I bought this amp open box described as good condition with no tubes from AV123 two weeks ago. The matched quad of power tubes (Valve ART 350b's) that I ordered arrived today. I ordered the other signal tubes from a different vendor and inserted them last week when they arrived. I inserted the power tubes into their sockets, connected my speakers and turned on the amp for the first time. Everything seemed OK. I waited 5 minutes and then measured the bias for each output tube. The first three worked perfectly and only had to be slightly adjusted to the recommended bias setting. The fourth tube is where the problem begins. It measured 12 volts on my meter and I was only able to lower this slightly by adjusting the pot. I turned the amp off, let the tubes cool and swapped the position of two power tubes including the one in the questionable socket. The bias problem did not follow the tube. This time, however, the bias in the questionable socket measures at 6.855 volts and, once again, I am unable to adjust it any lower. I am new to tubes and am wondering what to do. I'm concerned I might have made a bad purchase and have subjected myself to a $350 dollar lesson. Any help would be appreciated. Thanks in advance.
