Tubes are way too cold for the bias voltage. Normal tubes would be somewhere near 42-44ma with -15.5v. ma's work out as 25.5ma, 21.0ma, 21.9ma, 17.5ma. I'd get some new (NEW NEW, not "New old Stock" as those critters are as scarce as hens teeth and cost more per ounce than titanium! The problem with EH7868 (the only new production 7868 tube) is the pins are slightly larger than normal 7868 pins and stretch the socket leaves. So once you put EH's in, you stick with them or re-socket for 7591's. The EH's are good tubes, and sound very nice.