[AMPS] RE: Two Questions.

Tue, 9 Sep 1997 08:54:33 +0000

> From:          Peter Chadwick <Peter.Chadwick@gpsemi.com>
> Date:          Tue, 9 Sep 97 08:32:08 +0000

Rich claims:
> > Basically, VHF voltage-amplification is Mu*VHF-Rp

Peter replied:
> No, it isn't. 

Very true Peter, but you are beating your head against a wall.

> You cannot ignore the plate resistance, and strictly, you cannot ignore
> the non zero value of the tank circuit impedance when considering the
> VHF gain, although it may be (should be) very low. Indeed, the fact that
> it isn't is what causes the trouble.

Again absolutely correct.

> What value of shunt resistance is used in the W8JI type suppressor?

100 ohms special order carbon **composition** (NOT FILM)  type.

> Why
> can't you get the same lowering of impedance by dropping the value of
> the shunt resistor?

You can, but the PA will be less stable because the resistance has to 
dominate the VHF path impedance to minimize Q. Less resistance is 
LESS stability in this specific case.

> My (possibly faulty) understanding of the way the
> suppressor works is that the inductance is added to increase the amount
> of inductance in the circuit so that the shunt resistor is then damping
> a higher proportion of the parasitic circuit, thus reducing its gain.

Absolutely.

> The higher the stray inductance in the parasitic circuit ( bad layout
> etc), the bigger the suppressor choke has to be. 

Absolutely, at least outside of California.

>The bigger the
> suppressor choke, the more volts of fundamental appear across it, and
> the more fundamental power gets dissipated in the resistors. If the
> value of the resistors is  too small compared to the reactance of the
> choke at the parasitic frequency, then the effective impedance that the
> suppressor offers drops too far, and the assembly looks like a lossy bit
> of wire.

And the system Q and instability or the chance of instability  
increases at VHF! 

> So there's an optimum value for the impedance of the assembly
> at the parasitic frequency, and too low is just as bad as too high.

Absolutely Peter.

> After doing a few sums, I'm not convinced that the nichrome approach
> doesn't have advantages in some circumstances - at least you aren't
> searching for high power (5-10watt) low capacitance, low inductance
> resistors.

I also agree the nichrome has advantages. If the PA oscillates at low 
VHF, or near ten meters, it de-Q's the HF tank system and lowers HF 
gain. Unfortunately it usually INCREASES  VHF Q.

I measured the Q of a Ten Tec Titan with the nichrome mods, and the 
VHF Q was nearly 30% higher with the nichrome mods than with the 
stock suppressors. A similar thing occurs in the AL80 series of 
amplifiers.

A lot of people have been duped into thinking these "mods" improve 
VHF stability, when they often do exactly the opposite. When QST 
discovered that, they got pretty red in the face.

The AL-80B suppressor was designed with a network analyzer measuring 
the Q at the anode lead looking out towards the tank, and the values 
selected for minimum Q at the frequency of oscillation. It's sad to 
see good design work and accurate measurements tossed out by a 
quack with a GDO.

73, Tom W8JI

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