[AMRadio] Ameritron amps on AM (was Ongoing Quest: Legal Limit AM Amp)

[Rob Atkinson]

Every power rating has to have a time component.  N% duty cycle, at P
watts for T minutes.

Brand X may do P watts AM carrier but for how long.  2 minutes?  Ten
Tec provides specs based on 10 minute transmissions.  For 100% d c
they derate the Centurion from 1200 watts to 600 watts.  pair 3-500zg,
2.8 KV under load, 2000 W. input.  The stock plate iron is 500 ma CCS.
  FWB rectifier, 5 6A10 diodes on each side, 8 450 v. 330 uF caps

You have to study the Ameritron p.s.    First, I don't like 3800 v. on
3-500zg.   That's pretty close to the Eimac maximum.  Also, that high
a Ep probably results in poorer 3rd order IMD.  None of this has to do
with power over time though.  My experience is that usually, besides
plate dissipation and cooling, the limiting factor in a ham
manufactured amp is usually in the power supply.  What's the
continuous duty rating for current with the AL-82 transformer?  Do the
math on the tubes.  Assuming 60% efficiency, what is the power being
dissipated by the tubes?  Let's say the Ep sags to 3300 v. under load.
 To get 1500 watts, you'd need 2500 watts input and 757 ma plate
current.  Now, the plate iron can probably handle that.   (The Ten Tec
iron could not, hence the derating for 10 minute transmission--hats
off to TT for being honest).  What else is going on?  You might think
well the only continuous thing is the carrier which is less than 400
watts.  But the amp is dissipating even more than 1 KW dead carrier if
it is dissipating 1000 watts under the above scenario.  and 1 KW is
right at the limit for a pair of 3-500s.   You might get your "legal
limit" beefing up the cooling on the tubes, but what you really should
be after is the ability to make a 30 minute transmission or more.
This is an opinion, but I think of AM as a continuous duty no time
limit mode and one of the challenges is meeting that goal.   You can
get that amp and drop the power down to 300 watts, which is what I do
(but my Centurion has had some things done to it by me) and I assure
you, no one is going to notice the missing 75 watts.
That's a start, but the amp may still need a bigger plate transformer
(higher current handling not voltage!) and more cooling.

No matter what you do, you always run into a limit and that is the max
plate dissipation of the tube(s) you are using.  That's your physical
limit you cannot get around except by adding more tubes (no you can't
just add a 3rd 3-500) or using a different tube which usually means a
whole new amp.

What about the 800 w. figure from Ameritron?  It sounds as if they are
speaking in terms of voltage only, which is what you'll see with the
average watt meter.

Stop obsessing about the 375 watt 1500 watt nonsense.   Firstly, your
audio is going to be all over the place; your positive peaks may be
over 100% because of your voice.  No one can measure any of this with
precision.   It's a paper tiger.

73
Rob
K5UJ

On Thu, Jan 26, 2012 at 11:33 PM, Rick Poole <wa1rkt at arrl.net> wrote:
> At 09:11 AM 1/24/2012, Ross Stenberg wrote:
>
>  >>>>>
>>I used to run a Ameritron AL-82 easily at full legal AM limit. That amp,
>>which uses a pair of 3-500's run at 3800 volts or so with a very hefty
>>power transformer, could just as easily run 1500 watts PEP output all
>>day long.
> <<<<<
>
> Yup, that's what their specs say.  And, Ameritron has at least one
> other amp (the AL-1500, single 3CX1500/8877) that should loaf at 1500
> watts PEP all day and all night.
>
> Interestingly enough, though... I engaged Ameritron in a discussion
> on that topic over the last several days, and they completely disavow
> AM operation at anywhere near legal limit with any of their amps...
> not the AL-82, not the AL-1500, not anything.
>
> In our discussion, they continued to describe their amps in terms of
> X carrier and 2X PEP, e.g. 400 watts carrier and 800 watts PEP
> (AL-82) or 600 watts carrier or 1200 watts PEP (AL-1500).  I'm not
> sure they even understand the concept that on AM, PEP = 4x carrier.
>
> In any case, I'll certainly take their word for it... no Ameritron
> amps for me (other than the AL-811H I already have that is on its
> third set of tubes).
>
> Rick WA1RKT
>
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