CREATE TABLE table_name_56 (det_pre_amp VARCHAR, _number_tubes VARCHAR, rectifier VARCHAR)

Name the det/pre-amp with # tubes of 4 and rectifier of x
SELECT det_pre_amp FROM table_name_56 WHERE _number_tubes = "4" AND rectifier = "x"
SELECT "det_pre_amp" FROM "table_name_56" WHERE "4" = "_number_tubes" AND "rectifier" = "x"