Name the rectifier for Det/Pre-amp of x and # tubes of 4
CREATE TABLE "variations_on_the_theme" (
  "num_tubes" text,
  "converter" text,
  "det_pre_amp" text,
  "audio_amp" text,
  "rectifier" text
);
SELECT "rectifier" FROM "variations_on_the_theme" WHERE "det_pre_amp"='x' AND "num_tubes"='4';