I don't think you can say 'which' radio. But I sure would urge you to run
the test on the radio(s) you have. I do know most of the Icom's have this
characteristic, especially the IC-781. I lost the amp tubes in my Titan 425,
twice, mysteriously, before I found out my Omni 6 was putting out an 18
microsecond spike that was close to 300 watts. But to tell you that was the
cause of two 3cx800a7's going bad, together, I can not. An 18 microsecond
power spike would take a while to remove the plating on the grid of a tube,
it is not an instaneous failure mode, thus you seldom expect the xcvr to be
the culprit. VE3DC alerted me to it when he heard I was driving my Titan 425
with an IC- 781. He lost three sets of tubes before he discovered the real
problem.
Slowing down the rise time is not a cure, it's just that the ALC does not
act quick enough to stop the spike.
Tom - W4BQF
----- Original Message -----
From: <al_lorona@agilent.com>
To: <tentec@contesting.com>
Sent: Wednesday, August 18, 2004 7:22 PM
Subject: TT RE: TT Re: [TenTec] Ten Tec amps and ALC
Tommy,
Is this 500 W spike that "most radios" produce always there? If so, then
what protects the grids of the tubes in the PA under normal operation?
Or maybe I don't understand exactly which radios have this spike problem.
Are there modern radios in this category?
Al W6LX
> -----Original Message-----
> From: tentec-bounces@contesting.com
> [mailto:tentec-bounces@contesting.com]On Behalf Of Tommy
> Sent: Wednesday, August 18, 2004 4:16 PM
> To: tentec@contesting.com
> Subject: Re: TT Re: [TenTec] Ten Tec amps and ALC
>
>
> A word of caution about ALC....do not EVER think ALC
> completely protects the
> grids of your amps tubes. The 'little' problem that ALC has
> is that it is
> slow. Most radios that use ALC for controlling output power
> level, produce
> a very short (microseconds), high level RF spike, until the
> ALC circuit can
> grab hold of it and bring it back down to a 'regulated'
> level. More that one
> pair of 3cx800a7's have mysteriously failed, and
> investigation showed the
> HUGE RF spike (300-500 watts) present on the leading edge of
> the RF waveform
> going to the amp, be it SSB, digital, or CW. If you want to
> be sure, check
> your xmtr's RF output waveform with a scope, running at about 10
> microseconds per division. Otherwise just replace expensive
> grid driven
> tubes, which seem to fail for no apparent reason. The Eimac suggested
> replacement, 3cpx800a7, is supposed to be more rugged in this
> respect....just in case one would need new tubes.
>
> Tom - W4BQF
>
> ----- Original Message -----
> From: "Stuart Rohre" <rohre@arlut.utexas.edu>
> To: <tentec@contesting.com>
> Sent: Wednesday, August 18, 2004 6:37 PM
> Subject: TT Re: [TenTec] Ten Tec amps and ALC
>
>
> > An overdrive indicator on an amplifier seems an excellent
> feature, to
> avoid
> > the possible non-compatibility of ALC circuits in light of
> no universal
> ALC
> > standard.
> > -Stuart
> > K5KVH
> >
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
>
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>
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