Grounded Grid Amplifiers


Posted on Feb 5, 2014

Floating the grid above ground is bad for RF, and bad for arc protection. The control grid ground is also the single most critical connection for stability. The grid connection to ground should always be as wide and short as possible, and use as many pins as possible. The input and output circuit of a grounded grid amplifier are connected in series through the tube.


Grounded Grid Amplifiers
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Plate current is common through both cathode and anode, and only dc plate voltage is not. Back when we measured power as plate input power and not RF output power, the FCC even had a rule similar to this. The FCC wanted driver plate input power to be included as a full part of power amplifier plate input power. Thus kilowatt grounded-grid amplifiers, like the Heathkit SB220, when driven by 100-watt exciters could only run 900 watts input if the operator wished to comply with FCC rules. Probably based more on FCC conservatism than actual operation, a few widely accepted handbooks and authorities claim driver power adds to output power via feedthrough and is not accounted for in the metering system. Thinking unclearly, these books propose full driver power be deducted from output power when calculating efficiency. In actual operation, every single milliampere of anode current contributed by the exciter is fully accounted for in the plate current metering. The only thing not accounted for is a portion of the average cathode-to-grid voltage, which directly adds to the anode-cathode voltage during negative cathode swings. During positive cathode voltage excursions the gird is more negative compared to the cathode, so the tube cuts off. Since the tube is just "coasting" the positive cathode swing does not detract from effective operating anode voltage. This asymmetrical tube conduction causes the RF voltage between cathode and...




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