In my power system dynamo lights are regularly powered by a mixture of AC and DC, with the first taking over completely when riding fast and the second - when at rest. I remember that in the B&M lamps there was some functionality that employed only one polarity of the AC. I.e. when powering the lamp with DC, you need to test both polarities and pick the one for which the lamp works consistently.

P.S. It should be 6V but a bit up or down is OK.