[ARC5] Filament Question

[Richard Knoppow]

     I wonder how the battery tubes are constructed so as not to have 
inrush current.  AFAIK, this is due to the temperature coefficient of 
resistance of tungsten. Nearly all tubes use tungsten filaments either 
directly or as the heaters of a separate cathode.  Tungsten has a 
significant positive TC, that is, the resistance is much lower when cold 
than when hot so there is a current surge when power is first applied.  
The current surge can cause problems due to the magnetic field it 
generates but I think the more common reason for failure of filaments 
during turn-on is just thermal cycling; the mechanical effects on the 
filament from being cycled from cold to hot and back. I am not sure 
limiting surge current would make much difference except in very high 
power tubes but controlled warm up and cooling of the filaments might 
make a lot of difference.
      Some receiver tubes are made for series-string operation and have 
controlled warm up time. This means the current draw is controlled so 
that a slow heating tube will not draw extra current while other tubes 
have already warmed up and are drawing less current.  Since the voltage 
across the filament is proportional to the current through it a tube in 
series with another that is drawing high current will be subjected to an 
excessive voltage, at least for a short time.

On 10/3/2015 9:20 AM, Kenneth G. Gordon wrote:
> On 3 Oct 2015 at 14:53, Leslie Smith wrote:
>
>>    Hello Wayne,
>>    Lacking any specific knowledge about your question, my guess would be
>>    a definite "yes".
> I disagree for reasons I have previously stated.
>
>>    I guess that thermal cycling would reduce the life of the filaments
>>    and therefore the tubes.
> I reply: that would depend on the tube in question. In this case, a "blanket"
> statement does no apply.
>
>>    I suspect (from your question) you think the same as I do.
>>
>>    In an editorial in "Ham Radio", the editor Jim Fisk, wrote about a
>>    surplus receiver he had operated from the mid-40s.
>>    I may be wrong in detail here, but as I recall his editorial, his set
>>    was wired so the filaments were always on.
>>    He switched the B+ line when the set wasn't in use.  He believed (as I
>>    recall) that the thermal cycling of metal to glass (in the tube base)
>>    was the main source of failure of tubes.  He pointed to the 30 year
>>    life of his set, with the filaments constantly on, as evidence of his
>>    hypothesis.
> And I operated an RAL-7 receiver for over 12 years, only turning it off once
> to test the tubes. Finding nothing wrong with them, I turned it back on and
> left it on until I was able to aquire a more modern receiver...but in this case,
> the tubes, 6D6s and 41s, were 6.3 VAC filamented tubes which draw
> significant current when compared with battery tubes.
>
> Wayne is talking about battery tubes with 1.5 V and 3.0 V filaments. The
> tubes are DESIGNED for "instant-on", are designed to "ramp up" internally,
> and IMHO, leaving them on would result in REDUCED life.
>
>>    I don't know if he's right, or not, but his theory seems to have
>>    reason on-side.
> Well, yes, but not if he is dealing with battery tubes...which, btw, are, to me,
> a favorite category of tube.
>
> Ken W7EKB
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL