>> How can a series resistance of 100 Ohms or so be greater than the K-Ohm
>> impedance of the anode circuit?
>
>I assume you mean the time varying resistance of the anode as
>modified by stray reactances.
Time varying resistance of the anode? You must be suggesting that the
anode impedance changes over the RF conduction cycle. However, most
specs sheets always list the anode impedance around a couple of K-Ohms.
So I assume then, if you are correct, that this is the average impedance.
>
>The tubes anode system, so far as that time-varying resistance, does
>not come into play. We are only looking at a termination for that
>system, so source impedance does not concern us greatly.
Source impedance does not concern us??? Tom, I think you need to go back
and re-read your initial RF transmission line theory textbooks. If
source vs. load impedance doesn't concern us then why don't we just run
PAs and transmitters into any SWR? Source impedance is of extreme
importance when we are talking about terminating a network. Perfect
power transfer to a load occurs when the load is the complex conjugate of
the source. In other words if the source has an impedance of 1700-j35
Ohms the load needs to have an impedance of 1700+j35 Ohms impedance. So
it DOES concern us.
>
>The requirement is, if you could stand at the anode and look outward,
>that you would "see" a load resistance that is primarily resistive
>and dominates the lossless reactive part of the load. That guarantees
>controlled phase shift, and the high resistance reduces VHF gain (the
>mechanism for gain reduction is the shunting anode capacitance).
OK. But the resistor is going to have some inductance which will give a
varying reactance over frequency. But I assume though that you are
saying it is a small reactance. This makes some sense.
>
>There are two mechanisms at work.
>
>One is the reduction of Q because the resistive part of the anode
>load impedance external to the tube dominates, and resonance (and
>uncontrolled phase rotation with frequency) is reduced. To meet
>this criteria the system, looking from the anode outwards, must
>look like a resistance is a major portion of the load impedance.
>
>The second criteria is the resistance must be large enough
>that the shunting capacitance of the anode can reduce high
>frequency gain.
Ok. Makes sense too.
Although I must say that the explanation given by Ian White does make the
most sense. By applying series to parallel conversions and doing good
old fashioned network analysis, we can actually make a 100 Ohm resistor
look like a couple of K-Ohms at a given frequency. So in a sense it does
absorb the energy.
>
>
>> The conventional concept of a supressor is that the coil chokes the VHF
>> signal and the resistor then absorbs it. However, if you think about
>> impedances and such, that doesn't make sense.
>
>No, it makes absolute sense. The resistor dominates the system at the
>frequency of oscillation, because inductor impedance is high. It's
>tough to build a TPTG or TPTK oscillator when the anode tank is a
>resistance.
>
>If the resistor "absorbs" anything the system is less than ideal.
>It's function is to load the system, not absorb energy. Sometime we
>can't help having it absorb a little "good" power, but it certainly
>doesn't have to do that.
Well, looking at Ian's analysis, the resistor DOES absorb VHF energy.
Also a 50 or 60 nH inductor is a high impedance at VHF but not extremely
high. Due to my current vocation of technical sales and not engineering,
I do not have a scientific calculator at my office so I can't calculate
the impedance here, but I could figure it out later.
So inductor impedance isn't really a choke.
Looking at it all, I think that using the network analysis idea as Ian
did as well as what you say Tom, makes sense. And anyhow, Tom, you and I
were saying about the same thing originally and that is that the
supressor de-Qs the circuit and makes it less prone to oscillation as
well as providing some absorption. Of course, if that resistor IS
absorbing VHF energy, there is still an oscillation occuring! The whole
idea is to stabilize things in order to prevent the amp from taking off.
Now, I don't mean to start a war, but you and Rich Measures (AG6K) say
about the same thing regarding the function of supressors. So where is
your beef with him? You can answer that one personally to me if you wish.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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