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## [AMPS] RE: 4-1000A input Z

 To: [AMPS] RE: 4-1000A input Z measures@vc.net (Rich Measures) Wed, 3 Feb 1999 20:40:11 -0800
 ``` > >Gents, > >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 >> >> >>Rich... >> >>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures >> >> >>-- >>FAQ on WWW: http://www.contesting.com/ampfaq.html >>Submissions: amps@contesting.com >>Administrative requests: amps-REQUEST@contesting.com >>Problems: owner-amps@contesting.com >>Search: http://www.contesting.com/km9p/search.htm >> >> >> > > >-- >FAQ on WWW: http://www.contesting.com/ampfaq.html >Submissions: amps@contesting.com >Administrative requests: amps-REQUEST@contesting.com >Problems: owner-amps@contesting.com >Search: http://www.contesting.com/km9p/search.htm > Rich... R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures -- FAQ on WWW: http://www.contesting.com/ampfaq.html Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
 Current Thread [AMPS] RE: 4-1000A input Z, Rich Measures <= [AMPS] RE: 4-1000A input Z, Jon Ogden [AMPS] RE: 4-1000A input Z, km1h@juno.com