user: ### NOW ANSWER:
Question: Name the rectifier for Det/Pre-amp of X and # tubes of 4
Columns: ['# Tubes', 'Converter', 'Det/Pre-amp', 'Audio Amp', 'Rectifier']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['4', 'X', 'X', 'X', 'X']
SQL:

assistant: SELECT "Rectifier" FROM "Table" WHERE "Det/Pre-amp" = 'X' AND "# Tubes" = '4';