Bill,
Assuming the grid current is within specs realtive to the grid disipation
limit (which I believe it should be as the 500z need a strong drive), the
question is one of plate disipation limits.
2000V @ 700ma loaded is 1400w input. Subtract the Po and add the drive and
you have 685w of net plate disipation against a 1000w limit. A 54% net
efficiency.
Assuming you have sufficient air flow to keep the pins cool at the base of
the tube so as to not violate the seal temp limit, you should be able to run
the tubes at this level until you are quite long in the tooth.
If I'm not missing my guess, I think you would only approach the tube plate
disipation limit at up to something like 1100 ma of plate current and
perhaps 100w drive. Assuming the same overall efficiency, that would be
around 1200w out and a bit over 1KW of plate dis. I'm assuming the B+ line
would drop slightly off the 2KV level. The 3500z is a pretty tough beast -
if you can keep it's feet cool.
73/jeff/ac0c
--------------------------------------------------
From: "Bill W5WVO" <w5wvo@cybermesa.net>
Sent: Sunday, July 05, 2009 7:14 AM
To: <AMPS@contesting.com>
Subject: Re: [Amps] AL1200, RTTY contesting, temps, duty cycles (not 100%)
> 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@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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>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
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