The VK4YE 1 Kilowatt Linear Amplifier.

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Filament Transformer.

A 47k ohm 1W resistor connects the centre tap of the filament transformer to ground. Its purpose is to maintain the tubes at cut off when the amplifier is in stand-by mode. The pair of 813's require a filament supply of 10V at 10A. This transformer is centre tapped and is not exactly an 'off the shelf' item. You do what I have done - wind your own. You take a discarded microwave transformer, chisel out the high voltage secondary and wind the required number of turns through the window. This is easily done as the turns ratio is about 1 turn per volt. You cannot pull laminations apart as they are welded together.

In determining the exact number of turns to use, remove the secondary winding, then wind 10 turns of ordinary insulated hook up wire through the window. Using an accurate AC voltmeter, measure the voltage delivered by this winding. Exercise extreme caution when connecting mains power to these transformers, as terminals tend to be exposed spade types and careless amateurs could easily become silent keys!

Wind on or take off half a turn at a time until 10.0V to 10.5V is measured. Now that the correct number of turns is known, you can take some 2mm lacquered wire and wind the filament winding. It is a good idea before you start the winding process to wrap Teflon tape around the core to prevent accidental shorts to frame through nicks on the wire. You must make provision for a centre tap by winding half the turns, then start again to wind the second half. When it is completed, again check that the voltage produced is between 10.0V and 10.5V. The 813 tubes require 10V +/- 0.5V. The filament transformer must be mounted on the same chassis as the tubes. Use 2mm insulated wire to connect the filament transformer secondary to the tube sockets.

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