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Today's Topics:
1. Re: Failure of a pair of FU728F (Greg Wasik)
2. Re: Failure of a pair of FU728F (Joe Subich, W4TV)
3. Re: Failure of a pair of FU728F (dj7ww@t-online.de)
4. Re: Failure of a pair of FU728F (Leigh Turner)
----------------------------------------------------------------------
Message: 1
Date: Wed, 18 Sep 2019 14:13:39 -0400
From: Greg Wasik <gwasik@gmail.com>
To: "Roger (K8RI)" <k8ri@rogerhalstead.com>
Cc: amps@contesting.com
Subject: Re: [Amps] Failure of a pair of FU728F
Message-ID:
<
CA+KdLttcjXOE2E+Nij+YCpG7ORMy8_3yuLq8LG_6P9xPrhnH+Q@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
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...
Greg
K1YW
On Wed, Sep 18, 2019, 4:22 AM Roger (K8RI) <k8ri@rogerhalstead.com> 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
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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
--
Roger (K8RI)
_______________________________________________
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------------------------------
Message: 2
Date: Wed, 18 Sep 2019 17:13:41 -0400
From: "Joe Subich, W4TV" <lists@subich.com>
To: amps@contesting.com
Subject: Re: [Amps] Failure of a pair of FU728F
Message-ID: <02cd5628-ae76-411e-6ac9-29fe2338c40f@subich.com>
Content-Type: text/plain; charset=utf-8; format=flowed
"Legal limit" AM is 375 W of carrier -> 1500 W PEP. Whether the OM4000
can be tuned properly for that operating condition is another story.
73,
... Joe, W4TV
On 2019-09-18 2:13 PM, Greg Wasik wrote:
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...
Greg
K1YW
------------------------------
Message: 3
Date: Thu, 19 Sep 2019 01:18:01 +0200
From: <dj7ww@t-online.de>
To: <amps@contesting.com>
Subject: Re: [Amps] Failure of a pair of FU728F
Message-ID: <AF52D62FEE5247E782E83EF1C142AFAF@DJ7WW2>
Content-Type: text/plain; charset="us-ascii"
Yes, the pi net design values for 4KW PEP are quite different to 1.5KW.
If designed for 4k, circuit Q will be almost doubled for 1.5k.
And I doubt it can be tuned on 10m with that high tube impedance.
73
Peter
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Joe Subich,
W4TV
Sent: Mittwoch, 18. September 2019 23:14
To: amps@contesting.com
Subject: Re: [Amps] Failure of a pair of FU728F
"Legal limit" AM is 375 W of carrier -> 1500 W PEP. Whether the OM4000
can be tuned properly for that operating condition is another story.
73,
... Joe, W4TV
On 2019-09-18 2:13 PM, Greg Wasik wrote:
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...
Greg
K1YW
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
------------------------------
Message: 4
Date: Thu, 19 Sep 2019 14:02:42 +0930
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: <amps@contesting.com>
Subject: Re: [Amps] Failure of a pair of FU728F
Message-ID: <88975500115041D28F75A2B180A1F51F@Invertech>
Content-Type: text/plain; charset="us-ascii"
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
full-tilt....
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.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Greg Wasik
Sent: Thursday, 19 September 2019 3:44 AM
To: Roger (K8RI)
Cc: amps@contesting.com
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...
Greg
K1YW
On Wed, Sep 18, 2019, 4:22 AM Roger (K8RI) <k8ri@rogerhalstead.com> 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
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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
--
Roger (K8RI)
------------------------------
Subject: Digest Footer
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------------------------------
End of Amps Digest, Vol 201, Issue 12
*************************************