[Amps] Silly question - interim report on results.

[Tom W8JI]

> 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