John Devoldere wrote:
>
> I use FT1000MP's and have ALC with set potmeter on all my amps, and it works
> beautifully. No a watt of overshoot! My amps require about 50 Watts, and the
> ALC adjusts that beautifuly!
> 73
> John, ON4UN
Hi John,
That is another method. I use that with my IC-706 and Yeasu 757.
I have an external bias supply, and feed a fixed ALC voltage back into
the ext ALC port through a diode. The amps ALC can increase this
voltage, but not decrease it.
By the way, the only way to be absolutely sure you really don't have
this problem is to measure the envelope with a storage scope or a
storage meter.
It is normally NOT a problem with triodes, or with tetrodes with
adequate screens and bias supplies, but some of the recent articles have
used very loose supplies for those elements.
What happens is the pulse comes along, loads the screen supply heavily
(or cause a grid current pulse that pumps up the bias), and the brief
pulse that would be a low average energy click is lengthened into a
distortion causing voltage change in the PA control grid or screen
circuit.
The time constant of the screen and bias supplies, generally caused by
bypass capacitors and series impedances, is critical in determining how
much IMD is produced by the leading edge spikes.
Triodes are more forgiving, although Eimac got me involved in looking at
a new ICOM that could actually ruin a set of 3CX800's after a few weeks
of use. The peak RF spike at the envelope leading edge was 350 watts, no
matter what level the output was set at!
The ICOM owner was using a 3CX800 PA without grid overload protection.
That was an extreme case, but it illustrates the potential problem.
73 Tom
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