The VK4YE 1 Kilowatt Linear Amplifier.

Alternate Tubes.

The 4-125 tetrode and its derivatives such as the RS1007, QY3-125 and QB3/300 are very suitable for this design. They are physically smaller than the 813, and have a 5V filament drawing 6.5A. All have an ICAS plate dissipation rating of 125W. You will require a paralleled pair plus sockets. The plate connectors, which are the same as for an 811A tube are available from the Electronic Valve and Tube Company. Telephone (03)95711160.

The correct ceramic socket to use is a Johnson type 275. This series of socket also fits the 4-250, 4-400 and 3-500Z transmitting tubes. The QB3/300 tubes and sockets are available from Malcolm McIntosh, 26 Branch Creek Rd, Clear Mountain, QLD 4500. Telephone (07)32985454.

Because the QB3/300 has a much lower plate to filament capacitance of only 3.5pF, I have recalculated the pi-network values. The plate tuning capacitor will still need to be modified as explained previously, but it will connect to the to the start, not the centre of the 10m coil. I suggest the following coil winding data for the nine MF/HF bands.

As the input capacitance is lower than that of the 813, it may be possible to achieve a low SWR on the 20m band without having to switch in an inductor as previously done.

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