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Laboratory studies of power amplification using disk-seal tubes are reported for frequencies from 200 megacycles up to the vicinity of 3000 megacycles. Operational data in the 200-megacycle band agree well with the predictions from conventional class-C calculations making use of static characteristics. In the 3000-megacycle band, the output and efficiency are approaching zero, although appreciable power gains at appreciable efficiencies are still obtainable on most tubes with especially well-designed circuits. The circuit types and some physical examples are discussed in the paper. Also discussed are certain transit-time effects which may contribute to the poor performance at high frequencies. One such effect is back-heating of the cathode because of returned electrons.