Apologies to all, I thought I had fixed this text formatting problem a few
months ago.
Hi Mike
A few questions that might lead towards some answers:
1. Are the two amps completely separate, with no shared
control circuits?
2. Did the problems begin at exactly the same time with
both amps, or only "about" the same time?
3. Have either or both of the amps been moved recently -
in particular, have the GS35Bs been turned upside-down?
4. Have there been any HV arcs recently?
Q3 and Q4 are aimed at the possibility of oxide particles from
the cathode finding their way onto the grid [1]. Normal heat
dissipation of the grid would then cause some electron emission
from those particles, forcing the grid current 'backward' from the
normal direction. (However, I'm speculating here. I don't know if
that would explain your RF-related observations.)
[1] http://www.ifwtech.co.uk/g3sek/misc/gs35-arcs.gif
73 from Ian GM3SEK
On 28/10/2019 03:52, 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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