At 12:22 PM 1/21/2005, Dennis12Amplify@aol.com wrote:
Will,
I believe Chris is talking about converting from a PI network to a Pi-L
network. The Pi-L having two levels of impedance matching, and a little more
harmonic suppression, than the original Pi network.
Thus the original Tune C becomes the middle capacitor of the
Pi-L, allowing
for a larger value of C, and the plate C is now the new Tune C.
Also, If his tune cap is already backed all the way out, he needs LESS
inductance in the tank to tune it, not more... Therefore he would
be SPREADING
the coil turns apart, or removing a turn, or moving the tap towards less
turns,
any of which would result in less inductance in the tank circuit....
Regards,
Dennis O.
In a message dated 1/21/05 11:49:21 AM Central Standard Time,
craxd1@ezwv.com writes:
A 300 nH coil (0.3 uH) is not very much. Assuming this would be the tank
coil, the 10 meter end being the largest in size, one might just squeeze
the coils together to raise the inductance enough to bring it in tune. If
not, adding 1 to 2 turns of the same size wire, and same diameter would
most likely do it. A dip meter would be easier to use to find the exact
placing of the tap on the coil if it has taps. According to the way the
tube is ran and tube type, some at a higher frequencies dont need a tune C
in order to tune correctly because the tubes output C is high enough and
takes care of it. In the amps I've seen this way, a load C was all that
was used.
Actually it is the other way around. The pi-L normally refers to moving the
output cap (loading) to a higher impedance point and matching this
intermediate impedance to 50 ohms with the L network. This provides
additional harmonic suppression as well as reducing the amount of
capacitance required for this "shared" capacitor, shared between the
output of the pi section and the input of the L section. Downside is
that the load capacitor requires a higher voltage rating because the
impedance is no longer 50 ohms but a few times higher.
What Chris is trying to do is the exact opposite. He is trying to move the
Tune capacitor to a lower impedance point by adding a new L between
the original network and the anode of the tube. I guess this could be called
an L-pi network. Since the impedance is now lower at this new point
you need more C for the tune cap -- and that may allow a capacitor with
a higher minimum C to be used.
So this new inductor is to be placed between the TUBE and the TUNE
capacitor.
I have used the 3CPX5000A7s in pulse mode at 50 MHz for wind profiling
and that is what I did. The average observer would just think my plate lead
was pretty darn long. But in reality it is an inductor that is part of an L
network that steps down the impedance so that the Tune cap can resonate
the circuit without running into a minimum capacitance issue. The tune cap
is now "shared" between this L network on the tube side and the pi network
for matching the new impedance to the output impedance. In my case I
also used another inductor at the output, once again a long lead and not
a "real" coil, so my matching is technically called an L-pi-L network, I guess.
Some of this effect exists in any case just due to stray inductance in the
leads, but this is increasing the inductance to a value high enough to make L
networks that provide useful impedance stepping.
--John W0UN
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