I have recently built a two stage tube preamp (anode follower + cathode follower)using 12AY7 tubes. Upon its completion, I connected to one of my power amp (Summo 9+), and it fuctioned fine and sounded very good. However, when I connect this preamp to my other power-amp (Classe C200), the protective circuit was tripped. I have observed the practice of waiting for about 1 minute before turning on the power-amp but in this case, it did not help. Using my multimeter, I measured the DC in the output jack and recorded wide flucuation voltage (between 150Mv to about 0.1Mv). Having only the skill to built based on circuit, I can not understand what causes the problem that affact one power amp but not the other. I thought the capacitor before the output will block all DC. Any help will be appreciated.