>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.
But you have your network theory correct. Again, though we (you and I)
are sort of applying a linear network analysis to a very non-linear
system. As others in the group have instructed me, a tube running in
class AB does not have a constant input impedance. Rather it varies over
the drive cycle as the tube is cycled on and off. But I will still argue
an "average" value can be used to simulate a more linear system.
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.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
--
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
|