This transmitter is very  stable and will deliver up to seven watts of
power.  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 particular what type of valve you
want then there are lots of valves available.

Above is the circuit of the transmitter with the coil winding
information. The oscillatoar is a Pierce oscillator circuit 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.

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 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.  I
obtained mine from an old telephone as telephones nearly always have a
2.2uf 250v non-electrolytic capacitor somewhere in the circuit.

The oscillator is un-tuned so the only tuning procedure required is to
select the band (coil tapping) and peak it for maximum RF output.

The valves used were selected  because they had B7G bases and were the
first I put my hand on when I reached into my junk-box.  Virtually any
pentode valves will work in this application.    An EL84, for example,
will deliver over ten watts in this circuit but the valves quoted will
deliver seven watts from 3.5 to 30 MHz. If you should use another tube
then don't forget to check the screen voltage. If it is less than 250v
then you will need a resistor in the screen supply to the valve.

Have fun, de HARRY, Upplands Vasby, Sweden

I am very grateful to Harold, SM0VPO for allowing me to add his files to my web site. He can be contacted by email:

Frank, G3YCC

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