Jim Thomson wrote:
>
>EXCEPT when you get to the higher bands. (10 and 15. And sometimes 20).
>You need a way to cancel out the reactance. At least you do on a pair
>of 4CX800 Russian tetrodes.
>
>73
>Jim W7RY
>
>### agreed. Alpha achieves that by switching in a tiny bit of uh in
>parallel with the grid to chassis resistor. ## On these 8170/8171
>tetrodes, they use a small roller inductor in parallel with the 450
>ohm resistor. That cancels out the XC from the C between control
>grid and cathode. If you don't do this,,, you will have SWR on the
>input, between xcvr and input of amp.
>
>Jim VE7RF
This is surprising, because the reactance of the input capacitance can
be cancelled without any switching, by "absorbing" it into a 1:1 pi
network with C1 equal to the shunt capacitance of the tubes. Physically,
it looks like an L network because C2 is inside the tubes, but
electrically it behaves as a pi.
The network compensates for the effects of input capacitance on the
higher bands, and being a lowpass configuration, it becomes increasingly
"transparent" on the lower bands with no switching required.
Jim Tonne's ELSIE program is a good way to design the network. It isn't
very intuitive, so here are a few pointers...
From the "Design" dialog, choose Capacitor-input lowpass, Chebyshev,
Bandwidth = 50M (ie 50MHz - trust me, this is a good starting value for
HF), Order = 3, Rs = 50, Ap = 0.5.
Then click "Edit" on the top menu. This opens a schematic for the
pi-network that you have specified, with editable component values at
upper left. Click on item 3 (output capacitor) and change it to equal
the input capacitance of the tube(s), including a few pF for strays.
Don't forget to include the units of pF! Click on item 1 (input
capacitor) and change it to the same value.
Now click "Plot" on the top menu, and choose the "Transmission + VSWR"
plot option (6th from the top). You should now be seeing a reasonably
flat VSWR curve, up to at least 30MHz.
To flatten it even more, click "Tune Part". This opens a popup window
where you can click "Next Part" to select the series inductor, and set
the Stepsize to say 10%. You can now use "Increase" or "Decrease" to
flatten the VSWR curve across the whole HF range.
(Damn - I really didn't mean to provide detailed instructions. These
technical writing habits are hard to shake :-)
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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