2. amps-preamps
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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?

mazian

Showing 1 response by carlsbad2

carlsbad2's avatar
carlsbad2
2,023 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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