[ARC5] [Milsurplus] Cold Filament Inrush Current

[Bob kb8tq]

Hi

There are a lot of things that kill tubes. Filaments opening up are only one of many
things. I’ve seen *way* more tubes die of “other causes” over the years than of open
filaments. 

I’m not suggesting that fiddling filament supplies is a bad idea. The only point is that 
you are attacking a small percentage of the “typical” tube failures. There is still good 
reason to be concerned about the supply of replacements, no matter what you do 
with the filament supplies. 

Bob

> On Mar 16, 2018, at 10:42 AM, Bill Cromwell <wrcromwell at gmail.com> wrote:
> 
> Hi,
> 
> I haven't read the last message in the thread (yet) but I want to mention my selection of filament transformer for the National SW-3 that lives here. There are three tubes that each draw just under two Amps (not mA) at 2.5 volts. I had my choice of a 6 amp or a 10 amp transformer for not much difference in price. I often go for overkill but with the cold inrush in mind I bought the six amp model. There is no filament "bloom" during a cold start. That approach *DOES* work.
> 
> With some of my other gear I am now considering a (DC) solid state current limiter (not a foldback) that will see a rise in voltage as the heaters warm up but NOT a current surge. A foldback circuit shuts down the power source until the load is *removed*. So I will use the constant current circuits instead.
> 
> They aren't making new tubes by the millions any more. Let's take care of the ones we have.
> 
> 73,
> 
> Bill  KU8H
> 
> On 03/15/2018 11:14 PM, w8au at sssnet.com wrote:
>> At 03:22 PM 3/15/2018, Jeep K3HVG 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...
>> 
>> I guess BuShips took the FIL xfmr sag into consideration for the big 861
>> amp in the TBK/TBM series since the secondary was rated the same as the
>> tube...11 volts at 10A.
>> 
>> When helping out at USS Massachusetts in the past I noticed that not all
>> their stock of those big bottles had good filaments and wondered if the
>> shock
>> of a cold start contributed to shortening their life.  Then I realized
>> that the start button energized the Motor/Generator which had it's own
>> time-constant
>> in reaching full RPM and full voltage.  Those guys at BuShips were
>> pretty sharp in their specs for the Westinghouse and RCA products back
>> in 1939.
>> 
>> However, with my RCA TBK-12 being fed with a HB AC supply I added a
>> Variac ahead of the FIL xfmr primary to ease the 861 to full brilliance.
>> ;-) Those
>> bottles are rarely available and not cheap.
>> 
>> Perry     w8au
>> 
>> 
>> 
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> 
> -- 
> bark less - wag more
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