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Tube-based 40M VFO

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December 18th 06, 11:07 PM posted to rec.radio.amateur.boatanchors

Al, N1AW

Al, N1AW is offline

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First recorded activity by RadioBanter: Nov 2006

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Tube-based 40M VFO

wrote:

I've been having a lot of fun the past few weekends with my "new" Eico
720 and a couple of 40M crystals and a dipole strung between two trees.

But now I'm tired of QRM'ing the same frequencies all the time and want
to build a VFO. ...

If you are building a VFO in a separate cabinet you will do well to
make it solid state. Two advantages are much less worry about drift due
to heating, and powering it easily from low voltage batteries. That
will ensure a pure note, and, particularly if you are new to home
brewing, not having lethal voltages on the circuit is desirable.

I once had an Eico 720 but I don't remember the circuit. If it doesn't
have an input jack for a separate VFO you may need to make a slight
modification to be able to feed the VFO to the tube that was originally
a crystal oscillator.

I'm guessing that all I need is 5V or so to drive to the Eico, correct
me if I'm wrong! With the crystal oscillator in it I almost always have
to back way way down on the grid drive so I'm guessing I've got some
extra oomph if I need it.

A transistor oscillator of any sort (see the Handbook) followed by a
buffer stage should have no problem providing enough drive. Back in the
1960s and 1970s I used such a VFO to drive a Gonset Commander, a rig
more or less comparable to the Eico -- as I recall the Commander had a
6AG7 crystal oscillator driving a 6146 final.

- Al, N1AW

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