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Re: [Amps] Fwd: suppressors

Subject: Re: [Amps] Fwd: suppressors
From: "Robert Bonner" <>
Date: Fri, 13 Jul 2007 16:37:08 -0500
List-post: <>
Oh yeah hey guys...  I want you all to meet my good friend POINDEXTER
(Steve SF).
He was on TV when we all were kids.  His arch nemesis back then was MASTER

I don't think he was properly introduced before taking off on the
RE-engineering diatribe.  When he was working on his Masters of EE he
experimented with high doses of B24 vitamins and it caused 100% retention of
his textbooks.  Scary he can't remember his way home from work or his kid's
names but he can calculate PI to the last digit in his head. 

Steve I thought you were going to call about the trip?  Tom ZR is thinking
of riding down at the same time.


-----Original Message-----
From: [] On
Sent: Friday, July 13, 2007 12:09 PM
Subject: [Amps] Fwd: suppressors

Absolutely right Tom.

If whomever is dealing with passband stability, the suppressor is almost out
of the circuit.  It is some small series jXl, where it will matter little
with the high operating impedance of the tube, and the Pi L or Pi net it is
feeding !

Without going into S parameters and analytic stability analysis, I will give
the reader's digest version (math is easier, but does help explain the
physics of the problem).

Inband stability is probably much more a function of phase shift from input
to output because of the input and output tuned ciruits, gain and isolation
where enough feedback in an amplifier with little phase margin (the input
and output are almost in phase, when instead, they should be nearly out of
phase) cause an oscillation (isolation does not overcome gain).  Note that
each Pi or L network will introduce large phase shifts, and the sum of these
thru the amplifier can leave it such that the amplifier output is in a non
benign phase at the input such that poor isolation will make it sing inband.

I had an amplifier from a manufacturer recently that was not unconditionally
stable.  That is, some setting of tune and load settings would cause the
thing to take off on 10m.  The amplifier has a pair of 4cx800s and is high
gain (about 17 dB).  I reviewed the schematic and modified the input circuit
to improve phase margin by changing the tuned input to include some zeros
instead of poles (series L and shunt Cs are lowpass or poles, series Cs and
shunt Ls are highpass or zeros).  

Wa-La ! Amp fixed, it did not take off anymore.  I trapped the oscillation
on the scope, and it was in the low 30's of MHz, meaning it was inband.

My point is this: if you have an oscillation, you must first determine where
it is: inband or outband (VHF/UHF).  If it is inband, suspect bad phase
response because of too many poles (or too many zeros) in your matching
systems.  As an example, here's a rough count:

 - Coupling Cap: Outband Zero and essentially non inband phase affecting.
 - Input Match: 3 poles and inband affecting.  Consider changing this to a
highpass design (series C, shunt L, series C) to balance poles and zeros.

 - Plate Blocker:  Outband Zero, non inband phase affecting.
 - Suppressor:  Outband Pole at least until 10m, non inband affecting.
 - Output Match:  Pi or Pi - L almost always 3 inband poles for the Pi, and
4 for the L.  Definately affects inband phase response.

The low hanging fruit is to change the input circuit from low pass to high
pass which will probably improve the phase margin such that the amplifier
will be stable again inband.  

Having said all of this, a relatively low gain grounded grid amp (8 - 12 dB)
should not take off inband regardless of what tuned input is used; the
output circuit should be sufficiently isolated such that even -20 dB
isolation overcomes the gain of the tube provide there is some phase shift
from in phase.  But, in the case of a grid driven Tetrode, or high gain
triode, you may not have enough isolation, and if the poles and zeros in
your amp (plus the 180 shift of the tube) cause and in-phase relationship at
some frequency...that amp is on its way to the moon.


Steve, K0SF

----- Original Message -----
From: "Tom W8JI" <>
Sent: Friday, July 13, 2007 11:14:45 AM (GMT-0600) America/Chicago
Subject: Re: [Amps] suppressors

> Steve said:
>>In every case so far, copper gives

> better 'goodness'.<
> Which, from purely a theoretical viewpoint, is exactly 
> what should be expected.
> 73
> Peter G3RZP

N7WS measured identical magical nichrome and conventional 
Ameritron suppressors, and found at VHF near the frequency 
of suppression there was essentially no difference at all.

As frequency was reduced to HF and lower, the nichrome 
suppressor remained lossy.

A nichrome or other lossy resistive conductor suppressor is 
NOT a "low-Q VHF suppressor". It is actually a low Q low 
frequency suppressor.

This makes perfect sense and follows conventional wisdom, 
since the primary path for lower frequency signals is 
through the inductor. The primary path for VHF and highest 
frequency signals shifts into the resistor. Thus the coil 
loss affects dc the most, and UHF the least. The resistor 
affects UHF and the highest frequencies the most, and dc the 

By adding a resistive conductor we lower HF Q the most, 
while barely changing VHF Q.

If I had an amplifier that was unstable at or near the 
operating frequency and if I couldn't neutralize the 
amplifier, adding resistance that swamps the signal 
frequency might be a solution. If I really had a VHF 
oscillation in an HF amplifier then the solution would be 
increasing inductance and SHUNT resistance.

It's amazing how such a very simple system has become so 
misunderstood, almost to the point of being voodoo.

73 Tom 

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