My apologies for this general post but someone on the
list was looking for suggestions regarding using 50
watts of drive with an Ameritron ALS-600 and not
getting the output that they wanted (didn't say what
they were getting) and wanted suggestions to increase
the output without having to increase their drive.
Can't remember who it was but the following may be of
interest to others as well.
The ALS-600 uses a "swamping" resistor in the input
circuit to limit the drive and you might want to look
into changing it if you are using less than 75 watts
or so of drive - or - if you are using more drive but
not getting the output you expect - be reasonable
here!
I've done a lot of "playing" with the ALS-600 and
frankly if you are getting 450 to 575 watts output
(depending on band and drive) under steady-state
key-down conditions (measured with a known accurate
wattmeter into a low swr dummy load), you are probably
doing "OK" - and yes, I've seen the reports of 800 and
900 (rms? pep? accurate wattmeter? etc.) out and I've
got to wonder how long it would last (ready for the
email flurry??) under those conditions - maybe a long
time if the duty cycle is low enough but if you're
hitting it pretty hard (dxing with compression or
using digital modes with high duty cycles) you may be
in for an expensive replacement of the finals.
Ameritron advertises 600 pep and that's probably a
fair claim in today's market (note the "in today's
market" disclaimer).
Apparently Ameritron has tinkered with the "default"
value of the "swamping" resistor used in the ALS-600
over the years (compare early part lists with later
part lists) - perhaps to compensate for the gain of
individual units or other circuit changes. I believe
the values have been 100, 45, 35 and 25 ohms - or at
least values in those ranges. The resistor is a high
wattage non-inductive type that mounts in a couple of
large "fuse clips" and is located on top of the
amplifier board - easily accessible with the top of
the RF unit removed - you can't miss it.
Ameritron sells the resistors for about $25 plus
shipping(as I recall) and a call to their factory
number will put you in touch with someone (probably
"Willie") who can advise you. I know all this because
mine had the 45 ohm resistor in it and it was
attenuating the drive (90 watts) too much (evidenced
by lower than expected output from the ALS-600. I was
getting 275 to 350 watts depending on band). I
replaced it with the 35 ohm resistor and that resulted
in much better output (400 to 500 watts) under steady
state key-down conditions with about 90 watts of
drive. I have even heard of others going to values
lower than 25 ohms in order to drive it with qrp rigs
with good success.
I think it's a nice little amp - runs cool, "faults"
just the way it should when under duress and with an
antenna system that presents reasonable swr, makes for
simple band hopping with the twist of the knob and
little fuss.
Usual disclaimers - no pecuniary interest in
Ameritron, etc. etc. YMMV 73 es gud dx --don n4hh
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