> You may remember a couple of weeks ago, I asked for
> comments on the amplifier that was OK at 3.5MHz and went
> mad at 3.7MHz. At last, I'm at home for long enough to
> actually look at it. Someone suggested that the 'non
> inductive' grid shunt resistors wound Ayrton Perry style,
> might have had one winding go open, and this was indeed
> the case. There were three 80 ohm ones in series: one was
> bad. The bad one measured at 158 + j22, while a good one
> measured 80 + j7. Some of the inductance was doubtless
> down to the leads - I estimate about +j2. Interesting
> though, because we still have a Q of less than 1 for the
> whole grid circuit, so it's somewhat surprising we can get
> an oscillation.
Why? Q of one small part of a complex system doesn't
determine anything by itself. If it did, we'd never be able
to build a R/C oscillator circuit now would we?
Many of these resistor swamped grid driven PA's are hanging
on the very edge of the instability zone. Look at the grid
impedance, the anode impedance, and the feedback of the
system. About half the time I have to neutralize resistor
swamped PA's even when grid circuit Q is far less than
1....and I use a dozen or more real non-inductive resistors
mounted radially right around the grid collets, not the
pretend non-inductive mounted several inches away.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|