yes bti red led will light up on the power tubes if one operates outside of what the primaluna amp bias range allows
coughing tubes usually small signal tubes... try fresh set, or move tubes side to side to isolate
primaluna supplies sino shuguang tubes labelled primaluna... primaluna gets them with more quality control but the qc is far from perfect, and the modern tubes often wear out get noisy faster than old stock ones due to poor materials poor manufacturing of the tubes
coughing tubes usually small signal tubes... try fresh set, or move tubes side to side to isolate
primaluna supplies sino shuguang tubes labelled primaluna... primaluna gets them with more quality control but the qc is far from perfect, and the modern tubes often wear out get noisy faster than old stock ones due to poor materials poor manufacturing of the tubes