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