[ARC5] 6k8 - 12k8?

[Dennis Monticelli]

I agree that probability does favor the mislabeled theory.  But I would like
to point out that in the classic double-folded filament wire configuration
of an indirectly heated cathode there are two sharp folds at the top and one
fold at the bottom (you can easily sketch it for yourself).  At the crease
of the folds on top there is an increased liklihood of coating failure on
both creases over time.  Should the two top creases touch within the
sleeve the result is EXACTLY half the filament resistance.  Your example of
directly heated cathodes, while correct, does not apply to the indirectly
heated 12K8.

Dennis AE6C

On Sat, Apr 16, 2011 at 11:50 AM, Kenneth G. Gordon <
kgordon2006 at frontier.com> wrote:

> On 16 Apr 2011 at 10:22, J. Forster wrote:
>
> > That makes a whole lot more sense. I think tubes were always tested
> > before shipment and a 6 V tube would not survive 12 V for very long at
> > all.
>
> Although I agree that it DOES make sense, it is very surprising just how
> long
> a tube will operate at double the "normal" filament voltage without burning
> out.
>
> Thoriated-tungsten and tungsten filamented tubes are a very clear and
> historically important example:
>
> To reactivate thoriated-tungsten filamented transmitting tubes, one applies
> 2.5 times the normal filament voltage for a period ranging upwards of
> several
> minutes, ONE minute being the norm, then "cooks" it at 1.5 times that
> voltage for up to an hour.
>
> I have done this, repeatedly, to 304TLs (12.5 volts on a 5 volt filament),
> and
> 572Bs. Although it is really impressive how bright the filament is, I have
> never lost a single tube doing this.
>
> Besides which, in Les' case, the subject tube actually had only somewhat
> less than 5 volts on it, and never, so far as we know, a full 12.6 VAC
> applied.
>
> Also, we DO know of mislabeled tubes. In fact, it is not at all uncommon.
>
> Again, it is a fact that RCA recommended applying increased filament
> voltage to tubes of the size and type as the 12K8 to fix certain problems
> with
> such tubes. I have the document here somewhere, and it is on the web.
>
> And, the fact that when 6.3 VAC was applied to the subject 12K8, the
> current
> drawn was exactly 300 mA is interesting. How likely is it that an internal
> short
> would short out EXACTLY 1/2 the filament? I think it unlikely.
>
> Lastly, isn't a 12.6 VAC filament made out of smaller-diameter wire than a
> 6.3 volt filament, since it has to carry 1/2 the current? If that is the
> case, then
> such a filament could not carry 300 mA for long without burning out, or at
> least being seriously degraded.
>
> I conclude from these facts that the tube was mislabeled.
>
> I rest my case.
>
> Ken W7EKB
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