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.
Phil T
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
>
>
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>
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