The VK4YE 1 Kilowatt Linear Amplifier.

Input Circuit.

Now that the plate tank circuit has been taken care of, we need to ensure the input side can accept all bands from 160m to 10m. The unun consists of 13 bifilar turns of ordinary hook up wire on an 80mm section of standard ferrite rod. This works as well as any toroid I have experimented with.

Due to the input capacitance of the tubes, the SWR increases to around 1.5 : 1 at 20m. To maintain a low SWR from 20m to 10m, use slug tuned coils to form a parallel resonant circuit with this capacitance. They are switched in by miniature relays [ Altronics S 4160 ]. For 20m use 20 turns of 0.4mm wire on a 6mm former. The 15m coil consists of 12 turns of 1mm wire on a 6mm former, and the 10m coil consists of 6 turns of 1mm wire stretched to a length of 1cm on a 6mm former. All coils are fitted with an F14 slug.

Once the coils have been fitted, the SWR stays very low and flat across each band because of the low Q due to the swamping effect of the 200 ohm resistor. The earth end of R1 should be terminated very close to the braid of the coax bringing the input signal from the contacts of RA1. I have tried to make this obvious on the schematic of the input circuit.

I coupled a 12 position rotary switch wafer to the shaft of the band switch using a switch mechanism from Farnell Electronics. All relays were wired up so that they were switched in according to the band selected. This is a neater option than having to provide a separate 'input selector' for switching in these relays. The Farnell stock numbers for the switch mechanism and 12 position wafer are 146-033 and 146-034 respectively.

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