tube bias conundrum
I recently posted about burn in time for a set of 4 Shuguang KT 88Z Black Treasure tubes. After considering the diverse and often contradictory advice I installed the tubes into my Rogue ST 90 amp replacing KT120s and burned in 20 hours before a first listen. The tubes sounded very very good and if reviews can be believed they might have the potential to sound great. But here's the quandary none of the tubes bias properly. One tube sits on 25mA and won't move no matter how much adjustment I try. The other 3 set on the recommended 35mA setting but if tried again 15 minutes later they are all over the place, some right off the scale and some below 25mA. So what is going on here because the sound continues to be impressive despite the bias problem? I've had over 40 years experience with tube gear and never encountered this situation before. Any ideas anyone?
poor quality control in those chinese power tubes, resulting in the set you have not being able to hold bias consistently across the set this of course presumes your amp properly biases other tube sets without any issue |
i agree with the input from @carlsbad2 @decooney op, i wasn't being derogatory about the 'chinese junk'... i was saying those tubes you have in hand were not quality controlled in a way to ensure you as the user/customer can use them in real world conditions with proper biasing the tubes themselves could be / are nice sounding tubes (as you are experiencing) DESPITE lack of spot-on biasing |
