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 |
@jjss49 OK I know Chinese junk, poor quality control is the obvious reason the tubes won't bias. However it doesn't explain why the tubes sound good and getting better with 50 hours of burn in now. I would appreciate some informed opinion why this is the case and what impact failure to bias might have on long term viability of the tubes and sound quality. And yes the amp biases fine with Gold Lion KT88 tubes and Tung Sol KT 120s. |
Sounds more like a problem with your amp. It doesn't make sense that you can't adjust the bias. Swap 2 tubes and see if the problem moves to the new location. report back. Now when you say the other 3 move "all over the place" are you saying that they respond to bias adjustments? Bias at 25 vs 35 is hardly noticable sonicly, especially if 3 of 4 are set correcty. Thus the sound is good. Jerry |
Okay so you’ve tried other brand KT88s and it works fine. Perhaps the is why all of us do our best to try and purchase from a reputable tube re-seller, and the good ones actually test and check their tubes before sending them out. With "40yrs of experience", we suspect you already know this. Can you return them, try others? |
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 |
