G4AKW

The first steps in restoring and updating the old G2DAF linear amplifier commenced with the removal of the 2 x 6U4GT rectifier valves and the associated voltage doubler components / 300 ohm input circuit. This will be replaced with a solid state voltage doubler (probably a 1N4148 diode stack) and a 1:4 unun broadband transformer to give as close a match to 50 ohms as possible. Next step was to solder a 5000 ohm resistor from the 813 anodes to ground. Two 10k resistors in parallel were used (see photo) to represent the load that the valves should see under normal operation. An MFJ antenna analyser was then connected to the pi - tank output and the variable capacitors in the tank circuit altered until a 1:1 50 ohm match was obtained on 80m and on 10m. This checked  the output matching circuit.

This antenna is a dipole, each leg approximately 60 ft long (overall length ~ 120ft), fed with open wire feeder (ladder line).  Photo 1 shows the ladder line (constructed from plastic garden plant staples garden centre stuff is usually good at withstanding the outdoor conditions!), and the feed point insulator. A short length of 12 guage wire is used to take the wire tension and support the ladderline on each side of the insulator. The ladder line spacing, around 3 inches in this case, and its characteristic impedance, approximately 500 ohms here, is not important. (Spacing determines the maximum power handling capability (volts) and wire thickness (current)). The ladder line enters the house and is connected directly to the transmatch by two lengths of URM 67 coax stripped of its outer sheath and braid (photo 2).  These two 2 ft lengths take the feedline through two 15mm speed fit Polyethylene pipes through the wall and into the living room.  The impedance at the point ...