[ARC5] [Milsurplus] Cold Filament Inrush Current

[Bill Cromwell]

Hi Bob,

Melting the screen grid or the plate is not the least bit beneficial to 
our tubes <grin>. I run mine conservatively. Short of melting internal 
structures they can be warped by overheating and rendered useless. 
Bouncing them on concrete is right there, too.

73,

Bill  KU8H

On 03/16/2018 11:07 AM, Bob kb8tq wrote:
> 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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>

-- 
bark less - wag more