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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