And a cross-over pair of 'caps' for
neutrilisation. In the end I actually removed these cross-over wires, as
I found I could only get more RF feedthough with them when measured on a
diode detector with cold tubes, not less. It was stable without them -
Hi David ,
you were young , but quite lucky I think. I did some experiments on
neutralizing yesterday on a Motorola quarter KW , which is a PP conduction
cooled 4CX250 equivalent tube .
This amp was modified by myself on 144Mhz , and is in a stage close to be
"fired up".
I measured the reverse gain with the original neutralizing circuit ( abt
the same than W1SL design you are talking abt , but with serie inductors
in the feedback loop) and found it to be only -26db . The forward gain is
expected to be around the same figure , so the safety margin for stability
is 0db! The neutralizing circuit was probably OK on the unmodified amp
design frequency but not good on 144Mhz.
I took off all the feedback links , and measured reverse gain for fun : it
was then -13db , so with no neutralizing this amp is an expensive
oscillator.
Doing the same shape links like in the original amp , but without
inductors gave a -35db reverse gain , then perfectly OK , but consistency
of this figure with different tubes is questionnable , so I added series
variable caps in the links and could tune the reverse gain to -40db . I
verified on a network analyser this solution was not too " narrow band " ,
and found it was OK. Playing with loop shape can give -50db , but it is a
marginal improvement in my opinion.
Probably the reverse gain you get in the w1sl design with noneutralizing
caps was a bit higher than the one in Motorola's amp because your tube
sockets are better than the simple one used in the quarter KW , but safety
margin must be quite low.
Sorry for my " far from perfect " english
73's Dom/F6DRO
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