I think that it is not "if" but "when" in Toms first statement. Most moving
coil meters
wouldn't even start to react to a minor grid current spike. What is needed is
the drive
being backed off to the point that the grid can never show any current, a pretty
impossible situation with most transceivers/amplifiers.
However, running a small amount of grid current in some tetrodes actually
improves the
IMD, the GEC data sheet for the 4CX250B recommends 250uA of grid current for
best
IMD performance.
I have learnt from many years of operating tetrodes such as the 4CX250 that a
simple
shunt stabiliser using a bunch of zeners for the screen supply is no good or a
grid bias
derived from a simple supply with a mere potential divider is also asking for
trouble.
Having experimented extensively with active screen and grid stabilisers for
4CX250's
I have seen the vast improvements that can be made with a little extra
circuitry.
For those interested Ian, G3SEK, book contains suitable circuits which are
highly
recommended.
John ZS5JF
----------
> From: Tom Rauch <w8ji@contesting.com>
> To: amps@contesting.com
> Subject: Re: [AMPS] RE: 4cx10000
To: <amps@contesting.com>
> Date: 10 July 1999 04:05
>
>
> 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.
>
>
> 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.
>
>
> 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.
>
Wouldn't it be a perfect world if we could convince the average amateur to
invest in such
things!
>
> 73, Tom W8JI
> w8ji@contesting.com
>
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