>> I had seen somewhere a voltage doubler mod. for amps
>> that used
>>Svetlana tubes in the FL2100B , but I wonder about the
>>ant. relay
>>having twice the voltage on it than original.Can anyone
>>tell me if that
>>is safe
>>?? I don't want to burn up a relay.!!
>
> The voltage "doubler" mod by K1KP/K6XX is almost certainly
> safe because
> it uses the existing relay supply rail, and only gives the
> relay a very
> short kick of increased voltage. All the rest of the time
> the relay is
> energized, it is operating at its normal voltage.
Actually that relay speed up circuit has nothing to do with
the Svetlana mod.
The Svetlana mod in the standard FL2100 (not the later
2100Z) converts a NOMINAL 15 volt dc supply to around 30
volts by making it a voltage doubler. That mod adds a
resistor that hopefully reduces relay voltage to the proper
amount, but it looks to me like the resistor might be too
small.
All in all, this is a pretty poor fix. What really should be
done is a relay added to switch the CT of the transformer
filament winding OPEN when going to receive. This simple
change would take fewer parts and ensure the tubes were off
during standby.
In the later production unit (FL2100Z) Yaesu attempted to
correct stability issues as well as occasional oscillations
during standby by adding a voltage tripler for bias and
neutralization. When the relay is closed the grids run off
an adjustable voltage divider from a 15V nominal supply, but
when the transmit line is open the tripler kicks in and
holds the grids at around -50 volts. They added the worse
neutralization system I've ever seen in an HF amplifier.
It's a capacitor from the loading cap to the filament of the
tube!
It should be noted these oscillations are near the OPERATING
frequency of the 2100....not at VHF. Yaesu obviously knew
they had stability problems at HF, they apparently just
didn't know how to fix them.
73 Tom
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