A home brew 3 kilowatt antenna coupler
I constructed my amplifier around a 8189 (8166 pulse rated) tube operating in a grounded grid configuration. This amplifier is almost entirely hand made.
The box was made by obtaining pre-cut aluminium panels from a metal shop. These were assembled using ¾” aluminium angle for the corners, drilled the secured using aircraft rivets flattened with a small panel hammer and sanded so as to be invisible when painted.
The tank was made from copper tube and copper wire. The 40 & 80 mtr inductor was made from 1/8” hard drawn copper wire using a large beer bottle as the former. 10,15 & 20 were from ¼” copper tube. I made several attempts to wind the tube on a former and found it was prone to flattening. Finally, and laboriously, I filled the tube with fine white sand and after winding the inductor, removed the sand. 10,15 & 20
mtr inductors were connected directly to the band switch.
Behind the band switch and on the same shaft I attached a home made aluminium sprocket.
Lower in the chassis I manufactured a smaller sprocket on a shaft that went to the rear of the unit to the input matching switch. These sprockets were connected via a light duty bicycle chain so that when the band was changed, it also selected the appropriate input. The sprocket ratio took some time to sort out.
The tube socket and anode cap were manufactured and cooling was per a 4” muffin fan at the bottom of the planum chamber. Placement of the fan was determined by experiments using a similar sized shoe box. To determine the air flow direction I first tried to use smoke but even with huge amounts it was inconclusive. Finally the appropriate fan position was determined by using small pieces of fine cotton placed in various positions. This also enabled the size and height of the chimney skirt to be selected.
Plate capacitor was manufactured from an old Calstan unit. The shaft was extended and appropriate spacers machined. Additional plates were cut from 1 mm aluminium. This worked out perfectly, the only down being the discovery by my wife the instrument used to accomplish this were her dress making scissors.
Cathode transformer was made by stripping two B&W TV transformers and rewinding for the necessary 22 amps at 7.5 volts and other control voltages.
The amplifier I have described has been in use for a long time without problems. My thanks to Dick Zimic VK2BOT whose guidance and help made this project much easier.
This page last updated on Sunday, 26 August 2007