CREATE TABLE table_66728 ("# Tubes" TEXT, "Converter" TEXT, "Det/Pre-amp" TEXT, "Audio Amp" TEXT, "Rectifier" TEXT)

Name the rectifier for Det/Pre-amp of x and # tubes of 4
SELECT "Rectifier" FROM table_66728 WHERE "Det/Pre-amp" = 'x' AND "# Tubes" = '4'
SELECT "Rectifier" FROM "table_66728" WHERE "# Tubes" = '4' AND "Det/Pre-amp" = 'x'