[Amps] Failure of a pair of FU728F

[Roger (K8RI)]

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
> cathode.
>
> 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)