Tuesday 18 November 2014

More BK Building

My recent 'Building For The BK' blogs have tempted at least one ardent homebrewer, John (VE7BDQ), to dig into his junkbox stash to create a nicely-working BK rig....it's a Colpitts oscillator using a pair of 45's in push-pull. His lash-up is now working and ready to go on both 80 and 40m and he is contemplating 160m operation with it as well....very nice!

1935 ARRL Handbook. Source: http://www.arrl.org/
John and I had a nice QSO today on 80m CW and the rig sounds just great. He reports 240 volts on the plates and a current of 58ma. His scope-measured output is just over 8 watts for an impressive efficiency of just over 60%. It appears that the Colpitts-style is much more efficient than the more popular TNT or Hartley oscillators...well at least more efficient than the ones I built!

Although John did not have every period-appropriate component available to him, it didn't discourage him from building his BK rig. For too many, this is often a stumbling block that leads to nothing being built....as the boy's of '29 no doubt would  have done, he put something together completely from his junkbox.

It will be super to have another left-coast station in this years '29 Party...and a Canadian station to boot!

Anyone else building for the BK?  ....there's still a few weeks left.


Anonymous said...

After seeing Steve's previous blog about the Bruce Kelley QSO party
and the type of transmitters that other amateurs have built.

So I looked in my old tube box in my workshop attic and found
I had a couple of 45 tubes.. then checked my tube socket box
and found a couple of old four pin isolantite type tube sockets. Wow.
I could put together such a transmitter. More searching
and found more parts.. Hey, I'm good to go..

After throwing things together and trying various push-pull
circuits TNT, TPTG and finally settling on the Colpitts circuit
that seemed to work really well right of the bat.

Maybe it won't pass muster for the purists out there. But after several
QSO's with it on the 80m band, I hope to have a few contacts in the
Bruce Kelley QSO party.

Steve McDonald said...

Your Colpitts is performing extremely well I think. I'm sure that it probably wasn't popular back in the day because of the much higher parts-count, which evidently pays off in performance. You will be amongst a select few in the BK using a Colpitts!

Niel - W0VLZ said...

How is the antenna/load coupled to the tank coil? 60% is really impressive. I struggle to get 15-20% efficiency with my Hull Hartley.

Anonymous said...

Just confirming today on my measurements into a different 50 ohm load gives 60v peak to peak sine wave or 9watts.

Using two turns the same diameter as the tank coil #14 insulated wire at the center of the tank circuit about 3/4 way into the winding. Now to try for 160 meters.