I hadn’t purchased a new matched quad of EL34’s in probably like in 15 years and was using a mix of tubes. I saw the EH EL34’s for $37 for a matched quad. At that price I didn’t have anything to lose. I installed and biased them (39 mV on a 1996 Jolida 302). They were fine at first but after two weeks or so became very noisy, that rattling metallic sound. It seemed that whenever that happened the bias was out by a little, 1.5 mV or so. I’m curious why tubes slightly out of bias create so much noise. Is this a characteristic of cheap tubes? Tubes sound pretty good right now after 3 months.