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

Which Crystals to Use/Power Output

Small Front View

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
 Tune Up Procedure  Other Versions of the Transmitter

Which Type of Crystal to Use:
If at all possible use crystals in a real FT-243 pressure holder. The metal pressure plates that hold the crystal in this type of holder help to dissipate heat and minimize the crirp caused by the expansion of the crystal. Other crystals with the same pin diameter and spacing are available, and they will work, but these have plated on electrodes, which do not diissipate the heat very well, and thus they chirp or "whoop" terribly. This is especially true the higher the frequency of the crystal.

Choice of Crystal Frequency:
The power output of the 6CL6 transmitter depends on the band of operation and the frequency of the crystal used. The transmitter can operate on the crystal fundamental, or double, triple, or quadruple the crystal frequency to the desired output. In all cases, the lower the crystal frequency for a given band, the better the keying. Unfortunately, the usual tradeoff is involved: when multiplying a lower crystal frequency to obtain an output at a higher frequency, less output is obtained and the harmonic content of the output increases. Since there are no intermediate stages in the transmitter to filter out unwanted harmonices, good amateur practice implies that when operating the transmitter as a multiplier you should use an antenna matching network ("Antenna Tuner") or bandpass filter on the output to attenuate any unwanted harmonics and make sure that they don't get radiated. Also, be sure that you don't accidently tune up the transmitter on the wrong band! (This is also possible with many vintage commercial transmitters as well!)

The table below summarizes the crystal frequencies that can be used on each band:

 Band:  Crystal Frequency and Mode:
 80m  3.5 MHz - Fundamental
 40m  3.5 MHz - Doubler, 7 MHz - Fundamental
 30m  5 MHz - Doubler
 20m  3.5 MHz - Quadrupler, 7 MHz - Doubler
 15m  7 MHz - Tripler

I take the lower output as a challenge, and prefer the better keying, so I use 3.5 MHZ (80m) crystals on 80, 40, and 20m. On 15m I have found that 3.5 MHz crystals give no useful output, so I use 7 MHz (40m) crystals on that band. I always have an antenna matching network in-line to insure that no harmonics get to the antenna

By the way, I see no reason why you couldn't use 1.8 MHz crystals (160m) and double to 80m, or use 3.37 MHz crystals and triple to 30m, if you happen to have such crystals around. They could theoretically give better keying on those bands.

Power Output Measurements:
The transmitter was operated on several different bands in several different multiplying modes, and the output was measured with a Bird Wattmeter. The results are summarized in the table below:

 Band:  Crystal Frequency:  Mode:  Power Output:
 40m  3.5 MHz  Doubler  4 watts
 40m  7 MHz  Fundamental  5 watts
 20m  3.5 MHz  Quadrupler  2 watts
 20m  7 MHz  Doubler  3.25 Watts
 15m  7 MHz  Tripler  1.25 watts

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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