[Amps] Amps Digest, Vol 201, Issue 12

Mon Sep 23 05:05:09 EDT 2019

On 9/19/2019 12:30 PM, Jay Terleski wrote:
> 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.

Isn't that violating the tuning for Tetrodes?  The guide for Tetrodes 
for (best linearity/best IMD) is to PEAK the screen current at what ever 
power you are running, not set it at zero

73, Roger (K8RI)

> 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
> wall 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:
>
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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 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
>> Message-ID:
>>          <
>> CA+KdLttcjXOE2E+Nij+YCpG7ORMy8_3yuLq8LG_6P9xPrhnH+Q at 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 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)
>>>
>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>
>> ------------------------------
>>
>> 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
>> Message-ID: <88975500115041D28F75A2B180A1F51F at 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 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
>>>
>>>   _______________________________________________
>>>> Amps mailing list
>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>> --
>>> Roger (K8RI)
>>>
>>
>>
>> ------------------------------
>>
>> Subject: Digest Footer
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>> ------------------------------
>>
>> End of Amps Digest, Vol 201, Issue 12
>> *************************************
>>
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-- 
Roger (K8RI)