The CV4024 / M8162 is a special quality version of the ecc81 / 12AT7. Heater is center tapped to permit operation in series at 12.6V or parallel at 6.3V. These amazing tubes were designed for long life and are shock resistant up to 500g. They were also made to withstand bulb temperatures up to 200 deg C. The anode is made of pressed sheet as normal, the box is closed on all four sides. The anode box is 2 mm deep 5 mm wide and 11 mm high. This is significantly smaller than the commercial ECC81.