input matching to grounded grid amplifier

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NA3CW:
One tip about physical layout:  If at all possible, place at least the output capacitor side of the pi as directly on the tube socket as possible because this is part of the RF current return through the tube.  On the higher bands even a few inches of coax from a pi board to the tube can have major effects of efficiency.  It's probably not news to many but in a now-defunct amplifier company which shall remain nameless, we found the hard way that with a pair of 3CX800A7's in grounded grid, we had to put the 10 meter output capacitor right on the socket.  The rest could be a little farther away and that capacitance just subtracted from the output cap of the other bands.  It made a very noticeable difference in output.

YMMV. This effect may be different for different tubes. 

73,
Chuck
NA3CW

Dave K6XYZ:
Pat.....take a look at the tuned input ckts for the Collins 30S-1...those component values may get you into the ballpark.
You can dwnld a manual from the Collins Collectors site.

Opcom:
Quote from: DMOD on August 14, 2017, 12:23:24 AM

I am using the Wingfield Pi-Net equations.

I have to input an initial Qin = 10 in order to get a loaded Q near 5.

The -j1700 comes from the tube's input capacitance
Xc = 1/(6.28X27.2e-12 FaradsX3.5e6 Hz).

So the tube's input impedance at 3.5 MHz is 67 - j1700.

At 7.2 MHz it is = 67 -j813 ohms.


Phil - AC0OB




That explains something. Should the input Q be around 10 on all bands possible? I should shoot for that?

There is enough room under the chassis to put the output cap right next to the socket. How about an air variable? it'll probably fit.

ka1tdq:
I'm not the expert on this topic, but I remember an experienced amplifier builder telling me to use an tuned input Q of 2.

I'm in the process of building a small, 20 meter GG amp using a single 3-500 and I'm not going to use a tuned input at all.  The driver is a DX-60 and that'll match to the tube's input. 

Jon

KD6VXI:
A tuned input will give you much better IMD, Jon.

Not using a tuned input is bad Juju.



And yes, a tuned input of 2 to 5 is what Eimac recommends, depending on the data sheet.  2 to 3 is the most common values mentioned.

--Shane
KD6VXI

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