These tubes are carefully tested and matched. Each tube is paired for mutual conductance AND plate current using a GS-5A vacuum tube tester. Other quality checks include shorts and gas. Boxes are labeled with both mutual conductance and current readings. All tubes test at NOS levels and are very closely matched and balanced. The tubes are selected and matched in pairs with mutual conductance readings between two tubes within 200 micro-mho and plate current within 0.15 mA.