[Top] [All Lists]

[Amps] Fwd: suppressors

Subject: [Amps] Fwd: suppressors
Date: Fri, 13 Jul 2007 13:09:14 -0400 (EDT)
List-post: <>
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 

Amps mailing list

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>