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. I've got some random variable capacitors, a couple of
nice dials, and would love to spend a weekend punching holes in a
chassis and putting something together.

I look in the 50's and 60's era handbooks and am a little confused. The
"SSB exciter" construction articles almost exclusively use a Vackar VFO
made out of a twin triode. Looks really straightforward and simple. But
the cookbook section inevitably shows pentodes in usually both grid-
and plate-tuned circuits, and then with extraordinary amounts of
buffering (say, a cathode follower AND a buffer amp).

I don't anticipate doing anything but 40M and some 20M in the near
future, and the Eico will double up to 14Mc just fine from 7Mc.

Will probably use a 0A2 type regulator in any event.

Am I giving up anything by just using the simple twin-triode Vackar?
The Eico 720's crystal oscillator combined with the crufty old crystals
I had isn't perfect to begin with, remember :-).

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.

Tim.

wrote in message
ups.com...
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. I've got some random variable capacitors, a couple of
nice dials, and would love to spend a weekend punching holes in a
chassis and putting something together.

I look in the 50's and 60's era handbooks and am a little confused. The
"SSB exciter" construction articles almost exclusively use a Vackar VFO
made out of a twin triode. Looks really straightforward and simple. But
the cookbook section inevitably shows pentodes in usually both grid-
and plate-tuned circuits, and then with extraordinary amounts of
buffering (say, a cathode follower AND a buffer amp).

I don't anticipate doing anything but 40M and some 20M in the near
future, and the Eico will double up to 14Mc just fine from 7Mc.

Will probably use a 0A2 type regulator in any event.

Am I giving up anything by just using the simple twin-triode Vackar?
The Eico 720's crystal oscillator combined with the crufty old crystals
I had isn't perfect to begin with, remember :-).

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.

Tim.


always solid-state
http://www.minute-man.com/acatalog/A...adio_Kits.html

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. I've got some random variable capacitors, a couple of
nice dials, and would love to spend a weekend punching holes in a
chassis and putting something together.

I look in the 50's and 60's era handbooks and am a little confused. The
"SSB exciter" construction articles almost exclusively use a Vackar VFO
made out of a twin triode. Looks really straightforward and simple. But
the cookbook section inevitably shows pentodes in usually both grid-
and plate-tuned circuits, and then with extraordinary amounts of
buffering (say, a cathode follower AND a buffer amp).

snip

Hi,

You might look at the matching Eico 722 VFO manual on BAMA:
ftp://bama.sbc.edu/downloads/eico/722/


73,
Ed Knobloch

wrote in message
ups.com...
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. I've got some random variable capacitors, a couple of
nice dials, and would love to spend a weekend punching holes in a
chassis and putting something together.

I look in the 50's and 60's era handbooks and am a little confused. The
"SSB exciter" construction articles almost exclusively use a Vackar VFO
made out of a twin triode. Looks really straightforward and simple. But
the cookbook section inevitably shows pentodes in usually both grid-
and plate-tuned circuits, and then with extraordinary amounts of
buffering (say, a cathode follower AND a buffer amp).

I don't anticipate doing anything but 40M and some 20M in the near
future, and the Eico will double up to 14Mc just fine from 7Mc.

Will probably use a 0A2 type regulator in any event.

Am I giving up anything by just using the simple twin-triode Vackar?
The Eico 720's crystal oscillator combined with the crufty old crystals
I had isn't perfect to begin with, remember :-).

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.

Tim.


I always like John Seboldt's K0JD's approach to homebrew / build a VFO.
He used a military surplus T-368
http://www.seboldt.net/k0jd/analogvfo.html
http://www.seboldt.net/k0jd/t368vfo.html

Other VFO's
http://www.seboldt.net/k0jd/othervfo.html

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

wrote:
But now I'm tired of QRM'ing the same frequencies all the time and want
to build a VFO. I've got some random variable capacitors, a couple of
nice dials, and would love to spend a weekend punching holes in a
chassis and putting something together.

I look in the 50's and 60's era handbooks and am a little confused. The
"SSB exciter" construction articles almost exclusively use a Vackar VFO
made out of a twin triode. Looks really straightforward and simple. But
the cookbook section inevitably shows pentodes in usually both grid-
and plate-tuned circuits, and then with extraordinary amounts of
buffering (say, a cathode follower AND a buffer amp).

I don't anticipate doing anything but 40M and some 20M in the near
future, and the Eico will double up to 14Mc just fine from 7Mc.

Will probably use a 0A2 type regulator in any event.

Am I giving up anything by just using the simple twin-triode Vackar?
The Eico 720's crystal oscillator combined with the crufty old crystals
I had isn't perfect to begin with, remember :-).

As a followup:

Built a circuit using a 6AH6 for the oscillator and a OD3 for the
screen regulation and a 6C4 for an output buffer, and a little Allied
transformer (yes, they still sell tube power transformers) for the
power supply. The grid circuit resonates at 1.75Mc, the plate at 3.5Mc,
and the Eico doubles this up to 7.0Mc just fine.

Right now not everything is bolted down completely, but maybe over Xmas
it'll get a nice old Millen dial and a spiffy sheet metal box.

Had a very nice QSO on 40M last night with another guy who homebuilds
VFO's, he used the twin-triode Vackar type I was considering. I'm so
happy to be back on 40M with a VFO that I don't know if I'll ever
bother doing bandswitching, but I suppose I could double up in the 6C4
to get to 7Mc out of the VFO and 14 or 21Mc out of the Eico.

Result seems generally superior to the 720's built-in crystal
oscillator, which is actually pretty crappy with a lot of chirp on
dashes after the rectifier gets warm. Or my ancient 40M crystals were
crappy. The lack of screen regulation on the 720's oscillator is
probably what made it so bad.

Tim.