The AA8V/W8EXI 6CL6 One-Tube Transmitter
by Greg Latta, AA8V

Why Use A 6CL6?

Drawing of a 6CL6

6CL6 Transmitter Pages:
 6CL6 Home/Historical Background  Restoration Photos and Details
 Exterior Photos of the Finished Transmitter  Schematic Diagram and Circuit Description
 Interior Photos of the Finished Transmitter  Why Use A 6CL6?
 Photos of the Transmitter In Its Original Condition  Which Crystals to Use/Power Output
 Power Transformer Specifications  AA8V with Jim Trutko W8EXI
   Other Versions of the Transmitter

Why Use A 6CL6?

Won't just any tube work?
Why not use a big tube like a 6L6 and get lots of output?
Why use an electron-coupled circuit, rather than another type?
The 6AG7 is highly recommended in an article in the March, 1950 issue of QST
The 6CL6 is released by GE as the miniature version of the 6AG7 in May, 1953
So why use a 6CL6?
Why use regulated screen voltage?

Won't just any tube work?
At first glance it would seem that just about any tube would function fine as an oscillator, and to an extent this is true. But this isn't just any oscillator, it is a keyed oscillator that is connected to an antenna and subject to constant adjustment. The keying places severe limitations on the performance of the oscillator if a decent sounding signal is desired. Direct coupling to the antenna also affects the performance of the oscillator as well. Without careful design, antenna coupling can cause slow starting of the oscillator (that is, lousy keying!) and in some cases the oscillator may not even work. The design of a one-tube (or one transistor) transmitter is a compromise involving power output, keying, and the effect of antenna coupling on operation. The design of the transmitter involves the type of circuit, the choice of component values, and the choice of the particular tube used.

Why not use a big tube like a 6L6 and get lots of output?
One of the most important things in obtaining good keying and a good CW note is to keep the crystal current as low as possible consistent with adequate output. This excludes "monster" type one-tube transmitters using a 6L6, 6146, or TV sweep tubes like a the 6DQ5, 6CU6, and 6DQ6. These can give large output (30w or more), but their keying is lousy and it is possible to destroy the crystal with too much current. Still, such rigs were popular in the days when a good sounding signal wasn't as important as having a strong signal on a budget. I know, because I had one and it chirped like a bird and did a great job of heating the crystals!

Why use an electron-coupled circuit, rather than another type?
Keeping the crystal current low also dictates an electron-coupled oscillator circuit, which combines the functions of oscillator and amplifier into a single tube. With an electron coupled oscillator circuit it is also possible to obtain decent output (about 5w) while still keeping the crystal current low. The electron-coupled circuit also provides a measure of isolation between the load (antenna) and the oscillator.

The 6AG7 is highly recommended in an article in the March, 1950 issue of QST
In 1950, an important article in QST magazine, "Crystal-Controlled Oscillators, A Review of Modern Crystals, Circuits and Tubes" (QST, March 1950, C. Vernon Chambers, W1JEQ) addressed all of the issues above, as well as the choice of which tube to use. In that article, various electron-coupled circuits were tried along with a variety of tubes: the 6AG7, 6F6, 6V6GT, and 6L6. Among the many conclusions in the article, one came through loud and clear, which I quote here: "Of the four tubes tested the 6AG7 is by far the best from every standpoint." As a result of that article, virtually all crystal oscillator circuits in the ARRL handbook for the next 15 years featured or recommended the use of the 6AG7.

The 6CL6 is released by GE as the miniature version of the 6AG7 in May, 1953
On the first page of the May, 1953 issue of QST, an ad appeared by GE announcing the 6CL6 as the new, miniature replacement for the 6AG7. The new 6CL6 was the electrical equivalent of the 6AG7 in a cheaper, smaller, package:

Small 6CL6 Ad
Click On The Image Above For An Enlarged Version

So why use a 6CL6?
The 6CL6 was smaller, less expensive, and had the same performance and specifications as the 6AG7. It would thus behave the same as the 6AG7 in any circuit. So the choice of a 6CL6 was the result of the recommendations of a 1950 article in QST magazine, and the fact that the 6CL6 is the miniature version of the 6AG7.

Why use regulated screen voltage?
The same article recommending the 6AG7 also stated, and I quote: "Regardless of the circuit selection, use regulated screen voltage if good keying is desired." Needless to say, very few (if any) of the crystal oscillators in commercial gear produced in the 50's and 60's used regulated voltage on the oscillator screen. Good, clean keying was not a top priority in the 50's and 60's, so regulating the oscillator screen voltage was considered an unnecessary expense. That's why many crystal-controlled tube rigs have a well deserved reputation for lousy keying. (Note, however, that even with regulated screen voltage, the right tube, and the right circuit, a poor crystal will still result in poor keying!)

Wingfoot Exciter PictureClick here for pictures and information on the Wingfoot VFO 2E26 Exciter

813 Amplifier PictureClick here for pictures and information on the Wingfoot 813 Amplifier


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