A Grebe CR-18 Circuit Clone
Homebrew Regenerative Shortwave Receiver

I would never be able to recreate the original cabinet so I won't even try!



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 perch. 


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 assembly.





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 radio. 



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27 January 2007