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Re: [Amps] Failure of a pair of FU728F

Subject: Re: [Amps] Failure of a pair of FU728F
From: "Roger (K8RI)" <>
Date: Wed, 18 Sep 2019 04:21:51 -0400
List-post: <>
On 9/15/2019 4:39 PM, Paul Christensen wrote:
"A friend of mine ruined the finals in his OM4000 by running the amp at low
Array solutions told him the amp is designed to be run full power or very
close to it.
73 de Mike W9OJ"

Which is a bit confusing. As I understand as the amps are sold they require a lot of drive for the legal limit as there is a gain limitation requirement. The input circuit must be modified to get the amp's full capability

HOWEVER the FU728F requires about 60 to 70% of full power before the screen current flows in the proper direction. Although not large there is a reverse screen current area.  With the proper voltages and drive a single FU728F can hit 2500 PEP output, or 2000W carrier in the Emtron DX-2sp

73 Roger (K8RI)

I nearly ruined the FU-728F in my OM Power 2000A+ because the cathode was
running cold.  My QRZ page describes some details.  Bottom line:  don't
assume the power transformer tap (i.e., 250V, 240V, 230V) matches the
utility line voltage.  Choose the tap that results in 9.0 VAC on the
filament, exactly as to spec.  That will result in an adequately heated

My AC mains runs at 255V.  The remote site is the only subscriber on the
drop.  I initially set the amp's tap to 250V but the filament was starved as
the high tap resulted in 8.0-8.5V.  Filament voltage dropped even further
when the amp attempted to make full power.  The first clue to all this
occurred about six months after installation.  I noticed a fast 50% drop in
output power when keying the amp.  When keying the amp, it would initially
produce full power, then quickly drop.  At first, I was going to blame
component heating in the RF deck but the root cause was much simpler.

Another recent issue with the amp was step-start.  The step resistor blew
before its fuse could open.  The wirewound resistor's winding could only
take so many surge events before opening.  I ended up stacking two identical
resistors at 2X the value in parallel to equally divide the surge current
between them.

Paul, W9AC

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Roger (K8RI)

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