H03G9/00—Combinations of two or more types of control, e.g. gain control and tone control
H03G9/02—Combinations of two or more types of control, e.g. gain control and tone control in untuned amplifiers
H03G9/04—Combinations of two or more types of control, e.g. gain control and tone control in untuned amplifiers having discharge tubes
H03G9/06—Combinations of two or more types of control, e.g. gain control and tone control in untuned amplifiers having discharge tubes for gain control and tone control
H03G9/08—Combinations of two or more types of control, e.g. gain control and tone control in untuned amplifiers having discharge tubes for gain control and tone control incorporating negative feedback