From my inbox: 1, 2, 3-tube radios, cheap cap tester, cheap frequency synthesizer

  • Build a radio with three tubes or less. This article lists radios that you can build with three tubes or fewer. In general, this is a very cool website. (Thanks to Brad, AA1IP, and the Glowbugs mailing list.)
  • Cheap capacitance tester. The Glowbugs mailing list also recently had a discussion of cheap capacitance testers. I just bought a fancy, new Keithley DMM  that measures capacitance, but some of the instruments mentioned in this thread also measure inductance. The consensus seems to be that while they’re not as accurate as a “real” test instrument, for $20 or so, how can you go wrong?Brad, AA1IP mentions the article, “A Low Cost Automatic Impedance Bridge” by Dr. George Steber, WB9LIV, that appeared in the October 2005 QST. Brad says, “This arrangement uses a PC’s sound ports, a dual-section op amp, a few components and some software to deliver reasonably precise RLC measurements along with a graphic screen display.”
These frequency-synthesizer boards use the AD9850, and can be purchased on eBay for as little as $5 each.

These frequency-synthesizer boards use the AD9850, and can be purchased on eBay for as little as $5 each.


  • Cheap AD9850 boards. The AMRAD mailing list has a thread on these inexpensive signal-generator modules. One of the AMRAD guys has been experimenting with using them with an Arduino to make frequency control easier. They’re only $5 each and generate 0 – 40 MHz.On the Glowbugs list, they talked a little bit about these boards, too. One guy gave a link to the article, “An Improved VFO Driver Amp for Tube Rigs.” This article describes a circuit that allows you to easily use the AD9850 boards (or any other digital synthesizer, for that matter) as the VFO for a tube rig.

Having trouble finding tube sockets?

If you’re having trouble finding tube sockets, Hack a Day recently ran a story on a guy who uses a 3D printer to make his own tube sockets. How cool is that?

Here’s the video that shows how he did it:

There’s also an cool post on Hack a Day showing how to build a tube prototyping station.

Toying with tubes

As I mention in my Dayton report, I attended an interesting talk on homebrewing with tube circuits. This evening the speaker, Grayson, TA2ZGE, posted the following to the QRP-L mailing list:

I have had a lot of people ask me, after my talk on Hollow-State Homebrewing at FDIM about where they could find interesting theratron (vacuum tube) circuits to build.   So I thought I would pass along some references and links to get people started:

Dave’s Homemade Radio. This guy is a real craftsman and homebrew whiz with tubes.  A must visit!

Max Robinson’s Fun with Tubes. Max has a GREAT tube cross reference guide that lists types by type and application.

Jeff Duntemann’s Junkbox. Lots of interesting tube homebrewing ideas and circuits + LOTS of info on tube categories.

Electric Radio Magazine. If you are interested in tube gear, homebrewing with tubes, restoring tube equipment, you will definitely want to subscribe to this magazine and get the back issues.  It’s been around since the late 1980’s and is edited by Ray Osterwald, N0DMS, who write a lot of the more technical tube articles.   They have an index to search for back issue articles.  Look for projects by Bob Dennison (SK) and Bruce Vaughn.  These guys had many, many great thermatron projects!

If you have any questions on thermatron homebrewing, just let me know.


Grayson, TA2ZGE

Fun stuff!