Intro: 40M QRP Transmitter Using a 6CU6 Tube
This is a simple QRP transmitter modeled after a schematic from and OLD ARRL handbook. I have those manuals going back to 1950 and the older projects are much more interesting to me. Maybe its the glow of the tubes or the heat they produce. really comforting right now that its cold out here in NY. it uses a crystal but I had built a couple of DDS vfo units using cheap modules I got from eBay I want to try with it. Before you even consider building something like this you should know how to safeguard yourself and others from the lethal voltages used here. A vented plexiglass enclosure would be good or a preforated metal cage. This is not for the timid or those with little kiddies or pets in the house.
Step 1: Assembly
I used an old award plaque as a base and laid out the parts using terminal strips and octal relay sockets for the tubes. A scrap of aluminum plate made a nice mount of the vintage meter. It even says "Element Current" and reads up to 50Ma. That works out well since when tuned up it runs about 27 - 39 Ma.
Step 2: Coil Form for Tank Circuit
I cant seem to find the nice pre-formed coils for a reasonable price but I found this coil form on Thingiverse. I printed it out and wrapped it with some 16Ga enameled wire then paired it with a vintage variable cap.
Step 3: RF Output
I used a BNC for the output since I really don't expect much power out of it. I cheated for the initial tuneup and used a 5 turn output coil behind the tank. I have to rewind that as 2 to 4 turn coil centered over the tank,
Step 4: Initial Tuneup
I laid out the transmitter, Key, power meter, voltmeter on a bench. the cap uses a screwdriver to adjust it. I powered it on and the B+ reads about 360V and the VR-150 tube is regulating nicely at 149V. Key down and then tuning for least current it dips to about 28Ma at max RF out.