2009

How to make an 80m Valve Paraset Rx coil.....and band spread the tuning range if you wish.

First some underpinning knowledge:

1. The diameter of the coil former affects the amount of inductance you get. (use the one you have!)

2. The wire diameter affects the amount of inductance you get. (use the wire you have!)

3. The spacing between the turns affect the amount of inducantance you get. (close wound no spacing problems now)

4. The coil former must not conduct electricity or it will act as a shorted turn and will be useless. (plastic or paper tube but not metal!)

5. Sometimes the winding direction is important. (not for us, we wind in one direction only....a result!)

6. Bandspreading takes patience but provides superior performance for the band required. (oh yes it does!)

7. Varnishing the coil affects its' self capacity and so alters the tuning range.

And the lesson to be learnt from the above is that you can use any size coil former you can find to make a Paraset receiver coil. Something within the range 15mm to 28mm diameter will do nicely.

Wind on the turns as shown. All coils are wound in the same direction. Mount the coil vertically. There now follows a sort of pictorial showing taps and coil connections:

Antenna Earth Cathode Tuning Capacitor

coil turns 0 x x x x 5 x x x 9 x x x..............................x x x 40

Chassis end Top of former

Now you can use this coil and it will work perfectly. However to get the tuning range of an orignal Paraset you will probably have to take one or two turns off the tuning capacitor end of the winding. NOT the chassis end or the coil won't work properly.

Bandspreading the coil onto the 80m Band

Varnishing the coil affects its' self capacity and so alters the tuning range. A tuning capacitor of about 20pF to 25pF maximum will be needed. A fixed trimming capacitor of roughly 130pF across will be needed across the coil. The final value of this capacitor is found by trial and error. With the tuning capacitor unmeshed and 130pF across the coil remove mone turn at a time from the top until your RX tunes to roughly 3.7MHz. Now varnish the coil......

When the varnish is dry adjust the value of the trimming capacitor until the RX just tunes above 3.6MHz (say 3.63MHz) with the tuning capacitor fully unmeshed. When fully meshed you should easily tune below 3.5MHz.

If you think there is tuning capacity to spare, remove another turn...add some fixed capacity to bring the tuning back to just above 3.6MHz with the tuning capacitor unmeshed and now see how far down it tunes. Repeat this process and eventually you will have a receiver that tunes 3.5MHz to 3.6MHz with just a little overlap at each end.

Always place the receiver in its' cabinet before noting and making any adjustments as this does affect the tuning range because of residual capacity.

Band spread has been achieved. Once the receiver is placed in its' cabinet, since this affects the coverage slightly, you can download a chart from this website and use it to calibrate your tuning dial.