A 6L6 Classic

The venerable 6L6 was introduced by RCA in early 1936 as an audio power tube. The June 1936 issue of QST carried two articles featuring this new tube, one describing a 50 watt audio amplifier/modulator built around it and the second showing it as a high powered crystal oscillator. This second article in turn inspired a whole series of compact 6L6 transmitter designs in QST starting with the QSL 40 in February 1938 and ending with the QSL 25 in April 1941. The QSL Push-Pull (June 1940) ran 600 volts on the 6L6 plates and could light a 100 watt light bulb to full brilliance. The 5 watt version described in December 1939 was at the other end of the spectrum. It was a transformerless design using a voltage doubler off of the 110 volt AC line to get 220 volts B+. That particular article mentions that the operator should avoid touching the metal frame of his key. The 6L6 (and its big brother the 807) appeared in transmitter article after article for two decades.

I wanted to build a 6L6 transmitter but at the same time didn't want to melt down any of my 6L6 stock or fracture a crystal. The QSL line, while classic, was not easy on tubes or crystals. I found what I was looking for in the 1939 edition of the Stancor Hamanual.

"The Stancor 25-B transmitter rig is designed specifically for the beginner. Several types of tubes may be used but the 6L6G is recommended for giving the highest power output at a conservative rating. The schematic diagram shows a regenerative type circuit using an aperiodic coil in the cathode circuit. The 25-B will give the new licensee, or any operator for that matter, a compact experimental transmitter at a minimal cost, offering a maximum of operating enjoyment."

My version of the 25-B is pictured. I believe that old transmitters should be seen as well as heard so I assembled mine breadboard style on a 6 1/4" x 10" piece of wood. The 6" x 10" front panel is masonite hardboard painted black. Across the back are (left to right) antenna, antenna ground, B-, B+ and the two filament connections. The final tank/antenna link (L1/L2) coil is at the left rear corner between the two antenna terminals. At the right rear I included a wooden doughnut for tank coil storage. The crystal socket is a 5 pin tube socket. An octal socket can also be used. Wiring should be kept as short and direct as possible.

I found the circuit laid out best with the 6L6 socket oriented with pins 1 and 8 closest to the front panel. I added a parasitic choke to the original Stancor design as cheap insurance and simplified the transmitter a little by using link coupling rather than pi network output. Bud OEL or B&W JEL 160, 80, or 40 meter end linked coils will work as the tank coils. Table 1 is coil information if you need to wind your own. The antenna link should be wound next to the grounded end of the tank coil. You may need to cut and try the coils a bit to get the most out of the 25-B with your antenna system.

A power supply capable of 300 to 350 VDC at 50 to 100 mA is ample for this transmitter. I chose choke input filtering to help voltage regulation.

To put the 25-B on the air plug in a crystal and coil for your favorite band, key down and adjust the output capacitor for maximum output. A word of warning though. One attribute of this particular circuit is that it easily oscillates on harmonics of the crystal frequency. You need to be sure that you have the output tuned to the right band.

The 25-B design appears to be fairly tolerant of substitutions (I made several). Mine easily runs 5-10 watts into a 50 ohm load and I've used it on 40 meters with good success.

Need a matching receiver? Take a look at my 3 tube superhet based on the beginners's receiver described in the February 1941 issue of QST.

Parts List
R1 56K/1 Watt
R2 220/1 Watt
R3 12K/5 Watt
R4 6K/5 Watt
C1 50 pF Mica
C2 .001 mFd Mica/Tubular Paper
C3 .002 mFd Mica
C4 .002 mFd Mica
C5 150 pF Variable
RFC1 2.5 mH/125 mA
RFC2 2.5 mH/125 mA
PC1 Parasitic Choke - 8 tums #20 on 47 Ohm/1 Watt resistor.
L1 Final tank, see text and Table 1.
L2 Ant. link, see text and Table 1.

Table 1 Coil Information

160 meters
Tank 54 turns #22 enamel on 1.5" form
Link 8 turns #22 enamel

80 meters
Tank 35 tums #22 enamel on 1" form
Link 7 turns #22 enamel

40 meters
Tank 18 turms #22 enamel on 1" form
Link 6.75 turns #22 enamel

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