Hello Tom,
Thanks for the response. I don't exactly understand it fully,
where you are making a reference to a design flaw, or its one of those
things you have to live with when you ad a Tuner in the configuration.
I am more of an "Appliance User" so I guess I will live with my solution.
I just figured it was "Operator Error"..
It does look like from my explanation I found the problem and resolved
it in my own way or confirmed your findings.
Using a Tuner is my way of keeping peace with my Neighbors.
If I had it my way I would have many antenna's cut for each band and no
tuners.
I own 3 ATR-30's, and when you add that up in $, that's plenty antennas
including
cable and switches.
I have considered getting rid of my Sloper and Tuners and go with one of
those new "Fluid-Motion Verticals" (The Big one.)
where the antenna adjusts to resonance. I tried the Beam at Dayton they had
out in the Flea Market,
and I was impressed.
Keith, K4KAL
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Keith LaBorde - K4KAL" <k4kal@arrl.net>; "*Amps Group"
<amps@contesting.com>; "Gary D Elliott" <k7ox@pipeline.com>
Sent: Wednesday, March 05, 2003 9:30 AM
Subject: Re: [Amps] Alpha 99
> > A few months later my 2nd Alpha99 I had on order came in.
> > They are about 3 months apart in age.
> > I said to myself OK... lets see if this new one does the same thing.
> > "Trip" ! It did it again. Wow this is driving me nuts.
> > So I reduced power, and kept using it with reduced output.
> > Like a few 100 watts are going to make a difference anyway..
> >
> > So one day during a contest I switched from 80 to 40, and later
> > back to 80, and noticed my problem was gone.
> > It was the ATR-30 Tuner.... Maybe?
>
> I'm almost positive the Alpha 99 is one of the amps that have a tendency
to
> oscillate at very low frequencies when running into certain loads OUTSIDE
> and far below, **not inside or near**, the band you are operating on.
>
> Looking at ETO drawing SXX-X164 rev L there is a VLF feedback path through
> L1, the parallel combination of R33 and D4, the parallel combination of D7
> and R11, R10, and L3 directly from amplifier output to amplifier input.
The
> shunt impedances are virtually all capacitive reactances, and that forms a
> low-pass filter!!!!
>
> This forms a feedback loop directly from the amplifier output back to the
> input through RF chokes in the switching system. The chokes and bypass
> capacitors, in essence, form a low-pass network that connects the output
> directly back to
> the input.
>
> When used with certain rigs that terminate the amplifier input in a high
> impedance BELOW the operating frequency, and when the output of the amp
sees
> an almost open circuit at very
> low frequencies, the amplifier actually oscillates at a few hundred kHz!
>
> This causes problems similar to what you are describing. It also causes
> spurious
> signals on SSB and spurs and keyclicks on CW signals. There was discussion
> about this on the topband reflector, after a contest. I had severe
problems
> with a W4 who use a PIN switched Alpha amp into a "T" network tuner.
>
> If you add a 1/4 wl shorted stub for the band you are using on the PA
> output, or eliminate the C-L-C tuner, the output of the amp is terminated
in
> a low VLF or LF impedance. This loads the feedback path, and the LF or VLF
> oscillation stops.
>
> This is a common problem with PIN diode switched amplifiers or amplifiers
> that use RF chokes from output back to input, even when those chokes are
> bypassed.
>
> VLF and LF parasitics are actually more common than people imagine, and
are
> what cause the worse problems. The grid chokes in SB220's, 30l1's, and
> TL-922's set the amplifiers up for such problems.
>
> 73 Tom
>
>
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