[ARC5] [Milsurplus] Cold Filament Inrush Current

[Bill Cromwell]

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