> >Maybe, but then we must also think of all the wonderful signals
> >that would come from grid driven tetrodes with poor screen or bias
> >regulation.
> >
> ? Tom - - How can grid bias regulation be a problem when the grid
> current is zero?
Because if the user ever does draw grid current accidentally, say
when the transceiver has overshoot, and unregulated high ESR grid
supply can go into cutoff bias by charging the bypass and RF
decoupling capacitors.
If you properly regulate the grid supply, momentary operation in
grid current will not cause a longer time operating into improper
bias as the grid voltage slowly restabilizes through the R C time
constant provided by the grid bypasses and coupling caps and the
ESR of the poorly regulated control grid supply.
It's an imperfect world with imperfect exciters and operators, that's
why regulation reduces IMD in practical real-world systems.
The tiny overshoot TIME but large amount of power at the leading
edge, especially if the tetrode article gives you a design that drives
the grid with ten or twenty watts, can be greatly extended by the
time constant of the bias circuit. The overshoot turns into a sharp
peak followed by a long dip while the bias recovers and the tube
sits in class C.
One W2 on Long Island has a signal like this when he gets carried
away, and he often takes out 30 kHz or more of 40 meters with S9
splatter. You can watch his signal on a scope, and see the sharp
peak followed by a long abrupt dip where IMD galore is generated
until the envelope slowly slopes back upwards to normal shape.
I'd rather have millisecond pulse of spewing emissons than two
second bursts of garbage.
Not all big tubes are clean tubes, especially when they are grid
driven tetrodes with "unstiff" supplies that recover slowly.
Grid driven voice mode operated tetrodes need peak reading grid
meters and regulated supplies, and should be set so the exciter
runs at near full output in normal operation.
73, Tom W8JI
w8ji@contesting.com
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