[ARC5] 6k8 - 12k8?

[Kenneth G. Gordon]

On 15 Apr 2011 at 13:39, Leslie Smith wrote:

> Dear list,
> I have a valve (or "toob")

Isn't that properly pronounced "TSCH-OOB" ? :-)

> 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?

Sounds like it is to me. I have seen this before. Actually, from what I have 
read, it was fairly common at various times.

> 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 ....)

According to my calculations, if a 12K8 (12.6 VAC @ 150 mA) were in series 
with a 6K8 (6.3 VAC @ 300 mA) across 24 VAC, the 12K8 would have 19.2 
volts on the filament while the 6K8 would have 4.8 volts on its filament. If you 
measured the midpoint voltage to ground, it would depend on which tube 
was in which socket as to whether you would see around 20 volts or around 
5 volts at that point.

> 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?

I have definitely seen mislabeling before. For instance, Heathkit had a 
problem with the tubes it shipped with the HW-101 kit for a while, in that 
some tubes labeled 6EA8 were actually 4EA8s for TV service.

I can certainly see how in the "heat of battle" so to speak, such things would 
happen, and would never be discovered.

Ken W7EKB