TT RE: TT RE: [TenTec] Ten Tec amps and ALC

[Tommy]

Your correct Al.

My point is that just because an amp has a nice LED that's turned on by
external-ALC, one should not get too comfortable that their amplifier is
'safe' from a poorly designed RF output circuit in the driving transceiver.
Since it is not an instant death threat to the amplifiers tubes, most folks
will ignore it.

Tom - W4BQF

----- Original Message -----
From: <al_lorona at agilent.com>
To: <tentec at contesting.com>
Sent: Wednesday, August 18, 2004 10:51 PM
Subject: TT RE: TT RE: [TenTec] Ten Tec amps and ALC



Thank you, Tommy.

This thread started off discussing external ALC between rig and amp, and has
ended up talking about internal ALC in the rig only. I wanted to point out
this very important point.

I wasnt' doubting that the leading edge spike did or did not destroy grids;
I'm sure that over time it can cause failure of the grid in a tube. But I
wanted to be clear that we were talking about the exciter alone doing this.
It has nothing to do with what I am calling "external" ALC. In other words,
let's not condemn "external ALC" for doing something that "internal" ALC can
do all by itself!

Were we expecting "external" ALC to fix this problem (the power overshoot)
that many "internal" ALCs have?

I believe that W8JI is talking about internal ALC on his page, too.

Al W6LX



> -----Original Message-----
> From: tentec-bounces at contesting.com
> [mailto:tentec-bounces at contesting.com]On Behalf Of Tommy
> Sent: Wednesday, August 18, 2004 6:51 PM
> To: tentec at contesting.com
> Subject: Re: TT RE: [TenTec] Ten Tec amps and ALC
>
>
> Al,
>
> I doubt seriously that a single 20 microsecond RF pusle is
> going to melt the
> grids of present day tubes. However each of those short
> pulses, according to
> Eimac, will slowly destroy the grid structure. It may take
> years, it may
> take months, but it happens. Most of the time, when a tube
> goes bad, you
> don't know  why, and the manufacturer normally charges nice
> bucks to analyze
> dead tubes,  so we are normally not going to have it done. In
> each case that
> VE3DC had a failure, the tube failed because of the grid
> failure. So did the
> grid fail due to thousands of 20 microsecond pulses, or did
> it just fail?
> Who knows? The last time I discussed this with him, and after
> he modified
> the internal ALC in his IC-781's, he had six years on his new set of
> 3cx800a7's. His scope showed no more RF spikes from the xcvr.
>
> The culprit, in most cases, is the transceivers internal
> leveling circuit,
> or what we call ALC, but not the plug coming into the back of
> the xcvr.
>
> Tom - W4BQF
>
>
> ----- Original Message -----
> From: <al_lorona at agilent.com>
> To: <tentec at contesting.com>
> Sent: Wednesday, August 18, 2004 9:20 PM
> Subject: TT RE: [TenTec] Ten Tec amps and ALC
>
>
>
> So now I'm confused. When you talk about "the ALC does not
> act quick enough
> to stop the spike", are you talking about external ALC, that
> is, a cable
> from the amp's ALC output back to the radio, or are you
> referring in general
> terms to whatever internal ALC scheme the exciter uses to regulate its
> output power?
>
> If all of these radios have such huge spikes, how come we
> don't hear of
> millions of tubes being fried by the spikes? Is it only
> certain tubes that
> are susceptible?
>
> Al W6LX
>
>
>
>
>
> > -----Original Message-----
> > From: tentec-bounces at contesting.com
> > [mailto:tentec-bounces at contesting.com]On Behalf Of Tommy
> > Sent: Wednesday, August 18, 2004 6:05 PM
> > To: tentec at contesting.com
> > Subject: Re: [TenTec] Ten Tec amps and ALC
> >
> >
> > 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 at agilent.com>
> > To: <tentec at 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 at contesting.com
> > > [mailto:tentec-bounces at contesting.com]On Behalf Of Tommy
> > > Sent: Wednesday, August 18, 2004 4:16 PM
> > > To: tentec at 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 at arlut.utexas.edu>
> > > To: <tentec at 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
> > > >
> > > >
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