On Friday, June 30, 2017 at 7:34:24 PM UTC-4, Xtrchessreal wrote:   
   > On Sunday, February 12, 2017 at 10:54:08 AM UTC-7, Luigi Vogliobene wrote:   
   > > Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier   
   and it is time to replace the final tubes, I was suggested to replace the   
   original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I   
   wonder if the BIAS will    
   be OK if not exactely the same (this amp has a fixed BIAS).   
   > > Thank you in advance for your kindly replies.   
   > > Luigi   
   >    
   > The idea of throwing in 4 KT90EH tubes without any knowledge of quad output   
   tube stage or biasing would be reckless.  They need to be a matched quad and   
   as stated Jinvina v EH likely have different operating Max watt output.  If it   
   were me I'd put in    
   three trim pots one to bias each pair and then one for balance between the   
   pair (not if its a stereo output, 1 pair per channel) and then I would run   
   them very cool for long term use.   
   >    
   > I know it is a very different amp but I run tubes in my Marshalls as cool as   
   possible, to be more electrically robust since they are subjected to extreme   
   variances in input voltage 100-130 VAC RMS that range from no ground to well   
   made GFI and    
   Independent grounds.  I can also get more head room from input signal to   
   output signal, to some degree, depending on clean or distorted tones, the tone   
   stack seems to provide more range as well.  That may a subjective thing I'm   
   hearing, I have not put a    
   freq sweep across it and measured output.   
   >    
   > X   
      
   Variation of bias due to aging or adjustment will not effect the frequency   
   response much at all. But the THD & IM will be. All easily measurable & heard   
   by those who enjoy rock guitar or the classics. And by how much heat is   
   evolved!   
      
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