[ARC5] 6k8 - 12k8?

[J. Forster]

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.

-John

=================


> 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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