40M QRP Transmitter Using a 6CU6 Tube


Introduction: 40M QRP Transmitter Using a 6CU6 Tube

About: Disclaimer: The author is not responsible for loss of life, limb or property. The author is not responsible for your actions. The author is not responsible for anything. In fact, the author is completely i...

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.



    • Clocks Contest

      Clocks Contest
    • Water Contest

      Water Contest
    • Creative Misuse Contest

      Creative Misuse Contest

    7 Discussions

    If you put the 70V side of a Public Address speaker transformer (8 ohm side to audio amp out) in series with the B+ you could do simple AM Phone.

    Its about 10W plate current input. the output is a bad match for the 50ohm load so there's only about 3-5W out. I wasn't too careful in making the output loop because its meant to be paired with a tube PA. that will be the next instructable.

    I want to try this rig using Hellschreiber through its CW jack when I get the PA done.

    That could be interesting. I've always thought that was a cool mode but have seen much of it on the bands.


    I have a couple of arduino sketches for
    feldhell I want to post here. They are just simple beacons. One only
    transmits an ID and the other has a temperature / humidity sensor and
    transmits that data too. I want to add a barometric pressure sensor and maybe a GPS then it would be good for a balloon payload.

    That would be cool. I'm spoiled with my setup. I've got the Icom 7200 so with a USB cable and Ham Radio Deluxe so I'm set but I still love tinkering with building... it's what drew me to the hobby in the first place.

    This cooker reminds me of my amateur ham uncle in his radio shack, “CQ CQ Calling CQ....". Whenever he'd say something to us kids we'd always reply "we read you CQ, five by five, over", he liked it too.

    1 reply