Abstract: An amplifier circuit incorporating a valve or vacuum tube in the grounded grid configuration is disclosed. The anode D.C. potential is greater than the cathode D.C. potential which is in turn greater than the grid D.C. potential. The cathode is switched at an R.F. rate to a further D.C. potential below that of the grid. Preferably the grid potential substantially cuts off cathode current flow with the switch open. The amplifier finds particular application as an output amplifier in an A.M. R.F. transmitter with the modulation being provided by high level anode modulation. The tube or valve can be a triode, tetrode, or pentode.