>
>Hi Bob,
>
>Harmful advice was presented in the Hint&Kinks section of July
>2001 QST.
>
>One should NEVER *under any circumstance* reduce the filament
>voltage of a indirectly heated tube below the tube or equipment
>manufacturer's minimum recommended operating voltage.
/\ "NEVER"? For its UHF-rated planar triodes, Eimac states that as
operating frequency increases, back-bombardment heating of the oxide
cathode increases - so heater V should be reduced below the nominal range
if necessary. Also, indirectly-heated cathodes do not have filaments -
they have heaters.
>Any reduction below that point in a metal-oxide cathode tube can
>"poison" the cathode and permanently damage the tube.
/\ While it is true that thoriated-tungsten Filaments can be poisoned by
low V operation, this condition reverses after normal filament V is
applied, the only type of poisioning I have read about in Eimac info.
about oxide catode tubes is gold poisioning. /see Eimac Letter on my
Web site.
>
>Secondly, MOX cathode tubes can require inrush limiting, and are
>sometimes as much or more susceptible to damage than directly
>heated tubes.
/\ please name an Eimac oxide-cathode tube that they say is susceptable
to inrush-I damage.
> ...
>Some indirectly heated tubes with larger cathodes, like the
>3CPX5000 and its little brother, the 8877, have a tendency to
>maintain high starting current for a very long time until the heater
>temperature equalizes along it's length.
/\ In filament-type/direct heated cathode tubes, cathode temperature is
c. 1820º-kelvin. At this temperature, the filament R is
>
>One of the best guarantees of proper inrush performance is to NOT
>use "overkill" filament transformers, chokes, and wiring. Use the
>minimum size components necessary, and you will have built in
>filament inrush protection. Many amplifiers, the SB series
>Heathkits, the Ameritron series with separate transformers, and so
>on have absolutely no problem with inrush despite not having a
>filament step-start. Transformer and component resistances limit
>inrush current without external circuits. It is true that amplifiers
>using filament windings on large HV transformers are begging for
>problems unless a step-start is added.
>
>Finally, reducing voltage until CW power drops, and then bringing
>voltage up until full power is just restored is no guarantee that IMD
>performance will be within spec. Normal peak emission is several
>times the average emission current, and so the tube must be
>*comfortably* above the point where full peak power is reached on
>the worse case band. Once again, indirectly heated cathodes
>should NEVER be reduced in voltage below manufacturer's
>minimum recommended voltage.
>
>It is unwise to randomly modify amplifiers based on folklore and
>popular opinion unless we thoroughly understand what we are
>really doing. Reducing filament voltage is mostly a spin-off of
>commercial applications where NON-linear class C PA's with
>overkill tubes (like 4CX5000A's in FM transmitters operating at 20%
>of normal rated power) that simply don't need the full emission
>capabilities of the tube. Reducing voltage can be harmful to tube
>life, as well as the quality of our emissions in linear modes.
>
>In amateur service, very few (if any) failures are due to voltage
>correctable emission life of tubes unless the filament or heater is
>operated ABOVE or BELOW rated voltage.
>
>
>73, Tom W8JI
>W8JI@contesting.com
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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