[Amps] AL1200, RTTY contesting, temps, duty cycles (not 100%)

[Bill W5WVO]

I suppose right up there with RTTY is WSJT with its mode-defined 50% transmit 
cycles (30 seconds "on" for FSK441/JT6M, 60 seconds "on" for JT65). I'm running 
a 6-meter-converted SB-220 with new matched 3-500ZGs in the "throttled back" 
mode (lower anode voltage "CW/Tune" position). Here are the approximate 
operating parameters:

Power In: 85 watts
Anode voltage: 2,000V under full load
Anode current: 700 mA
Grid current: 250 mA
Power Out: 800 watts

Since I don't have access to accurate temperature-measuring instrumentation, I'm 
going by how stable it "feels" in long-term 50% (30-sec) WSJT duty. Power output 
starts at around 875W when the amp is cool and stabilizes at around 800W over 
perhaps 20-30 minutes of continuous 50% operation (30 seconds on, 30 seconds 
off).

Soliciting opinions from those with familiarity with the newer graphite 
3-500ZGs -- am I overstressing it in WSJT mode with these parameters? About 
right? Unnecessarily conservative?

Bill W5WVO


Jeff Blaine AC0C wrote:
> 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 at surfnetusa.com>
> Sent: Thursday, July 02, 2009 2:06 PM
> To: <AMPS at 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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