Vacuum tube classify Common vacuum tube Tri-element vacuum tube Three target vacuun tube Coating AL-N-AL ALN/AL N-SS/CU ALN/AL N-SS/CU+Ti Color Slant blue Slant red (contain Cu) More red and gold Structure All glass coaxial double-layer tubes All glass coaxial double-layer tubes All glass coaxial double-layer tubes Common models φ 47*1500mm φ 58*1800mm φ 47*1500mm φ 58*1800mm φ 47*15