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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?

128x128mazian
jjss49's avatar
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jjss49
6,612 posts
 

So what is going on here because the sound continues to be impressive despite the bias problem?

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

mazian's avatar
mazian OP
406 posts
 

@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.

carlsbad2's avatar
carlsbad2
1,623 posts
 

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

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