[Amps] Amps Digest, Vol 201, Issue 12 / Re: Failure of a pair of FU728F

Thu Sep 19 23:36:13 EDT 2019

All good comments here Jay. But you omitted to speak to the subject line and
offer an opinion on the likely reason why the pair of FU728 tubes both
failed with grid to cathode shorts?

What did the owner do wrong, and what teachings should other owners of the
OM4000 amplifier take note of when tuning up for low power or AM modulation
and avoiding an expensive tube replacement exercise?

73

Leigh
VK5KLT

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jay Terleski
Sent: Friday, 20 September 2019 2:00 AM
To: amps at contesting.com
Subject: Re: [Amps] Amps Digest, Vol 201, Issue 12

I get a kick out of this tube thread. But let me try to add some sanity to
the weirdness of it.

There is no way we, Array Solutions, told anyone to run the amp at or near
the high power limit to avoid cooking the tube. Just tune it normally and
keep the screen current at zero ma.  Your gonna be OK at lower power output.

Also with these tubes you want to make sure your filaments are very close
to 9V.   So watch the mains voltage. Many parts of the country are getting
well over 247V AC mains and that means your filaments may be running high.

Best wishes to the group.
Jay, WX0B

Jay Terleski
Array Solutions
214 954 7140

On Thu, Sep 19, 2019 at 11:00 AM <amps-request at contesting.com> wrote:

>
> 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 at 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 at gmail.com>
> To: "Roger (K8RI)" <k8ri at rogerhalstead.com>
> Cc: amps at 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 at 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 at contesting.com
> > > http://lists.contesting.com/mailman/listinfo/amps
> > --
> > Roger (K8RI)
> >
> ------------------------------
>
> Message: 2
> Date: Wed, 18 Sep 2019 17:13:41 -0400
> From: "Joe Subich, W4TV" <lists at subich.com>
> To: amps at contesting.com
> Subject: Re: [Amps] Failure of a pair of FU728F
> Message-ID: <02cd5628-ae76-411e-6ac9-29fe2338c40f at 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 at t-online.de>
> To: <amps at contesting.com>
> Subject: Re: [Amps] Failure of a pair of FU728F
> Message-ID: <AF52D62FEE5247E782E83EF1C142AFAF at 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 at contesting.com] On Behalf Of Joe Subich,
> W4TV
> Sent: Mittwoch, 18. September 2019 23:14
> To: amps at 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 at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
> ------------------------------
>
> Message: 4
> Date: Thu, 19 Sep 2019 14:02:42 +0930
> From: "Leigh Turner" <invertech at frontierisp.net.au>
> To: <amps at contesting.com>
> Subject: Re: [Amps] Failure of a pair of FU728F
>
> 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 tank inductor to
> accommodate proper tuning for widely disparate levels in RF power output.
>
> Leigh
> VK5KLT
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Greg Wasik
> Sent: Thursday, 19 September 2019 3:44 AM
> To: Roger (K8RI)
> Cc: amps at 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 at 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