>
>>Jon
>>KE9NA
>>But speaking of chokes and choke impedances, here is a question that
>>another member of the reflector posed to me a week or so ago. Maybe
>>someone can answer it:
>>
>>The average output impedance of the tube is about 2 to 3 KOhm. From my
>>RF knowledge, the tank circuit when tuned, looks like approximately the
>>same impedance. However, a typical RF choke at 80 m has an impedance
>>around 2 KOhms or so. This is right in the ballpark of the impedance of
>>the tube, it's not a "high" impedance relative to the tube's output.
>>So...why does the choke work???
>
>Yes, that's close. One would want to account for the inductive reactance of
>the choke, in the value of the first capacitor shunted with the tube
>capacitance. It reduces the capacitive reactance, being out of phase with
>it. The pi network or whatever, should account for this in the calculation
>of the cap value. Otherwise it will be off a bit, esp at the lowest
>operating freq.
>
well put, methinks
>In bigger tubes, the plate R is low, on order of <500 Ohms, so a 2500-5000
>Ohm choke reactance is great.
however, why would not a HV-RFC on the order of 500 ohms of X be used
with a tube whose RL was 500 ohms?
>That's what i just did, and using the network analyzer, I did a plot from
>1-100 Mhz of the pi. Then removed the plate choke. Very little if any
>effect was noticed on the through swept response. Since i have a narrow
>band amp, that's easy. I have two operating frequencies to work on, and i
>can live with 30 minutes to change freq. So built two different chokes,
>both optimized to be operated on the inductive side of parallel resonance
>(self resonance). That way I get high Z in circuit, even with strays of the
>cabinet detuning it a bit.
>
>As for the choke killing the RF power without self destructing, I am not
>sure I can envision it.
Me too. Maybe we should try to envision a HV-RFC made from a
superconductor (zero ohm ESR) immersed in liquid helium.
>If there is DC current and voltage being metered,
>and yet no RF output, then the laws of conservation of energy say that
>there has to be power dissipated somewhere. either as RF or heat or
>something. Perhaps there was a very high frequency oscillation going, that
>was sending RF out to the load? But not showing on a wattmeter?
>
not likely
>If the choke was a dead short circuit for RF, then yes, you may never get
>the plate swing. Still, you shouldn't see plate current then.
It matters not whether the anode potential is allowed to swing. The
instantaneous anode current changes in proportion to the instantaneous
driving potential between the grid and the cathode.
>And the plate
>bypass cap may be destroyed quickly, creating a DC short path.
not if the anode supply bypass can withstand the max. AC anode current
capability of the tube.
>THis is like
>the parasitic theories, a chicken and egg question. If you could stick a
>bad choke in a functional working amplfier at full power, with a magic
>switch, I would expect it to be blown up quickly.
Been there, done that. The choke caught on fire.
>But if the choke is already there, at series resonance, then the RF
>wouldn't even tune close, and your amplifier would be operating in some
>wierd mode, probably at a totally off frequency,
amen, John.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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