2 VALVE TRANSMITTER


Posted on Feb 6, 2014

This transmitter is very stable and will deliver up to seven watts of power with the components and tubes shown. Do not be put-off by valves as they are VERY easy to work with and it seems that there are quite a few valves around. Because no-one wants these obsolete things they may be bought for next to nothing at radio rally`s. If you are not too


2 VALVE TRANSMITTER
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particular what type of valve you want then there are lots of valves available. Above is the circuit of the transmitter including the coil winding information. The oscillator is a simple untuned Pierce oscillator which will oscillate at the fundamental frequency of the crystal. A 25pf variable capacitor between the grid of the first valve and earth will allow the final frequency to be "pulled" a little but I never used this facility. The RF choke in the anode of the two valves is made up of 34 turns of enamelled wire wound on a 1/2" ferrite ring (2. 2mH). The output tank circuit is wound on a piece of 2" dia plastic conduit tube. The object is to have as many turns of wire in circuit as it is possible to have on the band of interest. The output tuning capacitor should normally be set to about 10% when selecting the tapping for any particular band. The cathode resistor and 2. 2uf capacitor give quite a nice keying envelope to the keyed output. When the key is UP (off) there is about 100 volts DC across the 2. 2uf so this component must be rated in excess of 100v. Because of the high voltage you should NOT use a CMOS keyer circuit without suitable buffering. I obtained my 2. 2uf capacitor from an old telephone because they nearly always have a 2. 2uf 250v non-electrolytic capacitor somewhere in the circuit. The chassis is built using aluminium plate, drilled as shown above. An extra hole must be added to pass the connection...




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