user: Question: Name the det/pre-amp with # tubes of 4 and rectifier of X
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 "Det/Pre-amp" FROM "Table" WHERE "# Tubes" = '4' AND "Rectifier" = 'X';