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Re: [Amps] Failure of a pair of FU728F

To: <>
Subject: Re: [Amps] Failure of a pair of FU728F
From: "Leigh Turner" <>
Date: Thu, 19 Sep 2019 14:02:42 +0930
List-post: <>
Yeah running a tetrode based muscle amp as a linear for AM is problematic
and caution must be exercised in obtaining optimal plate loading and choice
of plate load line not causing excessive power to be dissipated in the
tube....and associated poor efficiency. That internal heating is what likely
warped the delicate and closely spaced control grid structure (yielding the
tube's high power gain) causing the unfortunate heater short and
catastrophic failure in BOTH tubes.

The OM4000 amp's tank circuit is optimised for running at or near

In a Pi or Pi-L tank network one ideally needs a variable inductor to
accommodate proper tuning for widely disparate levels in RF power output.


-----Original Message-----
From: Amps [] On Behalf Of Greg Wasik
Sent: Thursday, 19 September 2019 3:44 AM
To: Roger (K8RI)
Subject: Re: [Amps] Failure of a pair of FU728F

Guessing that if you wanted to run legal limit AM with the OM4000 you would
want to avoid any long pauses in your modulation so that the average power
is at least 1KW.  A dead key would be deadly...


On Wed, Sep 18, 2019, 4:22 AM Roger (K8RI) <> wrote:

> 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 power.
> 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
> 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
> 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
> 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
>  _______________________________________________
> > Amps mailing list
> >
> >
> --
> Roger (K8RI)

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