[Amps] Amps Digest, Vol 201, Issue 12
Carl
km1h at jeremy.qozzy.com
Tue Sep 24 18:57:57 EDT 2019
That is the correct way Roger, just peak at a low Isg using sufficient drive
for rated power. As the tube ages increase Isg.
I have been doing that with tetrodes from the 4CX250B/8122 to ones with
handles in commercial service over about 5 decades. The pairs of 8122's in
the 4 active NCL-2000/2006's here all are ancient by most standards but
still run full power.
SOME tetrodes are DESIGNED to run at negative screen curren, when in doubt
read the spec sheet.
Carl
----- Original Message -----
From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
To: <amps at contesting.com>
Sent: Monday, September 23, 2019 5:05 AM
Subject: Re: [Amps] Amps Digest, Vol 201, Issue 12
> 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:
>>
>>> Send Amps mailing list submissions to
>>> amps at contesting.com
>>>
>>> To subscribe or unsubscribe via the World Wide Web, visit
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>>> When replying, please edit your Subject line so it is more specific
>>> than "Re: Contents of Amps digest..."
>>>
>>>
>>> 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
>>> *************************************
>>>
>> _______________________________________________
>> 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
>
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