[Amps] SB-220 and Graphite tubes in high duty cycles

Sat Jul 4 20:31:03 PDT 2009

Carl,

That SB220 stock transformer is really a marginal design.  The SB200 is a 
far sturdy bird albeit at a lower voltage - even when compared under the 
same current draw.

And wow, when the winding shorts on that transformer, now that's an exciting 
time.  :)  I am sure you have had that same pleasure many times in your long 
history of playing with amps.

73/jeff/ac0c

--------------------------------------------------
From: "Carl" <km1h at jeremy.mv.com>
Sent: Sunday, July 05, 2009 10:26 AM
To: "Bill W5WVO" <w5wvo at cybermesa.net>; <AMPS at contesting.com>
Subject: [Amps] SB-220 and Graphite tubes in high duty cycles

> Id be concerned about the plate transformer temperature rise as its only
> rated at .6 kva in CCS service. Also filter cap temperatures.
>
> In the RF deck the problem could be RF heating in the input or output
> circuitry from undersized components or inadequate attachment of high
> current points.
>
> Graphite tubes like a higher airflow which is why Harbach offers the high
> speed fan.
>
> Carl
> KM1H
>
>
>
>
> ----- Original Message ----- 
> From: "Bill W5WVO" <w5wvo at cybermesa.net>
> To: <AMPS at contesting.com>
> Sent: Saturday, July 04, 2009 7:14 PM
> 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 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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