CREATE TABLE "variations_on_the_theme" (
  "num_tubes" text,
  "converter" text,
  "det_pre_amp" text,
  "audio_amp" text,
  "rectifier" text
);

Question: Name the rectifier for Det/Pre-amp of x and # tubes of 4
SQL: SELECT "rectifier" FROM "variations_on_the_theme" WHERE "det_pre_amp"='x' AND "num_tubes"='4';

Question: Name the # tubes with Det/Pre-amp of x and audio amp of x x (push-pull)
SQL: SELECT "num_tubes" FROM "variations_on_the_theme" WHERE "det_pre_amp"='x' AND "audio_amp"='x x (push-pull)';

Question: Name the det/pre-amp with # of tubes of 8
SQL: SELECT "det_pre_amp" FROM "variations_on_the_theme" WHERE "num_tubes"='8';

Question: Name the det/pre-amp with # tubes of 4 and rectifier of x
SQL: SELECT "det_pre_amp" FROM "variations_on_the_theme" WHERE "num_tubes"='4' AND "rectifier"='x';

Question: Name the # tubes with audio amp of x
SQL: SELECT "num_tubes" FROM "variations_on_the_theme" WHERE "audio_amp"='x';