The 3CX3000A7 has a thoriated tungsten filament?
"Each success only buys an admission ticket to a more difficult problem." --
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> -----Original Message-----
> From: Jim Reid [SMTP:jimr.reid@verizon.net]
> Sent: Friday, October 19, 2001 4:20 PM
> To: 2; Jim Thomson; w8ji@akorn.net; AMPS; Chuck Counselman
> Subject: [AMPS] 8k ultra Fil V.
>
>
>
> 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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