Sputnik QSO Party Transmitter Prototype

Posted on Feb 7, 2014

Here is my Sputnik QSO Party transmitter prototype. The RF output power is 450mW with 70Vdc @ 14. 4mA on the V2 anode. The V2 screen (G2) current is 1. 6mA. The V1 anode current is 1. 05mA with 45Vdc @ 176uA at the screen (G2). These little vacuum tubes arecapable performers, however, for long-life operation it`s important toheed the maximum electrod

Sputnik QSO Party Transmitter Prototype
Click here to download the full size of the above Circuit.

eratings as shown on the datasheets (links given below). The simple MOPA (Master-Oscillator -> Power-Amplifier) vacuum-tube radio transmitter circuit shown below was well-known in the mid-1950`s. A crystal-controlled Pierce oscillator drives a Class-C PA. The PA grid bias is derived from rectified PA grid current (thus; never run the PA stage without the oscillator drive signal present!). The CW keying and transmit/receive switching circuitry are not shown in the circuit below. Please click here to listen to a strong-signal recording of the original Sputnik-1. The oscillator signal bleed-though during "key-up" intervals is clearly audible. This implies that the oscillator stage was allowed to run continuously. Presumably, only the power amplifier (PA) stage was keyed on/off. As such, I plan to only key my PA stage (via the anode supply or perhaps using grid-block keying). My oscillator will be switched off only while I`m receiving signals. A low-pass filter may be required between the Pi-network impedance matching circuit and the antenna. I have not yet checked the RF output spectrum, however the inherent Pi-network 2nd harmonic attenuation is only ~28dB. The value of C1 may have to be adjusted for your particular circuit layout. This capacitor helps both to maintain the optimum level of oscillator feedback and provide the proper loading capacitance for the quartz crystal resonator. The drawing below shows the original...

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