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[AMPS] RE: 4-1000A input Z

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Subject: [AMPS] RE: 4-1000A input Z
From: (Rich Measures)
Date: Wed, 3 Feb 1999 20:40:11 -0800

>Most of the old articles on the 4-1000 in G-G, and Eimac's own newsletter
>on tetrodes in G-G,  indicate that the cathode driving impedance of the
>4-1000 is around 110 ohms  (104 - 110 ohms with 3KV - 5KV plate voltage).
>I don't really see what other option Jon has,  other than to assume the
>average driving Z is around 110 ohms - even Eimac says so.  I don't see
>anything wrong with the resistor substitution idea for the intitial set-up
>- as long as you don't expect it to be a perfect setup under actual
>operation. It's just an inital stab to get you up and running.
>Watch out for the component values you calculate for the input Pi when you
>use the standard "handbook" formulas.  If you use a Q of 2 with these
>formulas, you'll actually end-up with a Q closer to 4.  Those formulas
>assume that all the stored energy in the network can be accounted-for in
>the input shunt C, and therefore the Q of the input C is the Q of the whole
>network.  It isn't.  With a low Z, step-up transformation ratio like this
>(50:100 ohms), a significant portion of the network energy is stored in C2
>and therefore the Q of C2 must be added to Q of C1.  Having an adjustable L
>plus an adjustable C2 (remember Carl's suggestion from way back of using
>the ARCO mica compression trimmers) as Rich suggested provides lots of
>flexibility for unknown and unmeasurable factors.
?  There is more than one way to figure Q.  I use the method Eimac uses 
--  which is based on the input  Z divided by the reactance of C1.  Thus, 
for a Q of 2, the reactance of C1 =s 25 ohms.  

-  later, Phil
>At 09:52 AM 2/3/99 -0800, you wrote:
>>>>Would it not be possible to tune the circuit for min VSWR
>>>>using the exciters VSWR meter, as Rich suggested (real 
>>>>world operating conditions), then turn off the amp, terminate
>>>>its input port in 50 ohms and measure the impedance of the filter
>>>>looking back from the cathode with the MFJ-259B (cathode disconnected
>>>>of course)? This impedance should be approximately equal to the 
>>>>complex conjugate of the large signal cathode impedance. It 
>>>>would be interesting to see if this number comes close to Jon's
>>>>estimate of 110 ohms -j (2*pi*f*27pf)^-1.
>>>Theoretically, yes.  However, interfacing the MFJ-259 or any other 
>>>impedance measuring device to the cathode pins is a challenge and error 
>>>will be introduced there.  Rather, one could set the impedance as Rich 
>>>suggests, remove the tube and put the 110 Ohm//27 pF network in place and 
>>>check the impedance.  That is a little easier.
>>?   Agreed.  .  Perhaps it would be easier to measure the L and C of a 
>>Q=2, working-well driving the cathode, tuned input, and then scale the 
>>values for other frequencies?  However, since the Chebyshev filter in the 
>>radio and the length of the interconnecting coax are mitigating factors 
>>at the tuned inputs, my guess is that one would fare better by tweaking L 
>>and C2 as needed for each band. .   
>>-  later 
>>>If folks doubt me on the "average" bit, please define then how your AC 
>>>wall outlet can be "110 Volts" or "220 Volts."  Actually, these are RMS 
>>>values if I remember correctly, but my point is that AC voltages vary 
>>>wildly over their cycle as well, yet we call them by specific values.
>>?  However, in g-g amplifiers, cathode current Ceases when the input 
>>potential swings positive.  This is not the case with the electric-mains. 
>>-  later, Jon
>>R. L. Measures, 805-386-3734, AG6K,  
>>FAQ on WWW:     
>>Administrative requests:
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R. L. Measures, 805-386-3734, AG6K,  

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