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[AMPS] Suppressor effects

To: <amps@contesting.com>
Subject: [AMPS] Suppressor effects
From: w8jitom@postoffice.worldnet.att.net (w8jitom@postoffice.worldnet.att.net)
Date: Wed, 10 Sep 1997 14:34:08 +0000
Perhaps we should look at what we want in a suppressor.
I hope this ascii drawing looks ok after going through mailers.
The system looks like this:

                                     La                     Ltank    
             ___sup____(((((((___________(((((_____
         __I__                               _I_ 
 g  --- - - - -                                ----   C tune  
             ^                                     I
_^_____________chassis_____________________________


Between g and chassis we have an impedance that varies from being 
very low to fairly high. This impedance is determined by the grid 
lead and grid structure stray capacitances, inductances, and 
resistances.

Ltank impedance is a very high inductive reactance at VHF, while C 
tune is very low. The result is the primary impedance of the anode 
system (La) at VHF is inductive, and is parallel tuned by the anode 
to chassis capacitances. 

That makes the anode INSIDE the tube see a very high impedance 
(parallel resonant) at some VHF frequency.

The grid also has the same effect, with the grid to chassis 
capacitance parallel tuning the grid lead and structure inductance to 
resonance at some very high frequency. 

The combination of a parallel resonant grid system and parallel 
resonant anode system, and grid to anode capacitance forms a TPTG 
(tune plate tuned grid) oscillator.   

Of course at other frequencies the anode and grid leads are SERIES 
resonant and present a very low impedance inside the tube.  In this 
case the tube will be more stable. The problem with using a grid dip 
oscillator to look for "problems" is it tells us NOTHING about the 
impedance. It might indicate a strong dip at a good resonance that 
lowers grid and anode impedance, just as easily as it indicates a dip 
at a "bad resonance" that increases anode to ground impedance and 
grid to ground impedance. We'd have no idea which is which.

Only passing a signal through the tube from cathode to anode would 
tell us whether the impedances inside the tube are high (bad 
parallel resonances) or low (good series resonances).  

For example, a 3-500Z's "bad spot" is about 180-220 MHz in most 
good layouts with short direct grid leads. The bad spot becomes 
lower in frequency with longer leads (even if they are through 200 pF 
or larger capacitors). The 3-500Z, in a good layout, tends to 
oscillate between 150 and 220 MHz. In the AL-80B, the oscillation 
occurs somewhere around 200 MHz. This is repeatable and can be 
measured by SHORTING the suppressor (in some cases HV needs to be 
raised to get the tube to oscillate) and looking at the frequency of 
oscillation on a spectrum analyzer.

Feedback occurs from the grid to anode capacitance. This feedback is 
degenerative, and so requires appreciable phase shift to become 
regenerative. The feedback capacitance itself adds a phase lead 
somewhat below 90 degrees to the feedback path.

The main problem is resonances in the grid and anode also add 
additional phase shift, and on some frequency the total phase shift 
of ALL these effects might combine with sufficient feedback and tube 
voltage gain to exceed the circuit loss. If that is the case the tube 
oscillates, if that is not the case the tube will NEVER oscillate 
(even if "rung" by a transient).

What we generally want to do is add enough series resistance at "sup" 
to de-Q the anode path impedance. This resistance loads the system, 
reducing phase shift change with frequency and VHF gain. The reason 
it lowers gain is the system is left mostly with the anode to ground 
capacitance in parallel with the tube's internal resistive losses. 
This reduces VHF gain, and makes the phase shift less critical.

Remember even low values of reactance don't absorb energy unless there
is a resistance or load someplace in the system, so we don't want
infinite resistance either. That's why we generally want something 
between extremes.

As Peter pointed out, and as I have on numerous occasions, there is 
an optimum value of impedance at "sup" that maximizes stability. It 
is not when the parallel equivalent resistance of the suppressor 
system is zero ohms, because in that case the equivalent series 
resistance of the suppressor as a two terminal device is ALSO zero 
ohms.

A suppressor  is more likely to be effective if it's two terminal 
impedance presents a high value of  resistance in parallel with a 
very high value of reactance, or presents a high value of resistance 
in series with a very low value of inductance (either is the same).

What we really need to do is consider the two terminal impedance of 
the suppressor at every frequency, and try to "present"  a load 
resistance to the anode that properly dampens or loads the system. 

The worse possible impedance for the suppressor is a very low 
resistance, because it starts to look like it isn't even there.

Another problem if the suppressor's loading resistance is too 
high the suppressor's inductor just looks like an extension of the 
anode leads, LOWERING the anode parallel resonant frequency. 
That could move the anode resonance near the grid resonance in some 
cases, and form the undesired TPTG oscillator.

A nichrome coil in parallel with a resistor looks more like two 
resistors in parallel over a wider frequency range. That improves low 
frequency stability (where the amp is generally stable anyway, unless 
it's 811A's, 572's or other tubes with long internal leads). But at 
higher frequencies (where most amps oscillate) the Q is essentially 
the same with either suppressor! 

Unfortunately the hairpin suppressor (as used in the Titan 
modification kit) actually INCREASES VHF Q of the anode SYSTEM, 
because the suppressor simply doesn't have enough inductance and 
its two terminal resistance is a lot lower than Ten Tec's 
suppressors. The moderately high anode lead reactance combines with 
the lower series resistance of the nichrome suppressor at VHF to 
increase system Q. 

It's interesting to note that even if the Q of the suppressor, as a 
separate device, is very low... it can raise the Q of the anode 
system when compared to a suppressor with higher self- 
Q.  For example?

A one ohm resistor in series with an inductance of one ohm 
has a Q of one. A fifty ohm resistor placed in series with a 
inductance of twenty five ohms has a Q of two.

When they are connected in series with an anode inductance of 200 
ohms, the system Q becomes 4.5 with the fifty ohm (Q=2) suppressor 
and Q=201 with the lower self- Q suppressor.

That's the danger of looking at the Q of an individual part of a 
complex system, and deciding you have saved the world from 
all potential oscillations.

                      

73, Tom W8JI

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