First off, my apologies. I posted the first paragraph below over at Audiokarma but after figured here would be a better place for it sense there is a lot more traffic. I'll cut and paste/post any answer I get over there if it's okay.
Hi, I have an old Fet Valve EC preamp and was wondering what should I consider when selecting the right replacement 12ax7's for the best sound and most linear output from both the line and phono stages. I've noticed over the years that certain tubes will seem to push up the low end and/or roll off the highs a bit. Is it distortion that I'm hearing? My complete uneducated guess is that it could be due to some 12ax7's being lower gain then others and the circuit likes a certain gain range but of course I really have no idea.
Also, I finally changed over the 12AT7's in a Fet Valve 300EX from that of Chinese origin to Electro Harmonix brand, adjusted the internal pots via instructions that I found in this forum to 235ma and it was a large step up in sound quality. It was a great treat. I was expecting either a minor one or no difference.
Any input is greatly appreciated. Thanks.