What is the test for a motion for an evidentiary hearing regarding allegations that a jury improperly considered extrinsic evidence?

We review for abuse of discretion the denial of a motion for an evidentiary hearing regarding allegations that a jury improperly considered extrinsic evidence. See United States v. Hendrix, 549 F.2d 1225, 1229 (9th Cir.), cert. denied, 434 U.S. 818, 98 S.Ct. 58, 54 L.Ed.2d 74 (1977). In exercising its discretion, the district court must be guided "by the content of the allegations, including the seriousness of the alleged misconduct or bias, and the credibility of the source." Id. at 1227-28; see also Hard, 812 F.2d at 485. Unless the court is able to determine without a hearing that the allegations are without credibility or that the allegations if true would not warrant a new trial, an evidentiary hearing must be held. See Hard, 812 F.2d at 486.