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's Parallel 27s|
|WB2QMY's 80m 2W TNT|