Tuesday, 3 October 2017

A Weekend '29 Transmitter Project

A recent posting to the Yahoo AWAGroup, reminded me of a wonderfully simple transmitter that would easily get anyone on-the-air for the Bruce Kelley '29 QSO Party coming up in December. The AWAGroup is largely composed of '29 builders and operators and is often the source of much good discussion related to these activities.

When builders first tackle a '29 style rig, eligible for the BK Party, the first thought is often about finding an affordable tube that might make the transmitter a 'legal' entry. A suggestion of tubes and their relative availability can be found here in one of my earlier blogs. Although some of the tubes may not be typical junk-box items, one that is often very readily available and inexpensive is the '27' or the '227' / UX-227.

The '27 was a popular audio tube used in receivers back in the 20's and can easily be pressed into service as a self-excited oscillator on HF. When properly adjusted, the '27 can produce up to 4W of RF, more than enough for you to join the '29 fun a make plenty of contacts over the dual-weekend event.

Scott, WA9WFA, has produced a great article describing the construction of a 1929 Hartley oscillator using the 27. There is no need to utilize period-appropriate parts, other than the vacuum tube itself. If you are pressed for building time or just want to get something on the air, there's no need to be overly concerned about construction aesthetics ... there's plenty of time for that later, and besides, the uglier ones often work and sound the best!

WA9WFA's 27-Tubed Hartley

Power supply requirements for the transmitter are pretty minimal and could even be an old receiver supply ... something that delivers 250-300 VDC at 35ma. An effective way to easily double the power of this transmitter is to add a second tube in parallel with the original one, providing your power supply can supply the extra current (~35ma) required. Although Scott's information describes an 80m Hartley, there is no reason why you could not wind a coil for 40 or for 160 and take advantage of any activity on those bands as well. Like most of these link-coupled outputs, you'll probably squeeze a little better efficiency from the circuit by adding a variable capacitor (~365pf) in series with the pick-up link and the hot-side of the coaxial feedline.

Nick, WA5BDU, wrote his description of building the 27 Hartley which can be found here. He also includes information on running the 2.5V filament from a 5V transformer.

WA5BDU's 27-Tubed Hartley
ABØCW uses two 27s in parallel for the oscillator section of his 1929 MOPA. Details of his interesting Hartley can be found here.

ABØCW's Parallel 27s

As further proof that you don't need a lot of power to have fun in the 1929 BK Party, here is the rig that Kevin, WB2QMY, used when we worked on 80m CW. His little TNT, just barely putting out 2W, made it all the way from New York!

WB2QMY's 80m 2W TNT
When it comes to the BK ... if you build it, they will come. Hopefully you can join the fun this winter as well.

2 comments:

Anonymous said...

80 or 40 that is the question.
So I waited for the last minute to build a Hartley. I gathered most of the parts in the summer but stashed them away and forgot all about them.
I'm now ordering a filament transformer and some small bits that will be nice to have.
I'm going to wind my plate coil while I wait but I don't know if I should wind for 80 or 40M.
This transmitter will not have as much class as my little 6V6 transmitter ;)
Jon WS1K

Steve McDonald said...

Hi Jon. I'm looking forward to working you on the new Hartley! I think you will find more action on 80 so that would be my 1st choice. You may find that it will also reach 40 with the same coil.