[Amps] GS35B Amps Common Problem

[Alek Petkovic]

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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