[AMRadio] request for info on using the 6C21

[Donald Chester]

>Was it you, Don, who also said (at one time) that running too little of 
>voltage
>on filaments can have the same undesireable effect on tube life?
>

That's right.  The manufacturers say the filaments need to be within 5% of 
the nominal value.  I think the story of the 6C21 is that they willingly 
re-spec'ed a 450T for increased peak emission at the expense of life.  I 
would run the tube with a variac on the filament transformer, crank it up 
class-C plate modulated AM, and see what the postitive peaks do at 8.2 volts 
and at 7.5.  If  the emission holds up at 7.5, I'd leave it there.  The 
thing that ruins a tube is to starve the filament emission capability by 
using too low a voltage.  If the plate current, or peak output just begins 
to drop off due to reduced filament voltage, the voltage is low enough to 
shorten tube life.  You can try reducing voltage just to that point, then 
increase it just enough to restore full peak output, and that would be the 
minimum filament voltage it would be safe to run the tube at.

Don K4KYV

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