To make it possible to impress optional positive voltage to the control grid of any electronic tube and to use the electronic tube in the flowing state of an optional grid current by using a coupling transformer instead of a coupling pre-stage electronic tube.

A transformer 11 is used for coupling between a prestage AC signal source 9 and a post stage electronic tube 8. Namely positive potential, with an optional value is applied from a DC power supply 10 to the control grid of an electronic tube 8 through the secondary winding 12 of the transformer 11. The high voltage positive potential of the DC power supply 10 is impressed to the plate of the tube 8 through a load element. The AC signal source 9 is connected to a DC power supply 16 through the primary winding 13 of the transformer 11 and a 1st DC component 14 to which an AC component is superposed is allowed to flow into the primary winding 13. The flowing direction of a 2nd DC component 4 in the secondary winding 12 is determined so that the degree of DC magnification of the transformer 11 generated due to the 1st DC component 14 flowing into the primary winding 13 is canceled or reduced.