Date: Mon, 6 Feb 2012 11:47:42 -0600
From: Rob Atkinson <ranchorobbo@gmail.com>
Subject: Re: [Amps] Different amp recommendations
I have doubts about the AL82 doing 1500 watts 100% duty cycle for very long.
Knowing the p.s. in that amp you'll be right at its limit, right at
the Ip limit of the tubes, right at or a bit above their maximum
dissipation...
It's a "legal limit" amp only because Ameritron heaped up the Ep.
But you can call almost anything "legal limit" with no time factor considered.
Your enemy is time. do you want to go for 10 minutes or more at 1500
w. on RTTY? Do you want to be able to forget about the amp and not
have to look at it nervously every 15 seconds to make sure it's okay?
Personally, I like overbuilt stuff.
you can beef up the AL-82 in some ways and settle for 1.2 or 1.3 KW,
homebrew an amp or keep looking with more money.
73
Rob
K5UJ
## let's do the jr high school maths. The xfmr in the AL82/1200/1500 is
rated at 1.8 kva CCS. [32 lb]. To get 1.5 kw out, we require 1500/.6eff =
2500w dc input.
When those dahl xfmr's are maxed out, the temp rise is rated at 55 deg C over a
40 deg C
ambient. That also factors in another 10 deg C deep inside the core..for 'spot
heating'.
Deep inside the core, it will be 65 deg C over the outside 40 deg C ambient.
Now 40 deg C = 104 deg F. Now the hottest it will get, deep inside the
core is 105 deg C or 221 deg F....which is hot.
On CW, with dashes at a 3.6:1 dash to dot ratio, this
xfmr will be fine. RTTY for hrs on end, yeah, I don't think so. A 46 lb dahl
is only good for
2.7 kva CCS....and that would be the smallest size I would use in a RTTY
contest.
## If ur eff is lousy on 10m, like 50%... now you require 3 kw dc input to get
1.5 kw out.
IMO, the initial extra cost of a slightly heavier xfmr is well worth it. You
save nothing
in the long run by under sizing pwr supply components OR tank components like
coils+
bandswitch's, etc. A bigger xfmr not only runs cooler, it will also have way
better V regulation.
IF you burn up a xfmr, it will have to be replaced with the 46+ lb unit
anyway, so now you
not only didn't "save" any money....you lost money ! IMO, build it [or buy
it] like a tank,
then never look back.
Later... Jim VE7RF
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