Couple days ago, it was written about the 3CX3000A7:
> > The fil V should be below 7.5 V measured right at the
> >socket. Life of tube is dependent on fil V.
>
> // Excellent points, Jim. Few designers seem to realize that the
> emissive life of a thoriated-tungsten cathode/filament is cut in Half for
> each 3% increase in filament voltage above what is needed to achieve full
> PEP. Thus, the formula for emissive life vs fil. volts = (e1/e2)^23.4.
Per Henry, the Controller Unit filament voltage meter selection is
"calibrated" to actually "read" the voltage at the tube filament
pins.
My filament has been at about 7.6 v, above the mid-point of
7.5 +/- 5%, or 7.12 to 7.87 volts. This morning I operated,
(actually as a bit of a contortionist). Henry placed the filament
transformer quite buried within the PS assembly. However,
removing a side panel, laying on the floor with a flash light
and hot soldering iron at the tip of my nose, I was able to
move the primary tap to the end #6 tap. Result: filament
voltage is now 7.3 volts. So the emissive life, per the formula
ought to be a bunch longer! This action has lowered the filament
voltage 3.9% below the original 7.6v. I presume that even 7.12v
would be enough to reach full PEP, per the above.
Left the HV tap where it was, so when the SSB PS setting is
selected, the key-up plate voltage reads 5800v, and just under
4400v when lowering the voltage by the CW relay selected HV
xfmr secondary tap.
Thanks for alerting me to this tube life prolonging act.
73, Jim KH7M
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