Kevin,
You are right. RTTY is the big boy challenge for amps. But it tends to
make things fall apart in amps in the strangest ways...
You had asked some questions about determining temps. I had not seen any
replies specific to that and wanted to mention the tube consideration here.
The other things that break or melt :) are fixable. But tube damage, well
that's a pain that does not go away without some $$...
I believe the critical temps for the tube will be listed on the data sheet.
The issue for the tube, in an RTTY environment, is if the tube temp
stabilizes, or if it keeps creeping up. If it stabilizes, and the temps are
under the limits indicated in the data sheet, then the tube should serve you
well. A tube can be run hard - as long as "hard" is within the limits of
operation. And with 1200W of plate dissipation and maybe 1500w output, you
are not going to be pushing the plate dissipation limits. It's the body,
anode, pin temps that are the question. I would guess these are probably
fine as well as the tube is axially cooled. Put a thermo on the exhaust and
look at it vs. time at key down. the temp should rise up to a point and
then level off. If it keeps climbing, then that's trouble brewing.
As for the rest of the guts, Bill and Carl have books of stories. I will
add my own. A buddy of mine has a QRO 2500 he plays with on RTTY. Supposed
to be rated at 1500w NTL. Melted the 80 & 160m tank toroids. Quite a few
iterations later, he had a dedicated toroids for each band to lower down the
flux density (a problem with 2-band toroid configurations) and the thing
hums along fine at legal limit RTTY all day long now. K1TTT had the similar
problem so it's not specific to the guy. But other guys with the same amp
seem to never have problems.
With RTTY, there is only 1 rule. Your mileage will vary. And that's why
RTTY IS the big boy's mode.
73/jeff/ac0c
--------------------------------------------------
From: "Kevin Normoyle" <knormoyle@surfnetusa.com>
Sent: Thursday, July 02, 2009 2:06 PM
To: <AMPS@contesting.com>
Subject: Re: [Amps] AL1200, RTTY contesting, temps, duty cycles (not 100%)
> Bill, W6WRT wrote:
> "But here's the Achilles heel of the AL-1200. The low band tank coils used
> undersized wire and overheated."
>
> Thanks Bill. I've read some of your comments in the forum on the AL-1200
> before. I think one of the good things we can do as users is to just
> document what happens in as much detail as we can. There is definitely a
> place for the price/performance the AL-1200 provides, and I like it a lot.
>
> Sometimes the "complaints" about various amps lacks the detail to really
> assess what happened and why. It can be hard to sort out one-off issues,
> or generic operating environment issues.
>
> I'm just interested in making sure I operate within the AL-1200
> performance envelope. I was surprised to see all this talk about RTTY duty
> cycles, as if it was well defined/understood. When it seems not to be.
> i.e. if if was just RTTY duty cycle, do things melt on a 1 minute RTTY
> ragchew? I don't know, but as I described, don't care. I'm perfectly
> happen to limit my RTTY macros.
>
> So part of my post is this: There is no such thing as RTTY duty cycles
> when you talk about RTTY contesting. There is a desirable duty cycle, but
> people should define it better. It's not key down forever.
>
> And how do you characterize the difference between RTTY 50% duty cycle for
> two hours, versus CW duty cycle for two hours?
> Does two hours matter? Why isn't a 30 minute test enough?
>
> Now the stuff about the low band coils is important. What that says to me
> is that there's an airflow issue in the box. The wires are undersized for
> the airflow they see.
>
> What I don't understand, is how long it took your solder to come undone in
> a contest? What was the ambient temp in the shack?
> Is it possible that there's a constant slow rise of temp at the coil
> during a contest, such that the failure needs a combination of high
> ambient temps, and maybe an hour or two of contesting? That doesn't seem
> to match what I saw, unless it's a behavior that happens at 1500W.
>
> Hmm..I'm wondering if what I thought was temperature stability in the box
> after 15 minutes, isn't stable. I didn't measure the coil area. Maybe I
> need to? I have a hard time believing that the coil area didn't temp
> stabilize in my 30 minute test. No coil melted in my 30 minute CQ test at
> 1200W (80M) into dummy load.
>
> Any idea what kind of antenna and SWR you had on 80M? I've not used the
> amp on 160m yet.
>
> I appreciated Tom, W8JI's responses. They made sense to me. He described
> his use of the amp at 4000v with assisted air (outside fan) but stock
> blower. Although he apparently doesn't do RTTY.
>
> I don't care about the ads. The reality in the ham world is that the users
> sort out what works and what doesn't and in what environment. That's the
> nature of the beast. Stuff that really sucks: people eventually stop
> buying.
>
> Hey BTW: in looking at other amps that people seem to like, a bunch seem
> to have significantly higher CFM blowers (I don't know about noise). I
> always wonder if a fairer comparison to the AL-1200 would be with an
> AL-1200 with equal CFM blower/dBA.
>
> If you're right that it's a coil wire size issue (maybe other issues),
> then yeah, maybe it's not that simple. I'm musing on the higher CFM
> blower, maybe dialed back a bit from full speed.
>
> thanks,
> -kevin
> ke6rad
>
>
>
>
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