[Amps] parasitic oscillation techniques

Sun Aug 23 10:08:06 PDT 2009

Ive always enjoyed your comments about what happens when BIG tubes act up 
John. Especially the fun dumping the PS.

Having spent the last 10 or so years of my career in the 300 mHz to 75 gHz 
range I can appreciate the use of absorbent material to tame the little 
beasties, especially high gain multi stage MMIC's

The compact SB-220 layout minimizes plate lead heating from other 
resonances. The biggest other heat source there is the non RF rated blocking 
cap.

I have used 1/2" wide by .003"  mu metal ribbon in other amps to snub blips 
seen on the VNA. And yes the B+ choke can easily be part of the problem as 
Ive mentioned over the years.

Murphy will take any little misstep on the designers part to have his 
enjoyment. Unfotunately for hams they are usually the ones used as test 
beds.

Oh, getting back to Measures for a moment I remember calling Eimac shortly 
after the QST article came out as I was also a pre Richardson reseller at 
the time. The comment I most remember from the Eimac engineer is that 
Measures was responsible for more destoyed 8877's than all other sources 
combined since the tube came out. I couldnt take that lightly in a typical 
HAM fashion and included a warning with every tube sold that the warranty 
was void when following Measures "advice". I told that to Rich several times 
during the forum wars but only got a lot of 2 step dancing and plenty of 
magicians smoke in return.

All said and done Rich has come up with many better ideas in dealing with 
amps and shouldnt be completely dismissed because of  his views of 
parasitics.

Carl
KM1H

----- Original Message ----- 
From: "John Lyles" <jtml at losalamos.com>
To: <amps at contesting.com>
Sent: Sunday, August 23, 2009 12:15 PM
Subject: [Amps] parasitic oscillation techniques

> The discussion of they what, why and how of parasitic suppression has
> always been controversial on this forum, especially when AG6K was here
> with his own retrofit kits and QST articles. I think there has been a
> huge volume already said for a dozen years on this, and I will add my
> own two cents worth, based on experience, not what I read or heard. I'll
> first add to Lon's explanation of the dominant parasitic mode in many
> 3-500Z designs (and other medium power amateur tubes). The circuit
> inductance from the anode cap through the first shunt tuning capacitor
> tends to have enough L by accident (not by design) that it easily
> resonates with the stray C from the tube and the tuning capacitor C.
> This was discussed at length in the out-of-print book RF HEATING TUBES,
> from Mr. Dietrich of Philips Electronic in Eindoven, along with models
> showing how it happens. This one is easy to find with even a grid dip
> meter, but a network analyzer demonstrates its presence too. When we
> design for HF, we are looking at component values with uH and many pF.
> At VHF, it only may take nH and pF to allow sufficient impedance rise.
> This is why layout, lead length, lead diameter, are all important to
> good amplifier stability. Also, these hard-to-quantify strays are what
> makes it seem like hit-or-miss design to dampen elusive parasitic
> resonances. Once this happens, and the tube still has gain with enough
> feedback inside, you have a candidate for power oscillation or other odd
> behavior in the 100's of MHz. This seems to be the reason that certain
> tubes are more prone to the problem when circuited at HF.
>
> As a power amplifier designer of commercial/industrial circuits with
> large tubes, for nearly 30 years, I have had my own personal battles
> with elusive demons in amplifiers and power oscillators. In some
> circuits, the tendency for acting up has been minimal. But precautions
> have always been taken to watch where stray inductances are, and to
> measure, sweep the circuits BEFORE applying HV. I have made use of
> nichrome as a lower Q wire, for various 'suppressors' before I heard of
> their application in SB220s. When I shared this info with Rich Measures
> 15 years ago. But I am not an advocate of application of this technique
> in every tube amplifier, as it all depends on the individual circuit
> values and the requirements for power, frequency.
>
>>         The self resonance that causes most of the problems in the SB-220 
>> is
>> around 90 to 105 MHz  It is easily measured, both in frequency and "Q". 
>> In
>> the TL-922A there is also a high UHF self resonance that will vaporize 
>> 1/2
>> (only one side) of the double-sided twenty meter fixed contact on the 
>> band
>> switch.  Relate the size of one side of the contact to frequency and you
>> will get the frequency of the self resonance.  It is interesting to note
>> that the original TL-922 (not the 922A) dose not have this problem.  This 
>> is
>> because of a very slight, physical, re-design of the final compartment of
>> the later TL-922A that was sold on the American market.   Good parasitic
>> suppressors will de-tune/attenuate these self resonances, hopefully to a
>> point where they will be harmless.
> .....
>>  73 de Lon, K5JV
>
> As Carl mentioned here, there are other factors in amplifiers that can
> also cause RF instabilities, including the B+ choke, and VHF/UHF
> cavity-type resonances. These latter forms can be quite destructive and
> troublesome, especially when one is trying to design a VHF amplifier
> using coaxial or radial transmission line geometry. In my early career,
> I was able to stabilize a commercial amplifier that used a 4CX3500A
> tetrode in a coaxial output circuit by using a combination of nichrome
> wire L and stray C alone. This little LC tab was applied at the right
> place, to effectively quiet down a nervous amplifier that would have
> otherwise been a commercial failure. In another application years later,
> I did a somewhat thorough study of the application of nichrome, steel
> and copper wire, to large B+ chokes in 27 MHz power oscillators (30 kW
> CW). Not only was the  conductor compared, but also the geometry of the
> chokes, length and width, pitch, using a Hp Q meter followed up with
> power tests. The biggest problem there was that the resistive alloy was
> so lossy that it was glowing dull orange due to the plate current alone
> (5 amps DC). Then there are designs that never seemed to need parasitic
> suppressors, remembering one 8877 amplifier at 5 MHz that I built.
>
> Presently am finishing a large amplifier circuit design that has
> occupied me for a number of years as a part-time endeavor, now a full
> press project that has a commercial deadline. Due to the large geometry
> of the power tube and the need for it to develop 3 MW of VHF power,
> there is a significant money and time being allocated for suppression of
> parasitic cavity modes, even before the thing has been turned on. One
> missed high Q mode may destroy the $200K device. Big tubes (that require
> cranes and hoists for lifting) are prone to supporting UHF and L-band
> oscillations due to circular path geometry inside the radial beam tube
> itself. This one will not be easily stopped using lossy wire, instead
> using lossy bulk material (Eccosorb) in waveguide-style mode dampeners.
>
> So I guess my point here is that these things - suppressors - are not
> artforms, but are practical engineered devices, that have their place in
> power tube circuitry. It is unfortunate that their discussion among hams
> (and among some non-ham high power circuit designers - I have been there
> too!) tends to degenerate to the level of unexplained phenomena like
> UFOs. We have powerful tools that we can not use to demonstrate these
> problems in circuits. What we don't have is good knowledge and models of
> the tubes themselves, as to how they are easily tickled into laughter
> (unwanted oscillation).
>
> 73
> John
> K5PRO
>
>
>
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