>> // The great
>>puzzlement to me is why should one try to simulate the average cathode Z
>>when one has a perfect model at hand - a real cathode. With a resistor
>>substitute, one can have the Q wrong and there is no clue. With a real
>>cathode load, one can not take the wrong road.
>
>Because it can be VERY inconvenient to have to fire up the amp each time
>you are trying different capacitor values and finding out you have the
>wrong one!
? Use compression trimmers, a 5.5uH Collins Radio Co. 208T roller coil
($10 from Fair Radio Sales), and you can not have the wrong one. When
the swr is delightsome, stop, measure the values, and substitute fixed
ones.
> It takes mucho more time. And since a Q of 2 is defined by
>making Xc1 = j25 Ohms at the input (Q=Zin/Xc1) how is matching to an
>impedance at the tube going to make your Q be off?
>There is plenty of Q.
> If you pick the wrong value for C1 and get the wrong Q, it makes no
>difference of what is presented on the other end of the network - a real
>cathode or a simulated one. Please explain how one cannot take the wrong
>road with a real cathode?
>
? As we have seen on [AMPS], some folks do not choose to use a Q of 2 on
the first lap. Also, sometimes, a pecuilar situation occurs on one band
where the length of cable and the X of the Chebyshev filter combine to
upset the Q apple cart, especially on 15m and 10m.
>You seem to miss the point that tube manufacturers specify a driving
>impedance for their tubes.
? I know this.
>What does this value mean and why do they do it?
? It's a guide-line, however, it may not be the bottom-line in a
paticular amplifier/radio combo.
> Well, its meaning is as Dick so aptly put it. Why is so that you
>CAN know what impedance you need to match to for optimum power transfer.
>You'll get the best results from your tuned input circuit if you match to
>the driving impedance of the device. So why not simulate it? It makes
>playing with the circuit so much easier!
? For me, the easy thing to do is not to disconnect the cathode if I am
trying to match the cathode to the radio's Z, which ain't always
perzactly 50 +/- j0 ohms
>
>Of course, if you do it as you suggest by using a fixed value of C1,
>variable L and variable C2, it is possible to do it your way. But how do
>you know what value of L or what range for C2 unless you know what
>impedance you a matching to?
? The ^^^^Collins roller-coil^^^^ will cover 160m to 6m with an 8877.
I could not care less what the L is when I am looking for minimal SWR.
I tune it, stop, measure the values it took to get there, and sub. 'em.
>
>Simulations are done in companies all across the world every day. They
>tell us a lot about real world conditions w/o the pain of experiencing
>those conditions. So why not use it? You can play safely in the amp
>compartment in a passive manner - w/o all the pesky voltages being turned
>on!
>
? The high V is usually on the top side of the chassis.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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