>> hi all,
>>
>> > 'Super cathode driven' applies to a tetrode where the
>> > screen is grounded and the grid is operated at 0 RF-V
>> > with respect to the cathode. Thus, there is never any
>> > grid current in a super cathode driven amplifier
>> > circuit.
>>
>> not necessarily. 'super cathode driven' is where a fraction, call it "k"
>> of the driving potential applied to the cathode is applied to the grid
>> as well, thus providing some negative feedback. if "k" is zero, we have
>> normal cathode-driven operation (i.e. the grid doesn't move, the cathode
goes
>> up and down.) if "k" is unity, the grid chases the cathode up and
>> down, tracking it exactly. it is *definitely* true, however, that the
>> most reasonable thing to do when using this mode with a
>> tetrode is to make "k" equal to unity, i.e. the grid and cathode are
>> at the same rf potential, and therefore there is no chance of the
>> control grid drawing any current (assuming the tube is biased at a
>> reasonable operating point.)
>That's correct George. Orr (the person who coined the name "semi
>and super cathode driven") applies description to ANY PA (
>including triodes ) using a grid tied back to the input directly
>or through a divider (the divider also includes stay capacitance
>in the tube) and illustrates and describes the use of triodes in his
>July 1967 QST article.
George: So is 'semi' where K is less than unity, and 'super' where K is
unity?
>In the effort to make arguments, people have ignored the references
>and made up their own definitions.
I conceed that I overlooked the effect of stay capacitance, Tom.
>The important point I was making is the grids belong grounded in a
>grounded grid PA. Any time any grid is RF coupled directly or
>through a divider to the input or output circuit, stability is decreased.
Apparently, Collins didn't think so. I have never heard of an
instability problem with the Collins 30S-1 amplifier, and the grid is RF
coupled directly to the cathode.
>A second flaw is any internal tube arc is easily passed on to the
>cathode and other circuits when the grid floats.
Three out of the five 3-500Zs with bent filament helices were removed
from an amplifier (LK-550) wherein the grids are directly-grounded.
...snip...
>... ... A simple voltage division
>analysis that includes grid/cathode dynamic impedance will prove
>this.
Please show us the analysis, Tom.
>73, Tom W8JI
Rich---
R. L. Measures, 805-386-3734, AG6K
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