The VK4YE 1 Kilowatt Linear Amplifier.

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While the usual approach over the past twenty years or so in the design of a linear amplifier is to opt for a triode [ or triodes ] running class AB2 in grounded grid configuration, I chose to revamp a design rarely seen these days. This is the once well known G2DAF circuit. It was quite popular with British amateurs, and to a lesser extent, with USA amateurs in the days of rigs with a vacuum tube output stage.

For those not familiar with the configuration, a resistor or resistive network of 100 ohm to 300 ohm replaces the tuned circuit of a grid fed amplifier. Tetrodes or pentodes are used because they have a very low value of plate to control grid capacitance. Being un-neutralised, stability is achieved by the damping of any feedback signal by the low value of input resistance. This amplifier uses a pair of 813 beam power pentode tubes. Other suitable types like the 4-125, QY3-125 and QB3/300 are discussed later.

The passive grid configuration has a couple of advantages compared to a grounded grid design. The input resistor provides a constant value load for the exciter to work into, and there is no requirement for a filament choke and associated pi-network. Drive levels are modest at around 25 - 30W pep. This means that your rig's output stage can loaf along quite comfortably while the linear amplifier is called upon to do the heavy work.

The passive grid design probably fell from favour with the arrival of the solid state output stage in transceivers. If your rig has a fixed 50 ohm output impedance and you attempt to load it up into a 200 ohm resistive load, how much power is transferred? Probably so little that you qualify as a QRP operator! The rig's SWR protection circuit winds back the output so much that it is not possible to get enough signal to drive the amplifier. However, as I discovered, it is possible to overcome this objection by constructing a 1:4 transmission line transformer [of Guanella design] on a short piece of ferrite rod.

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