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[by way of KS9K Paul Hellenberg <paulh@truline.com>]

>From ks9k  Mon Apr 14 10:16:09 1997
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Date: Mon, 14 Apr 1997 10:01:40 -0400
From: "Tom Rauch (W8JI)" <W8JItom@worldnet.att.net>
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To: amps@contesting.com, "David B. Ritchie" <dbr@alumni.caltech.edu>
Subject: Re: [AMPS] 3cx800
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David B. Ritchie wrote:
> 
> FYI --
> My friends who work for Eimac and design their tubes have repeatedly told
> me that they are serious about their filament voltage specs and that if
> you are high or low, it will reduce cathode life, particularly on tubes
> such as the 3CX-type tubes (like 8877s, 3CX800s, 8874s, 8875s, etc.).

That is important advice.

If heater voltage is reduced durning operation of metal oxide cathode
tubes, it can more harmful than running too much voltage. Low catode
temperature can instantly ruin the tube!

Also remember the warm up time specs are for conventional filament
supplies, if you reduce the voltage or severely limit current tube warm
up time may need to be extended.

I have seen several 8877's with stripped cathodes caused by reducing
filament voltage below the specified range in an attempt to extend tube
life.

In amateur service, emission failures caused by excessive filament time
or temperature are almost unheard of. Since the tubes are generally
pushed right to the limit using the rated filament voltage makes sense.

73 Tom




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