RadioBanter - View Single Post - Tube-based 40M VFO

[email protected] · December 20th 06, 05:42 PM posted to rec.radio.amateur.boatanchors

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.