Hello,
Swapped out the GS35B in one of the amps with NOS GS35B and the amp. works
as it did. Why did the initial tube fail - don't know as HV supply, bias
system, filament voltage, etc. checked out.
I did see Ian's pictures and the same thing may have happened to the GS35Bs
in my amps. Again, all the HV supply parts in the common HV power supply
checked out okay including the glitch resistor.
Any ideas on what caused the problem?
One thing I might do is to put a fast blow fuse in the HV circuit although
not sure a grid to plate arc caused the tubes to fail or even if the fuse
can blow fast enough to "save" the tube(s).
73,
Mike, K4GMH
On Sun, Oct 27, 2019 at 11:52 PM Mike <k4gmh@arrl.net> 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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