I am trying to figure out AWG from a European/German schematic. The wire is listed as 0,3mm. (That's 0 (comma) 3mm.) I can find wire for sale in this diameter, but I need a reference to AWG. Is anyone familiar this measurement? The circuit is two parallel 807 vacuum tubes. The wire mentioned above is to wind a parasitic suppressor in both of the plates of the tubes. Thanks.
28.5AWG is exactly 0.3mm diameter. Not every manufacturer produces half sizes, and for most inductors it won't be that critical to be off half a size either way. For fill calculations (if it's a toroid or closed core) you need to be looking at the size over insulation, and not the nominal, and that will depend in the covering you choose. Carefully hand-wound parts may get away with thinner coatings that would be damaged by the forces involved with machine winding.
0,3 mm in English is 0.3 mm.
A Copper Wire Table tells me that #28 wire is 0.321 mm, and #29 is 0.286 mm.
There are online AWG-metric conversion sites. A comma in Germany shows the decimal place. You also should be able to figure out a safe wire size knowing the maximum plate current (e.g. from the power supply transformer rating). Somewhat larger-diameter wire is acceptable, and may be preferable from a mechanical standpoint.