I was able to increase the final tube life in a UHF television transmitter
(channel 63, 717.0 MHz 1000 watts peak output) running the filament at the
lowest setting....Just where the power dropped off then increase a
little....4+ years of continuos transmit with the same tube...I cannot
remember the tube number. It was a tetrode running 3800 volts on the plate
and about 800Ma of plate voltage and 375 volts of screen voltage....
I have spoken with many broadcast engineers and that is how they do it
too.....In low band TV (channel 2 and 4) and also in 100 MHz fm transmitters
25-35 kW range.
Good luck..
73
Jim W7RY
----- Original Message -----
From: "2" <2@vc.net>
To: "AMPS" <amps@contesting.com>; "Tom Rauch" <w8ji@contesting.com>
Sent: Wednesday, June 20, 2001 12:41 PM
Subject: Re: [AMPS] Filament voltage/bad advice
>
> >
> >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.
>
> /\ With thoriated-tungsten type tubes, the filament can be poisoned by
> low V operation. However, this condition reverses after normal filament
> V is applied. The only type of cathode poisioning I have read about in
> Eimac's info. about oxide cathode 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.
>
> /\ Mr Rauch - 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. (1573ºC) At this temperature, the filament's R is c.
> 8.3x higher than when cold. Oxide-cathode/heater-type tubes operate at a
> lower temperature, so the hot/cold R ratio is less.
> - I have seen no such precaution from Eimac about its heater-type tubes.
>
> >
> >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 Heath SB-220 uses a current-limiting type core filament
> transformer.
>
> >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.
>
> /\ ... and what is the measured filament inrush I on the Ameritron
> AL-82?
>
> > It is true that amplifiers
> >using filament windings on large HV transformers are begging for
> >problems unless a step-start is added.
> >
> /\ Good point, Mr. Rauch.
>
> >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.
> >
> /\ According to Econco Broadcast Services in Woodlawn, CA - a rebuilder
> of largish filament-type tubes -
> 1. filament potential should be set to 1 - 2 % above the point where a
> slight drop in peak power is detected.
> 2. emission is restored on tubes that have been operated with too-low
> filament V by operating the tube at rated V.
>
> >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.
> >
> /\ I know of three 3-500Z commercially-built amplifiers that apply c. 6V
> to the filaments.
> >
> - cheers
>
> postscript -- ¿perhaps Mr. Rauch would get more respect if he earned it
> by doing more homework?
>
> - R. L. Measures, 805.386.3734, www.vcnet.com/measures.
> end
>
>
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