>
>> >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.
? In practice, a bit of inadvertent grid current does not appear to
cause cutoff bias.
>
>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.
? What are you using for bypasses, 0.47F?
>
>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.
? Of course, none of this happens if the operator adjusts the grid
voltage so that no grid current flows, overshoot and all, when the
transceiver is sending bursts of full-throttle dits at 50wpm.
>
>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.
? Remarkable. I have never been able to see IMD on an oscilloscope.
>
>I'd rather have millisecond pulse of spewing emissons than two
>second bursts of garbage.
? Two seconds is nothin'. I have heard several minute bursts of garbage
from a pair of 572Bs that was driven by 115w.
>
>Not all big tubes are clean tubes, especially when they are grid
>driven tetrodes with "unstiff" supplies that recover slowly.
? But only with bozo operators and >0.1F grid bias bypassing.
>
>Grid driven voice mode operated tetrodes need peak reading grid
>meters
? For an 8171, a 1mA fs meter is plenty sensitive for checking grid
current.
> and regulated supplies,
? regulated screen potential is important.
>and should be set so the exciter
>runs at near full output in normal operation.
>
? running the exciter at full output is better. There should be no
situation - including overshoot - where it is possible to draw grid
current in AB1. .
>
- cheers, Tom
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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