CREATE TABLE table_66731 ("# Tubes" TEXT, "Converter" TEXT, "Det/Pre-amp" TEXT, "Audio Amp" TEXT, "Rectifier" TEXT)

Name the det/pre-amp with # tubes of 4 and rectifier of x
SELECT "Det/Pre-amp" FROM table_66731 WHERE "# Tubes" = '4' AND "Rectifier" = 'x'
SELECT "Det/Pre-amp" FROM "table_66731" WHERE "# Tubes" = '4' AND "Rectifier" = 'x'