This project came as a result of the failure of another. Many hours were spent constructing a 45 triode Hartley simular to the 1927 Hartley featured in ER for March '96. I could hear a rough signal in the receiver but the power out was not enough to get me out of my back yard. I determined my one and only 45 tube was a dud drawing about 3 times the plate current it should have been.
Howie, WB2AWQ had previously sent me a copy of the August 1928 QST article by Ross Hull on improving the Hartley for 1929 service. He had been successful using the article guidelines with a 211 triode, as had Gary, W7FG. Listening to them and the others in the 1929 AWA QSO party made me envious. When Gary offered a pre-tested VT-4C with socket, I got excited and busy on this project. After a few hours of fun, the circuit shown in figure 1 and the photo came to pass.
The circuit was built on a 10 by 16 inch board stained Walnut. The filament transformer at left began as a 17 volt secondary. Turns were removed to reduce it to 10 volts. A 75 ohm 25W balance pot provides the cathode connection since the transformer has no center tap. The plate choke is 3-1/2 inches of #30 DCC close wound on a 3/4" dowel rod. Small holes were drilled near the ends for short terminal studs of #12 bare wire to wrap and solder the small wire. Credit goes to Sandy, W5TVW for this tip. The tube socket is raised above the base by short plastic spacers. This permits the twisted pair heater wires to route under the socket, keeping them the same length and out of the way of the rear plate and front grid terminals. The real secret of a successful power Hartley is in the Hi-C tank circuit. The coil is 12 turns of 1/4 inch soft copper tubing about 5 inches long. I used a spray paint can as a form to wrap the coil over. The ends were flattened in a vise with holes punched for #10-32 machine screws which go thru the board. The coil mounts directly on the base for stability with the head of the screws on the bottom of the board. Short jumpers of #12 wire go to the ends of the 500pf variable. Flat and lock washers with nuts are tightened down to make a low resistance connection for the high value RF circulating current due to the low impedance Hi-C tank. A 25pf variable is piggyback on the main for fine tuning. It covers about 70KHz on the low end of 80M and makes QSY'ing a few KC much easier. Notice the main tuning cap is mounted on a piece of wood trim. A hole was drilled thru parallel with the base allowing the cathode or B- ground tap connection to route under the capacitor frame. This worked out well, keeping this connection as short as possible by not having to route the long way around the plate or grid end to reach the coil. The B- tap is made with a 45C Mueller clip that will not jump off. The two turn coupling link was made from #12 solid house wire. Holes drilled thru the 1/2" dowel coupling rod to hold the coil and the smaller stranded twisted pair going to the antenna terms. The coupling rod goes to the front thru a stand off bushing of 1X2 trim. The "T" handle control was Gary's idea and is nothing more than a short piece of 1/4 inch dowel. The link can be swung away from the main coil by twisting or moved further away by pushing the rod thru the bushing. The coil is held in position beautifully and simply by common friction. Additional 1/4 inch dowel rod shaft extensions are used on the recessed variable capacitors to mimimize the hand capacitance effect since the rotors are above ground. The exposed B+ connections are covered with electrical putty in case the cat gets a wild idea to step on it when hot. The plate current meter began as a 0-1 ma and was shunted with a length of #26 wire wound on a wood dowel sufficient to obtain X200 multiplier or 200ma full scale. With 750vdc B+ and the coupling adjusted for 100mils, it was possible to obtain about 30 watts into 50 ohms according to my custom walnut case Bird Thruline with the tap set 5 turns from the grid end. Best keying tone and stability seems to be with coupling reduced to 15-20 watts out and 80mils dc of plate current.
Operation of the Hartley has been much fun when the band is quiet with good T-8,9 reports from all over. Several DX contacts were made into Europe on the low end of 80 on the incredible night of Dec. 28, 1997. More than one operator (those being around long enough to know what a Hartley is) have asked if I am really using such a transmitter. I only have to wave my hand over the top of the set to demonstrate hand capacitance modulation. One local old timer I showed it to asked me how I modulated it. I would have to say now, "only manually". Also I have noticed that the older the key (bug), the better it plays. I hope this will inspire others to join in the fun with the rest of the self-excited oscillator gang. I look forward to meeting more of you on-the-air.
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luvcats@ionet.netThe contents of these pages is copyrighted © 1997 by Ann Maloney. Use only with permission. luvcats@ionet.net
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