David Lisney wrote:
>Hi, I stumbled across an enlightening article discussing the construction of
>a twin 4-400 amplifier, it seems the prototype had a parasitic problem at
>160Mhz, it seems the fix was found referring to an Eimac engineering
>bulletin of the time. The article appeared in QST, April 1957... completed
>amplifier used a series tuned circuit with damping resistor from the tube
>grids to ground, incidentally it used NO parasitic stopper in the anode.
>
>
The fundamental thing is that an oscillator (parasitic or otherwise)
requires gain>1 and phase=0. There's more than one way of preventing
this in an amplifier. The usual parasitic suppressor between the plate
and tank circuit only works where the parasitic resonance in the tank
circuit is low Z, or series resonant. It does nothing if the oscillation
is at a frequency where the impedance is high/parallel resonant. From
the circuits I've measured, series resonance is usually lower than
150MHz, so 160MHz might have been a parallel resonance. In that case, a
tuned circuit that provides a notch in gain, feedback, or both is a
reasonable approach. Likewise, feedback capacitance (with angle -90
isn't the whole story) - you have to get the other 270 degrees from
somewhere if it's going to hoot.
Steve
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