So I put a resistor across the key for some
keep-alive current and the light rise-time then worked very well for morse
training.  Hooyah!

73, Tim
N6CC

On Thu, Mar 15, 2018 at 12:45 PM, Bobby Drummond <alphak4ja at gmail.com>
wrote:

> Here is my poor man's answer to inrush filament current potential problems
> (especially with series string filaments)
>
> I power the rig first through a 25 watt incandescent light bulb (bulb in
> series with the 120 volt power to the rig) for 30 seconds (long enough for
> the filaments to warm sufficiently at the lower voltage allowed through the
> series incandescent light bulb), disconnect and then quickly power the rig
> with full normal voltage.
>
> It's the cheapest way I know of the reduce the effects of initial filament
> inrush current on the rigs I use.
>
>
> 73 de AK4JA
>
>
> On Thu, Mar 15, 2018 at 3:33 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:
>
>> Yes, blooming filaments = bad!
>>
>> If you are using the gear pretty heavily and are concerned about the
>> on-off stress on filaments, there are several solutions. NTCs have already
>> been mentioned, and are probably the most expedient solution in most cases.
>>
>> Broadcast transmitters generally keep the filaments warm when not in use.
>> Since the lifetime of a filament is proportional to something craxy, like
>> the 12th (!) power of voltage, it doesn't take much voltage reduction to
>> prolong lifetime to whatever value you want. Then the shock of going full
>> out is moderated, taking care of that concern.
>>
>> "Instant-on" TVs simply inserted a single rectifier in series with the
>> filament string so that the tubes were kept warm even when the set was off.
>> The reduced filament temperature had negligible impact on tube lifetime.
>> Bridging across the rectifier turned the set on "right away" without much
>> thermal stress. This trick might be a viable option for some usage
>> scenarios.
>>
>> --
>> Prof. Thomas H. Lee
>> Allen Bldg., CIS-205
>> 420 Via Palou Mall
>> Stanford University
>> Stanford, CA 94305-4070http://www-smirc.stanford.edu650-725-3383 <(650)%20725-3383> (public fax; no confidential information, please)
>>
>> On 3/15/2018 12:22 PM, DSP3 wrote:
>>
>> One of the factors mentioned in the Eimac power tube book is to address
>> in-rush by ensuring that the filament transformer (especially in thoriated
>> tungsten types) is rated for the actual current of the tubes, and no more.
>> That way the lower, in-rush resistance it initially mitigated by sag in the
>> transformer.  The opposite scenario is that an overrated transformer will
>> sink whatever current necessary during the cold, heat-up period.  I have
>> seen tubes "bloom" when power is applied where the extra current is
>> available.  Not a good situation...
>>
>> Jeep - K3HVG
>>
>> On 3/15/2018 2:37 PM, Tom Lee wrote:
>>
>> Exactly. Resistance is a function of temperature, so tubes that operate
>> at white-hot incandescence (pure tungsten) will have a larger hot-to-cold
>> resistance ratio than oxide-cathode tubes. For the former, a 5:1 ratio is
>> not uncommon, where for the latter, something around 2:1 is more typical.
>>
>> --Tom
>>
>>
>>
>>
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>
>
>
> --
> 73 de AK4JA
>
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