[Amps] Failure of a pair of FU728F

[Guido Tedeschi]

Thanks Joe,

    but the manual says that the amp protection is activated from Grid 
current too high...

73 de Guido, ik2bcp


Il 16/09/2019 00:20, Joe Subich, W4TV ha scritto:
> On 2019-09-15 4:40 PM, w9bs at w9bs.com wrote:
>> I was running the OM4000a at 200-300w out (with around 3-4w of
>> drive), and tuning it as normal to the "carrot" on the tuning
>> indicator.
> Sounds like the "carrot" is not optimum tuning for low power and
> results in some parameter (G1, G2, etc.) running out of tolerance
> at that level.  I would never rely on a "tuning indicator" as the
> sole indicator of proper tuning for a tetrode.
>
> The FU728F G1 is rated for a maximum dissipation of 1W.  If the
> amplifier is not loaded heavily enough (particularly when operated
> at low power output), it is very easy to exceed the grid dissipation
> limit.
>
> 73,
>
>    ... Joe, W4TV
>
>
> On 2019-09-15 4:40 PM, w9bs at w9bs.com wrote:
>> I am the person that Mike is referring to.  I was running the OM4000a at
>> 200-300w out (with around 3-4w of drive), and tuning it as normal to the
>> "carrot" on the tuning indicator.  I noted that the amp ran hotter 
>> than it
>> did at 3000w out, but ignored this warning sign that there was a problem
>> running it that way.  Then one day I was using it closer to rated power
>> (3000 out into a dummy load of course!) all of a sudden the tuning 
>> shifted
>> (no trip out though) and I noted that plate current had nearly 
>> doubled for a
>> given output.  Instead of the usual 2.2a at 3000 out, it was around 
>> 4.2a.
>> At 1500w out, the normal 1.2a that I had been seeing became 2.2a.  At 
>> this
>> point I wasn't sure what was going on, since the amp would still put out
>> 3000w, but using twice the current.  So I sent it off to Array 
>> Solutions,
>> and was told that the finals (both) were bad.  I was told that 
>> running the
>> amp at low power levels was a no-no.  This lesson cost me an arm and 
>> a leg.
>> I have had no problems since then, but I never run the amp below 600w 
>> now,
>> and very rarely even that low.  There shouldn't be any problems using 
>> it at
>> 1500w, but I  wouldn't run it below 500w out under any 
>> circumstances.  It's
>> just not designed to run that way I was told.
>>
>> I also had my output power on the Flex clamped to 50w out, so the amp 
>> had
>> never been overdriven.
>> Hope this info helps someone avoid extremely costly repairs.
>>
>> 73 de Gary - W9BS
>>
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of
>> w9oj at indy.rr.com
>> Sent: Sunday, September 15, 2019 3:24 PM
>> To: 'ka4inm at gmail.com'
>> Cc: 'amps'
>> Subject: Re: [Amps] Failure of a pair of FU728F
>>
>> 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
>>
>>     -----------------------------------------From: "Ron Youvan"
>> To: "amps"
>> Cc:
>> Sent: Sunday September 15 2019 1:36:28PM
>> Subject: Re: [Amps] Failure of a pair of FU728F
>>
>>   Guido Tedeschi wrote:
>>
>>   > A friend asked to me why the pair of almost new FU728F tubes on its
>>
>>   > commercial PA goes south.
>>
>>   > It seems that, when tubes are warm, there is a short between Grid1
>> and
>>   > Cathode on both tubes and, when the PA PTT is activated, it reboots
>>
>>   > again with the 3 minutes warmup delay.
>>   > So, he asked to me why both tubes were damaged but I was not able
>> to
>>   > answer, because:
>>   > - The PA is less than 3 years old and it has been used for less
>> than 300
>>   > hours.
>>   > - The amp was used 99% at about half of the maximum power
>>   > - The TX power is limited by menu to 50W (FTDX-5000MP)
>>   > - Mains voltage was always between 220 and 230V
>>   > - Antennas are fine
>>   > - The PA is claimed to be protected against almost everything...
>>
>>   > Normally, I think that it would happen when the tubes grid is
>>   > overdrive, but the power was every time much lower than 50W and,
>> anyway,
>>   > the amp "is protected", so, why it did happens?
>>
>>   > Any idea?
>>
>>   You did not mention the actual heater power.
>>   --
>>   Ron KA4INM - Understanding is much better than
>>   knowing how.
>>   _______________________________________________
>>   Amps mailing list
>>   Amps at contesting.com
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>