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one tube 2 watts Mopa


I have been experimenting with 1920s designs and 1930s tubes. After a lot of experimentation I came up with this little gem which although not powerful works very well. The osc is one half a 6sn7 and is lightly loaded giving it stability. It is tuned by a ceramic formed slug tuned coil wound with 16 turns of light copper wire and has a fixed 150pf ceramic cap across it. All freq selection is done with the slug whose screw protrudes from the front panel marked PTO(permeably tuned oscillator). It is mounted below chassis and is not affected by hand capacity in anyway. Tuning rate is very comfortable and it covers all of 40M. The osc is cathode keyed and after a suitable tube warm up does not stray. (Even on warm up less then a khz). The signal is fed to the grid of the other triode section by a suitable for RF ceramic capacitor. 100pfs would be ideal. That section is tuned with a large 1 1/4 diameter coil wound with no 12 insulated wire and a dual section variable capacitor using both sections. A 3 turn loop at the lower end of this coil feeds the antenna. Power supply is on board being a transformer for the filament and a doubler for the B plus. Osc is run at about 1/2 the voltage as the final. The meter is used to indicate the dip in plate current when the PA tank is in resonance with the osc. Dip is quite pronounced. The meter is any meter shunted to give mid range indication so the dip can be detected. This is important for a mopa design. A fun little rig.

I should finish the story. I have now had a successful QSO with it. I also built a two tube regen to use with it.
don VE3LYX


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