>> what reason is there for a specific value of resistor in a
>> parasitic suppressor. ?? I have seen 50 ohms, 100 ohms, and two 50
>> ohms in parallel.. ! Is there a valid reason for using any particular
>> value ??
>Sure, here is how it works.
>The resistor is the component that mostly dominates the system at
>higher frequencies, while the inductor dominates at lower
** The purpose of a parasitic suppressor in a HF amplifier is to de-Q
the VHF anode-resonant circuit formed by the anode-C, the anode lead
inductance, and the Tune-C. The reason we would like to de-Q this
circuit is that the VHF ringing-V produced by ordinary rapid changes in
anode current is proportional to Q. Since the tube has internal
feedback-C, plus VHF gain, oscillation is not impossible.
- Typical anode-resonant frequencies for HF amps are 80 - 180MHz. To
complicate the matter, the anode lead appears to act like a 1/4-wave line
as well as a 3/4-wave line, on and on. The 3/4-wave resonance is not
much of a problem with 3-500Zs and the like because the tube has little
gain in this region. However, compact ceramic tubes have gain into the
UHF region and may be less than stable. For example the Dentron MLA-2500
amplifier has three anode-circuit resonances below 300MHz, any one of
which could support an oscillation because the tubes  used are
rated at 500MHz max.
>The reason is the frequency slope of reactance of the
>inductor, which shifts more current into the resistor with increasing
>The resistor value has to be reasonably large compared to the overall
>impedance of the path from the tube to the tank capacitor to ground
>and through the chassis back to the tube.
>Since the parallel equivalent impedance of the resistor and inductor
>is in series with the anode path, a larger value of resistance and
>inductance will de-Q the system and add more loss at higher
>frequencies where the tube might tend to oscillate.
** The best compromize seems to be when the current through the resistor
and inductor are equal at the anode-resonance (X-ohms =s approx R-ohms).
>A tube that tends to oscillate very high in frequency (very short and
>wide grid leads) requires less inductance, and a tube with long thin
>grid leads requires more inductance.
>A tube with a long thin anode lead or a long connection to the
>chassis through the blocking cap requires more resistance to de-Q the
>system, because series impedance is higher.
>A larger resistor value also requires more inductance, since the
>"current shift" into the resistor is proportional to the the ratio of
>resistance to reactance.
>The inductor value has to be large enough to be sure the resistor
>dominates the path from the tube to the tuning capacitor at the
>frequency where the tube is unstable.
** good point, but the trade-off is that more L-sup means more heat in
the R-sup at 29MHz. If R-sup burns out, Q increases and the chance of
oscillation increases. ["Calculating Power Dissipation in
Parasitic-Suppressor Resistors", March, 1989 *QST*]
>In a grounded grid amp, that frequency is right around the frequency
>where the feedthrough attenuation is minimum. With a 3-500Z with
>short directly grounded grid leads, that is about 180-200 MHz.
** With my dipmeter, the resonance of a 3-500Z's directly-grounded grid
is a bit under 90MHz. Above this resonance, the grid looks more and more
transparent to the damped-wave ringing in the anode resonant circuit
being produced by sharp changes in anode current. In other words, above
grid-resonance, the feedback signal progressively increases with
frequency as the gain of the tube decreases. Thus, using a tube with
mo' VHF-gain may not be mo' bettah unless one is trying to build a VHF
>811A or 572B tube, it is around 60-100MHz.
>With a 811A or 572B (or 3CX1200A or D7) with long thin grid leads,
>you need more inductance and more resistance.
The 3cx1200D7 has minimal grid-L. The 3cx1200Z7 has even less.
>The tube tends to
>oscillate lower in frequency. That makes the system hard to
>stabilize, because the maximum operating frequency is generally only
>about 30-50% of the frequency of instability.
>With a tube like a 3-500Z, you need a modest resistance and much less
>inductance. The amp is much easier to stabilize, and the suppressor
>(if the anode and grid leads are kept short) can have very few turns
>and use a modest value of resistance.
>With a tube that has no appreciable feedthrough way up into or near
>the GHz range, like the 8877 or other coaxial grid tubes, you can
>often not worry at all about any suppression
** To see the result of not worrying about using a parasitic suppressor
on an 8877, see Figure 24 on my Web site. As I understand it, a medical
MRI-RF-amplifier manufacturer in Colorado lost a lucrative contract with
General Electric Co. because he refused to improve the VHF stability of
his 8877 amplifiers. (In Sept. 1935, G. W. Fyler of General Electric Co.
"In the elimination of parasites from a transrnitter, the circuits should
be kept as simple as possib1e to prevent complex resonance condition..
Radio frequency choke coils and shuntfeed circuits should be kept at a
minimum. Wide band neutralization circuits are desirable. The grids of
vacuum tubes should be effectIvely by-passed capacitively to the cathode
through a capacity, and inductance added next to the plates of the tubes
to eliminate shortwave parasites. If necessary, the plate or grid
parasitic circuits should be damped with resistance. Inductively coupled
rather than capacitively coupled Input and output circuits should be used
wherever possible. The mechanical layout of a shortwave transmitter
should be well planned with short lesds and compact tank circuits to keep
the current where it is supposed to be and to minimize stray coupling
>or use a very minimal
>suppressor (like a length of brass strip) since normal transit time
>of electrons as well as external circuit loss reduces gain far below
>critical levels without any or with minimal suppression.
>You can also, in some cases, add enough anode lead of the correct
>width to simply move the anode resonance far below grid resonance,
** Which sounds plausible but the rub is that this solution can reduce
the 3/4-wave anode resonance to a point where oscillation is not
-- Murphy was right: things are often more complicated than they look.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,