20A power cord on 15A amp connector?

DT, No, you cannot use a 20A connector on a 15A jack.
The blades are oriented at a 90 degree angle, so it will not work. 20A connectors on power cords are much more common today, but you would need a modern power conditioner or amp with 20A input jack. These items are relatively new, I would say designed mostly within the last year or two.

Regards,
John

John is correct -- I have both types of connectors and they only work for gear designed to accommodate that connector. However, I've seen ads for power cords with 15 amp connectors that claim 20 amp cord capabilities (i.e., the wiring can take it). You may wish to double check that -- i.e., is it a 20 amp cord or a 20 amp connector?