There will be virtually no difference in loaded Q of a suppressor using
silver plated ribbon or thin nichrome wire or anything in between. The 33-47
Ohm resistor is the Q controller.
Where nichrome has an edge is with multiple resonances and a single tuned
suppressor only controls one of them.
Multiple resonances can include the effect of the RF cage creating a basic
cavity resonator.
Deciphering it all really requires a spectrum analyzer and extensive amp
design experience in the real world.
Replacing RFC-2 with a 15-20 Ohm 25W real enamel wirewound (nicrome wire)
eliminates the problem caused by that chokes resonance a bit above the
3-500Z's parasitic resonance. Plus acts as a HV current limiter.
BTW, the strays have minimal effect on a parasitic. If you note, the
suppressor is before any LC circuitry. Tubes have their own built-in natural
parasitic resonance and if it is below the frequency of the tubes ability to
amplify it must be suppressed.
Carl
KM1H
----- Original Message -----
From: "kingwood" <k5jv@kingwoodcable.com>
To: "Amp Reflector" <amps@contesting.com>
Sent: Sunday, August 23, 2009 9:28 AM
Subject: [Amps] Design VS parasitic
> Greetings to all,
>
> A couple of comments to my last post have prompted another reply.
>
> The question must be asked, "What causes a parasitic oscillation,
> in
> the first place?" The very basic answer is "strays"; stray inductances,
> and stray capacitances that combine to produce stray resonances. The very
> best design is going to have numerous stray resonances (some say infinite)
> .
>
> Even with the best design techniques, it is impossible to design an
> amplifier that is totally without stray, thus self resonances. Thank
> goodness, most of the resonances are completely harmless and unless you
> have
> similar resonances in the input and output circuit, at the same instant in
> time, (remember, they change due to time constants in circuit components),
> they are all harmless.
>
> 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.
>
> The next question that has to be answered is, "Why do these self
> resonances cause damage to the amplifier?" The answer is very simple,
> LOAD,
> or the LACK of a LOAD for the resonance. Remember we are not talking
> about
> fundamental parasitic, but VHF/UHF parasitic. The antenna supplies the
> load
> for the fundamental, but there is no load for the VHF/UHF parasitic. This
> energy must be dissipated somehow, thus the "big bang", burned up band
> switch contacts, and what ever else, that we are so familiar with.
>
> Now, if we know the frequency of this unloaded parasitic, why can't
> we build a trap, or load, inside the final compartment to absorb it before
> it does any damage? This idea has been around for years. I spent a lot
> of
> time trying to doing just this about thirty years ago and finally gave up.
> The idea is not practical for several reasons, but mainly because the self
> resonances change, as mentioned above. This brings us back to good
> quality,
> low "Q", traditional suppressors in both the anode and cathode of the
> amplifier.
>
> 73 de Lon, K5JV
>
> 1110 Golden Bear Ln.
> Kingwood, TX 77339
>
> 281-358-4207
> 281-358-4234 FAX
> 281-795-1335 CELL
>
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> Amps@contesting.com
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