[ARC5] 6k8 - 12k8?

[Kenneth G. Gordon]

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