> Collins has used negative rf feedback over the tube final and driver
> stages for years. More of this in W6SAIs RADIO HANDBOOKs. The SB 220 har
> rf feed: the grid capacitors and grid to anode capacitances are used as a
> rf voltage divider.
The "Orr system" is useless.
It adds uncontrolled RF feedback with undesired phase shift to the
PA. Feedback level varies with drive power, and from band to band.
It adds instability, and if you use capacitors with long leads (as in
the SB-220) it moves the frequency where the grid is a poor
isolation device lower in frequency.
It's very very easy to prove the Orr "capacitor divider system" a
poor design idea. All you need do is calculate the resistances in
parallel with the capacitances, and calculate the grid-anode
capacitance in triodes, and you'll see it does everything you don't
want.
The correct and reliable way to add negative feedback in a
multiband grounded grid triode PA, especially one with grid current,
is to add a resistance right at the cathode. The Orr circuit belongs
in zero grid current cathode driven tetrodes ONLY.
73, Tom W8JI
w8ji@contesting.com
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