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Re: [Amps] Failure of a pair of FU728F

To: <>, <>
Subject: Re: [Amps] Failure of a pair of FU728F
From: <>
Date: Sun, 15 Sep 2019 16:40:14 -0400
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I am the person that Mike is referring to.  I was running the OM4000a at
200-300w out (with around 3-4w of drive), and tuning it as normal to the
"carrot" on the tuning indicator.  I noted that the amp ran hotter than it
did at 3000w out, but ignored this warning sign that there was a problem
running it that way.  Then one day I was using it closer to rated power
(3000 out into a dummy load of course!) all of a sudden the tuning shifted
(no trip out though) and I noted that plate current had nearly doubled for a
given output.  Instead of the usual 2.2a at 3000 out, it was around 4.2a.
At 1500w out, the normal 1.2a that I had been seeing became 2.2a.  At this
point I wasn't sure what was going on, since the amp would still put out
3000w, but using twice the current.  So I sent it off to Array Solutions,
and was told that the finals (both) were bad.  I was told that running the
amp at low power levels was a no-no.  This lesson cost me an arm and a leg.
I have had no problems since then, but I never run the amp below 600w now,
and very rarely even that low.  There shouldn't be any problems using it at
1500w, but I  wouldn't run it below 500w out under any circumstances.  It's
just not designed to run that way I was told.

I also had my output power on the Flex clamped to 50w out, so the amp had
never been overdriven.  
Hope this info helps someone avoid extremely costly repairs.

73 de Gary - W9BS

-----Original Message-----
From: Amps [] On Behalf Of
Sent: Sunday, September 15, 2019 3:24 PM
To: ''
Cc: 'amps'
Subject: Re: [Amps] Failure of a pair of FU728F

A friend of mine ruined the finals in his OM4000 by running the amp at
low power.
Array solutions told him the amp is designed to be run full power or
very close to it.
73 de Mike W9OJ

        -----------------------------------------From: "Ron Youvan" 
To: "amps"
Sent: Sunday September 15 2019 1:36:28PM
Subject: Re: [Amps] Failure of a pair of FU728F

 Guido Tedeschi wrote:

 > A friend asked to me why the pair of almost new FU728F tubes on its

 > commercial PA goes south.

 > It seems that, when tubes are warm, there is a short between Grid1
 > Cathode on both tubes and, when the PA PTT is activated, it reboots

 > again with the 3 minutes warmup delay.
 > So, he asked to me why both tubes were damaged but I was not able
 > answer, because:
 > - The PA is less than 3 years old and it has been used for less
than 300 
 > hours.
 > - The amp was used 99% at about half of the maximum power
 > - The TX power is limited by menu to 50W (FTDX-5000MP)
 > - Mains voltage was always between 220 and 230V
 > - Antennas are fine
 > - The PA is claimed to be protected against almost everything...

 > Normally, I think that it would happen when the tubes grid is 
 > overdrive, but the power was every time much lower than 50W and,
 > the amp "is protected", so, why it did happens?

 > Any idea?

 You did not mention the actual heater power.
 Ron KA4INM - Understanding is much better than
 knowing how.
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