A Tale of Two Tubes

A couple of weeks ago, I worked N4QR on 40m CW. I could tell by the tone of his signal that he was operating a homebrew transmitter. There wasn’t any 60 Hz on his signal, and it didn’t chirp exactly, but I could tell it wasn’t the pure tone you get out of today’s radios.

I asked him about his rig, and he told me that it was a one-tube transmitter made with a 6L6. I forgot to ask him where he got the schematic, but a quick Internet search turned up the following:

  • The May 2005 issue of the K9YA Telegraph has an article written by N4QR titled, “The Wonderful One-Tuber,” that contains the schematic for the transmitter. The K9YA folks don’t make issues of The Telegraph available on their website, but I was able to get a copy of the issue by e-mailing them.
  • A 6L6 Classic (shown below)
  • WB2MIC 6L6 Transmitter Project

This one-tube transmitter is made with a 6L6 pentode.

The 6L6 is a pentode that, according to Wikipedia, was introduced by the Radio Corporation of America (RCA) in July 1936. Apparently, it was used quite a bit in public address systems.

After the tube became successful, tube manufacturers introduced a number of variations, including the venerable 807. The original 6L6 was capable of delivering 19 W; the latest variation, the 6L6GC is rated for 30 W. The 6L6GC is still used in guitar amps, and is still manufactured in Russia, China, and by Groove Tubes in the U.S. They sell a number of different 6L6 variants; the cheapest is $16, the most expensive $180!!

Tube #2
One of the reasons I was interested in the 6L6 is because about a year ago I came across a schematic for a transmitter using 6A6 dual triode. I had just come into possession of a couple hundred tubes, and while I didn’t have a 6A6 (at least I haven’t found one yet), I do have a couple of 6J6 dual triodes. They’re not quite as high power as the 6A6, but I’m still thinking about building a little transmitter with one.

As you might expect, there’s a bunch of information on the Internet about this tube:

One interesting fact about the 6J6 is that IBM used it in the 604 computer. Unfortunately, they found it to be not as reliable as they wanted it to be, but at first none of the tube manufacturers were interested in making a more robust version. This led IBM to set up a tube-making laboratory where they could experiment with designs. They developed a more reliable version of the 6J6 and finally convinced RCA to manufacture the tube. According to the author of the history of the 604, part of the concern is that IBM would decide to get into the tube business.

So, the next time you hear a signal that doesn’t sound so perfect, remember that there just might be a story behind it. Ask the op about his transmitter, and listen to what he or she has to say.


  1. I don’t want to be contentious, but the 6L6 was invented by GE, not RCA.
    The British branch of GE, the GEC-MOV company, originated the Kinkless Tetrode design around 1935. For reasons of company politics, they passed their research findings across to the US parent, and they launched the 6L6 in 1936. The British company launched their version, the KT66, in 1937, when the 6L6 had been shown to be a successful design.
    I would amend that Wiki entry, but they always want references, and “my memory” is never enough for them to accept.

  2. Something went missing in my previous comment – “It was GE, not GEC-MOV, that licensed the design to RCA”.

  3. I read recently in “Turing’s Cathedral” by George Dyson, that when Princeton’s Institute of Advanced Study (IAS) constructed the first electronic digital computer, they decided to use 6J6 tubes, using “worst-case analysis” for use of the 6J6 in flip-flops and other digital switching elements. I bring this up because of the comment about IBM first using the 6J6 in an early digital computer … only because they inherited pieces of the IAS’s computer project (ENIAC). The idea about “worst-case analysis” was that parameters varied all over the place: noise, gain, threshold, etc. But surplus WW-II 6J6s were abundant. So digital design concepts were based on being able to use any 6J6, whether “hot” or “weak”. Otherwise, the project may have been stymied if instead it was decided to be necessary to develop an appropriate switching element to be specially developed. Such a tube that had specs within a certain tolerance on each parameter would have probably killed the budget and maybe IAS would have been unsuccessful in developing the ENIAC. Hats off to John von Neumann et al for having made such practical judgments and choices.

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