> In your example, you used a 68pF cap in series with the
> resistor. I noticed
> that your results showed this caused the current to shift
> to the resistor much
> faster and it occured between 50 and 60MHz. At 6M or
> 50MHz, would this shift
> be unusable for a 6M amplifier, or would it actually occur
> at a point that
> would be beneficial to quickly cut off frequencies above
> 60MHz?
Suppressing a tube that oscillates on 180-200MHz without
hurting the resistor on 50MHz can be a problem.
I can think of three ways to do it:
1.) For a single band 50MHz amplifier, you might try a
parallel resonant 50MHz trap in the resistor side of the
suppressor. This would notch 50MHz from the resistor. The
problem is you still need a high Z across the "inductor" at
180 MHz to make the resistor a large part of the anode load
impedance, and the capacitor might have some fairly high
voltage.
2.) Another idea is to parallel tune a section of the anode
lead for 180MHz by shunting a small section with a
capacitor. The C/L combination would resonate that small
area of anode lead at 180MHz, and you could put the resistor
across that section.
3.) You could do a flapper plate capacitor near the anode,
and have a series L and R to ground resonant at about 180
MHz (for the 3-500Z). I've used something similar on very
high power 100 MHz amps (50kW).
Maybe someone else has a suggestion, but most amps running
near the self-oscillation frequency (a 5:1 spread or less)
need special treatment.
Of course if the anode path presents a very low Z or a very
high R at the anode, it might be stable without any
suppressor. The normal oscillation mode is TPTG, where the
anode high Z is at the same point as the grid to chassis
high Z.
73 Tom
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