6AQ8 | ECC85 | 6BQ7 | 6BZ7 | 6BC8 | 6DT8 | 6N1P

The 6AQ8/ECC85 is a popular preamp tube The Russian 6N1P has become quite popular and can be replaced by any of these tubes. There aren't any in current production and with most of them being of European origin, they are getting expensive. There is a long series of compatible but slightly different tubes that will work in place and for a fraction of the cost. These include:

6AQ8, ECC85, 6BQ7, 6BZ7, 6BC8, 6DT8, 6N1P (Russian)