>Thanks to all who replied to my question about converting Heathkit
>amps to six meters. It seems the conversion is not that difficult, so
>I think I'll try to convert my own SB-1000. There is one thing I
>don't quite understand, though, and that is the use of an "L" network
>in the plate circuit. Perhaps someone could help me out.
>
>The existing tune capacitor apparently has too much minimum
>capacitance to use it as-is on six, since the tank Q would be
>excessive. Many conversions replace it with a vacuum variable but
>I'd like to keep the original if possible.
>
*** Removing plates from the original tune-C would work.
>What I'd like to do is use a coil from the plate of the 3-500Z to the
>tune capacitor (through the DC blocking cap) to step down the 3500 ohm
>plate impedance of the tube to something around 1000 ohms, which would
>be within the range of the tune cap. As I understand it, this coil
>forms a simple L network with the output capacitance of the tube
>(including stray capacitance). So far, so good. The problem is the
>numbers don't come out right. Perhaps I have made a math error, but I
>can't see what it is.
>
>The ARRL handbook, 74th edition, has an example of how to do this on
>page 13.21, using an 8877 as an example operating on 29.7 MHz. They
>make the following assumptions:
>
>1. Tube output capacitance, including stray, is 15 pF.
>
>2. Tube output impedance is 2200 ohms.
>
>3. The "step down" impedance which the pi-net sees is 1205 ohms.
>
>Using the classic formulas for an L network, matching 2200 ohms to
>1205 ohms, requires a capacitive reactance of 2421 ohms paralleled
>with the 2200 ohm resistance, and an inductive reactance of 1095 ohms
>in the series arm. At 29.7 MHz, these values are 2.2 pF and 5.9 uH.
>Obviously, the 2.2 pF value can not be achieved, since the parallel
>capacitance already present is 15 pF. This is where I get lost.
>
>In the ARRL's example, they show how to convert the parallel circuit
>to an equivalent series circuit, insert an appropriate coil, and reach
>a solution. My question is why are their numbers so different from
>what an L network requires? Apparently there is more going on here
>than simple L-type impedance transformation.
>
*** Converting a parallel circuit to an equivalent series circuit
(admittance to/from impedance) is not a piece of cake.
For an explanation see:
http://www.somis.org/YtoZ.GIF
>And that's where I need some enlightenment. All comments will be
>greatly appreciated.
>
>73, Bill W7TI
>
>
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Rich, AG6K, 805-386-3734, www.somis.org / new e-mail adr r@somis.org
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