The basic SSB transmitter wasn’t too difficult. Up to the 5mW level anyway. Trying to get from there to the 1 or 2 Watts level caused considerable grief. There are lots of push / pull circuits on the internet to give guidelines for construction. I found a circuit using a pair of 2N3866’s. Since I had a handful of these in stock I decided to use this circuit, driven by an NE46134. The same transistor used in the Elecraft K3 front end. This produces plenty of drive for the push / pull stage. At least a couple of volts of RF.
After the unexpected demise of a number of 2N3866’s I suspected that the output transformer turns ratio probably wasn’t correct. I tried optimizing the turns ratio, but only succeeded in blowing further pairs of transistors. I eventually concluded that the transistors were not up to spec. I had bought a quantity, from an eBay vendor based in China, at a bargain price a couple of years ago. A quick look around ebay revealed lots of negative feedback issues concerning fake parts. Once bitten …..
I modified the circuit to take some VN1210 FET,s, that I also had a quantity of. Without any changes other than to the bias circuit, this circuit worked first time. By mis-setting the bias, the output pair consumed in excess of 750mA at one point during testing. The solder joints melted. But the devices survived the experience. These transistors are in a TO39 plastic case with a small metal tab that is connected to the drain pin. Not easy to cool due to the shape. After some further fruitless heatsink experiments I shelved the whole transmit idea for the time being.
I arrived back home on 16th Sept. Maria returned from her UK trip on 19th. Over that weekend a search of my gear in my storage shed revealed an old Codan 7727. This has a solid state 100W linear PA module that only needs a few mW of drive, and it has ALC on the PCB. I have pulled this module, and intend to modify the output transformer in order to reduce the output power.
(Vcc^2/2*Pout) gives the collector impedance. At 13.5v and 65 Watts the turns ratio will be very close to 6:1. Currently the windings are 4:1 so I will need to remove the existing turns and replace with 6 turns. According to theory this should work. I hope it will reduce the heatsink requirements. The PCB is currently mounted on a heavy duty ( 10mm thick ) angle bracket that will allow me to mount it vertically down one side, on the inside, of the transceiver case. Adding a heatsink would not be impossible, but it will have to be cut to fit.