[Amps] VHF suppression systems

[Tom W8JI]

> 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