Note : Cathode follower pre amp circuits like some marshall amps cant use the RI tungsol or a few others. The chinese tubes work and seek advice on forums or research by reading other articles. The voltage on the cathode is too much for some tubes. Excelent article Phil. I revoiced a Egnater Rebel 30 channel 2 with a 12au7 and now the gain at 8 gets nice breakup and not heavy metal mayhem which i dont play. Yep its like another amp now ! Very good blues & rock & roll going !