These are TWO rf decks using ONE HV power supply in a SO2R situation.
Sent from my iPhone
> On Oct 29, 2019, at 8:16 PM, Alek Petkovic <vk6apk@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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