04/25/2015 14:57

VFO Driven MOPA

 

 

MOPA transmitters are in my view the least of the maximum. If built with tubes, they give to me the maximum fun also.

Here in the picture my main transmitter, which is going to be put aside in favour of a remake that I am about to build on the next months.




Its look, so humble and approximate, hides many years of experiments. It covers from 1.7 MHz up to more than 29 MHz. It is VFO driven (see on this website which one ...) and has only one frequency multiplier. I use this 90% on Radiotelegraphy and I can get, throughout all its frequency coverage, a PERFECT CW TONE: stable in frequency, chirp-less and click-less. The secret is to put at least two aperiodic class 'A' buffer amplifiers between the VFO and the multiplier/driver tube. The first buffer being a cathode follower and the next being a common cathode stage with aperiodic plate exit. Furthermore, the VFO anode voltage is kept extremely low (30-35 V) and the control grid keying is set to have a negative voltage level just below the tube cut-off.

The output stage impedance matching circuit is a PI-Network featuring an 11 positions rotary switch that taps a 2 inches coil with about 50 turns. The Plate capacitor is 325 pF and Load capacitor is about 700 pF.       I  can tune everything in the 1.7-29 MHz range very well. Because that is its frequency coverage, achieved by use of only 5 tubes: VFO(1/2 6BQ7) + DOUBLE BUFFER(1/2 6BQ7 & EL803S) + DRIVER/MULTIPLIER (EL803S) + POWER AMPLIFIER(2 x 6BG6G). It results, then, flexible to use and to set, and damned comfortable to use on ALL the Shortwaves, no frequency excluded ! And it puts out 15W on the worst bands, that is 12 mt, 11 mt (yes, CB band) and 10 mt bands.

At the time of writing, I've just done a QRP QSO on 21 MHz band very successfully, with my frequency being stable during all the contact and the tone quality being without compromises. Imagine: a very few cheap components, very well optimized, assembled in a circuit that is absolutely not critical, providing reliable and high quality Radiotelegraphy.

 

    

Radioman, April 2015  

 


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