>
>> >The tank is typically a virtual short for VHF and UHF energy,
>> >because it has a capacitor shunting the input.
>>
>> That's not quite right, Don. The tune C has inherent inductance, so you
>
>The series inductance LOWERS the impedance. It forms a virtual
>short in the cases I have measured.
>
>> get resonances. The resonance frequencies vary with shape, size and
>> capacitance setting, but in all that I have put on the network analyser,
>> the lowest frequency resonance is 'series', or low impedance and the next
>> resonance is 'parallel' or high impedance. I'm using the term resonance to
>> mean points where the impedance goes purely resistive.
>>
>> The one I looked at yesterday was 100pF max, .06" spacing and about 2.3"
>> cube overall. The first, series, resonance came in at 90-150MHz, depending
>> on C setting, and the higher one was in the 200-300MHz region. The higher
>> frequency one was low Q.
>
>I measured a transmitting capacitor commonly used in PAs. It was
>series resonant, and a virtual short, at the frequency where Rich
>claims considerable VHF voltages occur across the terminals of
>the capacitor. We were talking about a specific tank system, and a
>virtual short of a few ohms.
>
>I didn't find a high impedance (above the operating frequency) within
>the range of my equipment, which is 1.2 GHz maximum.
>
>The tank circuit with the transmitting type capacitor typically used
>that I tested was a virtual short for VHF and lower UHF energy.
>
>> The impedance at the capacitor terminal is low either side of the series
>> resonant point. How low depends on how far you move in frequency, but it
>> doesn't suddenly go high impedance at any frequency 'close' to the
>> resonant point.
>
>Exactly. The series resonance produces a wide smooth dip in
>impedance. Rich's guess that it suddenly goes high is a bad
>guess, even though it fits his "theory".
>
>> Adding inductance in series with the capacitor (to simulate the lead from
>> an anode) changes the resonant frequencies, but doesn't change the overall
>> characteristic.
>
>The lead from the anode affect the impedance at the anode, not
>across the capacitor or into the tank. That assumes the tank is laid
>out with some common sense, and the lead is routed from the tube
>to the tuning cap, and then from the tuning cap to the switch and
>the rest of the tank circuit.
>
>The best lead arrangement generally is in one stator terminal of the
>capacitor and OUT the other lead. This lets the capacitor act like a
>low pass feedthrough.
>
>> Adding a L/R parallel circuit in series with the capacitor introduces loss
>> as the frequency increases, but does not introduce multiple resonances. As
>> I understood what he said, Rich suggested that the separate current paths
>> through the L and the R should produce multiple resonances. I said then I
>> thought his analysis was wrong. I still do.
>
>Rich's analysis is wrong.
Rich did not say this. The discussion of current division in L-supp and
R-supp had Nothing to do with the discussion about series resonances in
C-Tune.
> I can't measure multiple resonances here
>either. There are some small ripples in impedance, but nothing like
>Rich claims.
>
I have made no measurements of series resonance in Tune Capacitors. I
lack an Impedance Analyzer. I depend on others' measurements.
>I think the problem is Rich only has a grid-dip meter. All of his
>resonance theories, for grids...anodes...whatever, are based on a
>dip meter.
>
>Dip meters do not measure system impedance or the resonance
>mode or Q,
... duhh
>they measure signal suck-out through magnetic field
>coupling to an oscillator to whatever is in the field of the GDO tank
>coil.
>
>Why not measure the impedance at the path loss where the switch
>contacts "arc" from VHF energy?
say what?
>I did that in one PA, and it would
>take a tube driving many thousands of amperes at the frequency of
>instability to arc the switch.
>
bad math.?
>
cheers
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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