Sunday, 16 November 2014

BK Building...

March 1926 QST. Courtesy: ARRL
The year's premier operating event for '29-style transmitters (well pretty much the only one) is fast approaching. The Bruce Kelley QSO Party runs during the first two weekends of December and hopefully catches some good early winter conditions. In a preemptive attempt at eliminating my normal (and usually justified) pre-contest wind-angst, I have begun work on a small '29-style amplifier for my Hull Hartley oscillator.

Every year as contest time approaches, I anxiously follow our local weather patterns in hope that the winds remain quiet for the event. For the past few years, the wind has been non-existent...right until zero-hour, when, almost as if a switch were thrown, the wind ramps up to gale-like conditions. Now if you've never heard a self-excited oscillator directly connected to a wind-blown antenna, then be sure to take a listen to this year's BK. These '29 rigs are often not very pretty-sounding, even into a dummy load, but couple them to the wind and things get very 'nostalgic-sounding' quickly. Frequency shifts in the order of several hundred Hertz between letters are not uncommon, making copying a weak signal difficult and almost impossible if listening with a narrow filter. The amplifier will effectively isolate the effects of a shifting antenna impedance from the oscillator stage, making life very much less stressful, come BK time.

My plan is to follow the wisdom of the '29ers themselves, carefully explained in "More Power With Better Frequency Stability" by George Grammer and published in the February 1931 edition of QST. I'm always impressed at how quickly those folks back in the 20's figured out so many of the important "basics of radio" that have withstood the test of time. It must have been an exciting time to be a ham! A glance through any of the numerous  radio magazines from the late 20's and early 30's reveals a vibrant market for the homebrewer, with every type of component available. Whether the average ham could afford to buy them in the hungry-thirties is another question.

I've chosen to use another type '10' power triode as my amplifier, as shown below.

Courtesy: ARRL

I see the main challenges being coupling conveniently from the oscillator as well as neutralizing the triode amplifier...I wonder, will neutralizing it on 40m (for example) also make it stable on 80m?

As well, since the type '10' uses a directly-heated cathode, if I wish to let the oscillator run while I key the amplifier, a separate filament supply will be required for each tube. With this in mind, I've got the ball rolling this week with the construction of a dedicated '29 rig filament supply. The supply uses two center-tapped 5V transformers with their secondaries in series and brought out to the back panel. This will allow me to use tubes with 2.5V, 5V, 7.5V and 10V filaments...up to 3A.

New filament supply
The amplifier will be breadboarded, likely with mainly newer parts, to shake-out the circuit and see how it performs during this year's BK event. If things go well, I plan to rebuild the amplifier using '29-style components and physical layout...maybe a vertical panel that the Hartley can slide into.  Hopefully I'll get the breadboard version done in time for the Party.

Are you building or planning anything for this year's BK Party?


Buzz goes to Europe said...

Sounds like so much fun. Another project to put on my to-build list. Have fun Steve.

Steve McDonald said...

There's still time Markus!

Anonymous said...

keep 1 hand in back pocket?

Steve McDonald said...

Yes! Lots of HV floating around these things but at least not on the big tank coils any more.