A voltage stabilizing circuit employing one thermionic pentode vacuum tube and one standard 45 v dry battery is described. When used in connection with a conventional a.c. to d.c. rectifier the stabilizer fully compensates for fluctuations in the a.c. mains and provides a d.c. output of zero to several thousand volts in which the fluctuations are less than 1 part in 5000. No current is drawn from the dry battery, hence its useful life is a year or more. Circuit constants are given for stabilizers supplying 85, 135, 600, 1000 and 1400 volts at currents from 15 to 0.5 milliamperes. A general theory of all similar triode, tetrode and pentode stabilizer units is derived, and the impracticability of eliminating the single dry battery is pointed out.