Heathkit B-1 Balun

Heathkits baluns purchased used at hamfest.   The one on the left I bought in the early 80's.  The one on the right was bought in the late 60's and was used outdoors unprotected for about 15 years.  It was still working well when removed.

Interior of the left balun.   This one is wired for a 4:1 ratio.

Interior of the right balun.   This one is wired for a 1:1 ratio.

I don't have the manual  for these baluns but was glad to see all the info needed wrote inside.  I suspect the old timer who assembled this balun made these notes.  The call W9IIU is also on the inside.

Right after I got the second balun I did an efficiency test by hooking them up back to back with both wired for a 4:1 ratio.  Although they are speced at 75 ohm the test was done with a 50 ohm source and load.  I don't recall the exact results but remember being surprised how efficient the power transfer was.  Maybe I will do the test again and document it this time.  For many years I used the balun with a balanced fed 160 meter dipole running about 1000 watts from a Dentron MLA-2500 amp.  The balun was just outside my shack with a short length of 7/8 inch Heliax connecting it to a Dentron MT-3000A truner.  This arrangement would tune and worked well on all bands 160 thru 10 meters.  I even worked some DX with the antenna on 6 meters using a different tuner.

Both coils are identical with two individual parallel wires, approximately 19 gauge bifilar wound with a total of 21 turns.  Coil diameter is 2.5 inches with a length of 4.5 inches.   This design wouldn't be hard to reproduce.  I wonder if the 75 ohm impedance is due to the spacing of the two parallel windings?  The wire spacing looks close to the 75 ohm twin lead I used in the early 60's.  If anyone can provide any info on this it will certainly be appreciated and included here.