TAD EL84-STR Quartet(4pcs) Premium Selected valves. The all new TAD-EL84-STR does combine best of both worlds. We did choose the thickest glass option (Mounting Diameter 22mm) and most heavy construction like the russian EL84M and sonically most promising cathode and plate option to meet and exceed the tonal quality of the EL84-Cz. Finally we added the gold grid wire to limit stray characteristics and to improve overall reliability. The result is the best EL84 currently produced. No compromise!