[ARC5] 6k8 - 12k8?

[Dennis Monticelli]

I have another theory.  That may be a correctly built 12K8 with half the
filament shorted upon itself.  Your typical filament is folded (often more
than once) before it is inserted into the cathode sleeve.  When the
insulating oxide coating on the filament breaks down or flakes off, it
sometimes makes contact with the sleeve (heater to cathode leakage) but it
can also make contact with its folded self.

Dennis AE6C

On Fri, Apr 15, 2011 at 1:39 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

> Dear list,
> I have a valve (or "toob") marked 12k8.  Manufacturer: KEN-RAD,  The
> cold filament resistance is around 3.9 to 4 ohms.  This is out of the
> set, unconnected.  Is this a 6k8 that is mis-labelled?  My experience to
> date has been that the cold resistance of any of the VT13x "toobs" is
> around 12 ohms.
>
> I'm uncertain about the source of this valve.  It came from a box with
> valves, IF transformers, chassis screws etc that was in another box
> containing a "scrap" BC-453 I am in the process of rescuing from a
> morbid fate.  In other words I don't know where it came from.
>
> I found it in the following manner.  Last night I was at the stage of
> re-fitting the valves to the "scrap" chassis; I inserted a pair of
> valves, turned the filament supply "on" and measured the voltage at the
> junction of the two filaments.  In this case, that turned out to be
> about 20 volts.  Hmmmm.  (Thinks ... this is unusual, Mr. Ohm is doing
> strange things ....)  Do various things - make various checks.  Mutter
> about socket resistance and promise to never wash another chassis in
> Dot's Bath Cleaner.  Got another 12k8 (this one was made by RCA) and
> measured the cold filament resistance.  Around 12 ohms.  Put this 12K8
> (by RCA) into the chassis, and measured the voltage at the center point
> of the two filaments.  Near enough to 12 volt.  (Thinks:  Dot's bath
> cleaner may not be so bad after all.)
>
> I imagine this valve somehow got from the 6K8 batch into the 12K8 batch
> before the labelling machine at KEN-RAD.  Any-one seen this before?  Or
> maybe it's just a broken filament in a 12K8?
>
> 73 greetings from Australia
>
> Les
>
> --
>  Leslie Smith
>  vk2bcu at operamail.com
>  (end)
>
> --
> http://www.fastmail.fm - Send your email first class
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