[AMPS] Suppressors

[Tom Rauch]

> >Why is it necessary to aim for this specific 'optimum' circumstance. 
> 
> Because the staggared-resonances are optimal when there are equal 
> currents through R-supp and L-supp at the VHF anode-resonance freq. 

Staggered resonances? 

The only resonance important to "stagger" for stability is the 
parallel resonant frequency of the grid and the parallel resonant 
frequency of the anode.

> If one wants to reduce 10m dissipation in R-supp and lower VHF-Q roughly
> 50%, nichrome  is a sound engineering choice.  .  

Suppressor Q is a tiny fraction of the anode system's impedance. If 
you look at VHF amplifiers, they have extreme values of VHF Q yet 
are almost always perfectly stable. 

It's important the anode impedance be a value that produces very 
little gain or the phase shift is such that the amplifier does not 
become an oscillator at the frequency of instability, where the grid 
is no longer an effective shield.

There are dozens of ways of accomplishing that.

Nichrome simply adds series-resistance in the primary path of low-
frequency currents. It offers the largest change in loss in the 
suppressor at DC, and the least change at the highest frequencies. 

Nichrome is a good choice when the PA has instability near the 
operating frequency, and you don't want to correct the design flaws 
making the PA unstable near the operating frequency.

The biggest flaw in the after-market suppressors sold is they have 
low values of series inductance, making VHF Q higher than a 
conventional suppressor with more inductance and a modest value 
of shunt R in some systems.

I have to chuckle when low Q HF nichrome suppressors are 
installed, and people talk about how "smooth" the amplifier then 
tune on ten meters. Of course they do, the tank system unloaded 
Q at 28 MHz has been noticeably lowered!


73, Tom W8JI
w8ji@contesting.com

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