[Amps] GS35B Amps Common Problem

[Steve, NR4M]

These are TWO rf decks using ONE HV power supply in a SO2R situation. 

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> On Oct 29, 2019, at 8:16 PM, Alek Petkovic <vk6apk at bigpond.com> wrote:
> 
> Bad input coupling or tuning network capacitor? Leaky and heating up with RF drive.
> 
> 73, Alek. VK6APK
> 
> 
>> On 28/10/2019 11:52 AM, Mike wrote:
>>  Hello,
>> 
>> My two homemade GS35B amps exhibit the same symptoms of a problem and can't
>> seem to figure what  is the "common" problem.  This happened recently after
>> several years of operating both amps without a problem in various RTTY
>> contest running 1500 Watts output (SO2R).  The symptoms are seen initially
>> tuning the amps up.
>> 
>> The symptom(s) occur after the amp is warmed up, filament and blower both
>> on for over 5 minutes before high voltage is applied to the amp.  RF is
>> applied to the input and plate tuning starts.  The amp output gets to ~1200
>> Watts output when the amp input impedance start changing causing the radio
>> driving the amp to fold back its output which in turn decreases the amp
>> plate and grid current.  The radio's output decrease continues until the
>> radio is putting out very little RF.  The amp is correspondingly putting
>> out very little RF.  The amp's plate current has gone from ~500 mA to less
>> than 100 mA along with the grid current going from 200 mA to less than 50
>> mA.  Once this condition is reached, removing the input RF from the amp and
>> immediately reapplying the input RF does not change the last condition of
>> amp's input impedance.
>> 
>> Usually a long period without the amp being keyed has to occur before the
>> amp's input impedance seems to start returning to normal.   However, once
>> RF reapplied, the amp immediately starts to return to the high input
>> impedance.
>> 
>> The amp plate voltage goes from ~3400 Volts at 1200 Watts output to ~3600
>> Volts at lowest output.  Quiescent plate voltage is ~3700 Volts.  Note: a
>> single power supply is used by both amplifiers along with a common blower.
>> 
>> The things check and found okay are the W4ZT bias circuit in each amp, 120
>> VAC to each amps filament xfmr, power supply's start up resistors by-passed
>> after HV power supply start up, HV diode string,
>> different radios used, each amp uses a separate antenna, etc.
>> 
>> Again, these amps have operated okay for several years without an issue.
>> Then both amps start exhibiting the same symptoms of a common(?) problem at
>> the same time.
>> 
>> Appreciate any ideas on where to look next for the problem.
>> 
>>   73,
>>   Mike, K4GMH
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>>   
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