09/11/2015 14:14

12BY7/6DQ6 MOPA

 

 

My preferred one. First, the 6DQ6 is unrivalled with regards to amplification: only 1 mA control grid current for full drive, as compared to the 2.5 mA typical of the 6146 family. Then, the PA works as straight through on all bands. And the oscillator does all the job for the generation of the driving signal ... up to 28 MHz.

12BY7 tends to be expensive. The same job, probably better performed, is done by a 6CL6, EL83, 6GK6, 6BQ5. The 5763 in my opinion is a bit more drive-avid than the above tubes, which mutual conductance is in the range of 10-11 micromhos, and above. So, the latter has never been my preferred choice for such application.

As far as I know it is the only schematic in which the final stage can be fully driven on all the Shortwaves with no limitation in performance.

6DQ6 is TOUGH! Die hard! Yet plentiful and cheap even in 2015! Can withstand up to 7 kV peak plate voltage, and at relatively low plate voltages can deliver comfortably about 50 W rms. A bit more voltage ... and at 700 VDC can run at 75 W rms output.

The above schematic, alongwith a convenient load of crystals for the Shortwaves, can offer a tremendous fun.

12BY7 and its cited sisters, can be full driven by a 5 to 10 V peak signal generated by an elementary analogue oscillator, for example made by valves or even some FET. Strictly speaking, with such tubes is practically possible to cover all the shortwaves. Only three active devices are needed for a nice performance in frequency stability and tone quality in case of CW emissions.

The only thing missing here is a convenient break-in system, for properly managing an antenna changeover relay and a set of biasing voltages for the tubes, VFO included if available, to achieve a good semi break-in operation. It will be described soon. Hold on, then ...

 

Radioman, Sept. 2015


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