The VK4YE 1 Kilowatt Linear Amplifier.
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Split Stator Plate Tuning Capacitor.
We need to be careful about ensuring that the minimum capacitance of the plate tuning capacitor is as low as possible. It is not an easy task to have on one hand, a capacitor that has a maximum capacitance of at least 220pF so that the 160m band can be tuned, yet have a really low minimum capacitance of say 10pF to ensure we can tune the higher frequencies while maintaining an acceptable loaded Q.
It is possible to divide the capacitor up into two unequal sections. The first one can have a maximum capacitance of around 50pF while the second contains the balance of the capacitance. A miniature relay [ Altronics S 4197 ] can switch the two sections together for operation on the 80m and 160m bands. Transmitting type tuning capacitors are easily pulled down into stator and rotor plates. The stator plates are usually held together by two long threaded bolts on which plates and spacers alternate. If the stator plates are soldered in position, then it is unlikely you have a tuning capacitor made for high power RF work.
Provided you have the correctly constructed capacitor, remove the rotor assembly. It is quite easily done by removing screws that hold the back plate to (usually) four side bars. The spindle retaining nut then can be removed and the rotor should slide out. The stator plates alternate with spacers on long threaded stock. Remove nuts which secure the stator assembly to the ceramic or Plexiglas insulating blocks. The aim is to have three or so stator plates and their associated spacers at one end of the threaded stock, then a gap where two or three stator plates (and spacers) have been removed. The threaded stock is then cut so that you have a small stator section and a separate main stator section. A solder lug and a nut will complete the job - both stator sections should be quite rigid. Of course, the smaller tuning capacitor you are creating should be located at the end closest to where the 10/15/20m tank coils are to be positioned. Next, you remove a couple of rotor plates corresponding to the gap between the two stator sections. You may need to include a washer as well as the spacers to retain the correct position of the last three or four rotor plates relative to the smaller separate stator assembly. If everything aligns satisfactorily, reassemble the modified rotor and secure the back plate. My original capacitor tuned from 30 - 320pF and now tunes from 8 - 55pF and from 15 - 260pF.
The load capacitor should ideally be a triple ganged job that was common in valve type radios. Generally their capacitance ranges from 15 - 500pF per section. Plate spacing is more than adequate as you will be working into a 50 ohm or 75 ohm load. Accidentally transmitting into an open circuit should produce some nasty fireworks! For 160m you will need to switch in an additional capacitance of 1500pF. This additional capacitor must be a high voltage ceramic type to handle the RF current. It is switched into operation by the 160m contact on the band switch. Refer to the circuit in Figure 2(b).
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