[AMPS] Feedback path 3-500Z

[Peter Chadwick]

Tom says:

>Placing unloaded chokes in grid leads is very poor engineering.

Hmm......I think you mean 'at HF, using usual chokes'. It's not an uncommon
practice in VHF amps, and used to be pretty usual in 'good ol' fashioned' Class
C designs at HF, always with the caveat about using different chokes in plate
and grid circuits, and getting the plate choke 'in circuit' resonance below the
grid choke 'in circuit' resonance. I reiterate that these are different and
separate resonances to those caused by parasitic effects in the choke itself.

With the above caveat, I'd agree: personally, I've always been a believer in a
series resistor with a grid choke to damp it down if it isn't meant to be
broadly self resonant at the operating frequency.

Interestingly, the 'good ol' designs' sometimes used a plate choke with a
coupling capacitor: if the grid leak was too high, you could get a gain peak at
LF where the choke in shunt with the plate and grid and stray capacities
resonated. I remember one tx that used resistance wire in the choke to get
around this; unfortunately, it had a tendency to suffer with corrosion, and
unless it was replaced by a manufacturers correct replacement, there was
trouble! As the choke looked like a standard 4 pie 2.5millihenry choke, you can
imagine that a number got replaced with the wrong (and much cheaper) part.....

73

Peter G3RZP



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