Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| # Tubes | Converter | Det/Pre-amp | Audio Amp | Rectifier |
| --- | --- | --- | --- | --- |
| 4 | X | X | X | X |
| 4 | X | X | X | (selenium) |
| 5 (standard) | X | X | X | X |
| 6 | X | X | X | X |
| 6 | X | X | X X (push-pull) | X |
| 8 | X | X X (push-pull) | X X (push-pull) | X |
| 8 | X | X | X X (push-pull) | X |
Question:
Name the rectifier for Det/Pre-amp of x and # tubes of 4
Answer:
X, (selenium)