Name the det/pre-amp with # of tubes of 8
CREATE TABLE "variations_on_the_theme" (
  "num_tubes" text,
  "converter" text,
  "det_pre_amp" text,
  "audio_amp" text,
  "rectifier" text
);
SELECT "det_pre_amp" FROM "variations_on_the_theme" WHERE "num_tubes"='8';