A Grebe CR-18 Circuit Clone
Regenerative Shortwave Receiver
would never be able to recreate the original cabinet so I won't even
John Dilks was nice enough to post the entire CR-18
manual and it was a nice source of information and an inspiration
for a new project. Nothing particularly novel about the 2-tube
circuit other than it utilized some very good coils and had a neat
adjustable antenna coupling scheme.
The following describes my project.
Looks like every other
homebrew rig, huh? I kept it simple. Front panel is made of
3/16" Garolite (phenolic). Beneath that innocuous big knob is a
National Velvet Vernier control so it tunes very nicely.
Here's the business side of the
panel. I utilized a National DX tuning capacitor,
a fine one indeed, and removed plates to get it down to the desired 130
mmf value. I did not use a bandspread trimmer as in the
original CR-18 owing to the vernier tuning assembly. Sockets are
modern chinese equivalents. I wanted to use ceramic since it is a
shortwave receiver and I didn't want sleeved sockets since I intend to
experiment with some other tubes. The coil assemblies are made
with strips of imbuya (imbullia) that have been coated. This is a
fairly dense tropical hardwood and seems to be adequate for
insulation. Coils are wound with #16 enamelled wire.
Chassis wiring is 1/16" square brass...no rf wiring touching the
wood. The base is typical 3/4" cabinet plywood.
There are two resistors in the circuit
so I restuffed some old grid leaks for the 7 Meg and 25k . My
audio xfmr is a homebrew 'can' with a Stancor A-53 inside. My
poor planning left
me without room for a transformer footprint hence you see it up on a
This shows the banana jack assembly
for the plug-in coils. I used modern jacks and plugs.
Here's a closeup view of the plug-in
This is the regen cap. Not of
very good quality. It seems to be
interminably out of alignment and I had to address the flaky rotor
contact. You can also see the gridleak in this photo with a new
silvered mica cap of 40pf hiding underneath. This particular
brand of resistor holders suck, btw, they have little or no
springiness in the contacts.
Yeah, thats a ST style 01A. I
was using it while experimenting as it has proven to be one of my
better tubes on shortwave frequencies. I tried some various types
of tubes. It seemed to work pretty good using 30s. No cigar
with UX-99s, though. I couldn't get any regeneration.
Probably needs more tickler to make the 99s work?
Here's the circuit. There is a
larger copy on the previously mentioned site along with full coil data,
etc. I went with 3-1/2" coils using a piece of PVC as a form to
make the 'skeleton' forms. That meant recalculating slightly but
my coils (I only did 2, 3 and 4) came out almost right on the money and
the coverage is very close to what is shown in the coil chart. My
tuning goes right up to 17890 +/- with no problems. In fact the
only problem I have noted is at the bottom of Band 2. Mine tunes
somewhat lower than the 9.7 called for and it does drop out of
regeneration for about 3 dial divisions at the end of the range,
somewhere around 9200. Not a problem, though, since there is
plenty of overlap from the next coil.
I think Grebe went a little heavy on
the antenna winding. Could probably drop 2 or 3 turns. The
angle that you see it at in my photos was the 'sweet spot' with my
antenna. Much closer and you lose regeneration at certain
freqs. That simple physical adjustment is somewhat critical and
is really handy to have.
Battery wise I was losing regeneration
on the top band when trying to use 16-17 volts on the detector. I
redid my battery pack (I'm using a 22 and a 45 in series all made with
9 volt batteries) and now have 23-24 or so and its very solid.
That results in about 67-68 on the audio tube. I guess a guy
could go higher if desired. Something I noticed when fooling
around is that at the lower detector voltage I was getting enough
hand-capacitance on the regen cap to where I was ready to make a shield
and look for a larger knob. Once I got up to 22 volts, though,
the hand capacitance doesn't seem to be an issue.
This set is very stable, btw.
Its built solid as a rock with that DX cap and the brass wiring and the
good quality coils. I'm sure the original Grebe CR-18 was a fine
27 January 2007