[AMRadio] AM linear operation

[Rob Atkinson]

On Thu, Dec 30, 2010 at 9:51 AM, D. Chester <k4kyv at charter.net> wrote:
>
> A  rule of thumb is that for 100% modulation capability, the carrier output
> is limited to half your total plate dissipation.

I'm afraid that it is not quite that simple in my opinion and I'm only
bringing this up so someone with a SB-220 does not think he can put
out a 500 w. carrier.  (Please don't reply with anything about the
phantom "375 w. power limit" as that is another issue.)   I believe
the limit of an amp has a lot to do with a number of factors with
plate dissipation being one of them.  I know Don qualified his figure
with "rule of thumb" but I fear some will use this to oversimplify
their estimate of their amp's operating envelope on AM and wind up
with something zorching or melted pin solder or worse.   It pretty
much depends on the amp--for example you can take two or three amps
using a pair of 3-500 and what you can do with them will vary.   From
the looks of it the Drake L4 stock will let you get away with more
carrier than the Ten Tec Centurion for example.  I'm using 3-500 amps
as an example because they are what I am most familiar with.   One
factor is how long you want to transmit.   Another is the way the amp
gets cooled including air around the pins.  The current rating for the
plate transformer and the tube plate current vs. carrier you want to
run have to be considered.  These things all vary from one design to
another.   If I have to give someone a quick and dirty way to estimate
what they can do with an amp (I admit I am conservative on all this
because I tend to run my mouth a long time when I inflict myself on
someone in a QSO hi hi) I usually advise carrier at 25% total plate
dissipation.   Before I did a few mods to my Centurion it would get
extremely hot after just a couple of minutes at 200 w.  All this
hassle I've been through is why I advised the new guy to skip the
whole amp thing and just get a 100 w. plate modulated rig like the
DX100.    I've worked a guy up in Minnesota (I forget his call sign)
who has one fixed up with a decent antenna and he had me thinking he
was running some kind of broadcast rig.   It is amazing what you can
do with 100 w. and a good antenna.

73

Rob
K5UJ