From: info@turneraudio.com.au   
      
   On Monday, 13 February 2017 04:54:08 UTC+11, 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 Jinvina KT90 looks like a fancy kind of KT90. Any other brand of KT90   
   should be able to be biased like Jinvina, if the advice in Ming-Da user manual   
   is followed.   
      
   HOWEVER, nobody should assume Jinvina is exactly the same as KT90 EH.   
      
   THEREFORE, be sensible, and fixed bias should be adjusted so that each KT90   
   has Idle Pda <25W, so that if the Vadc between anode abd cathode = 450Vdc,   
   then each KT90 has cathode dc current no exceeding 66mAdc. This allows for   
   Pdg2 = 5W at idle, Pda = 25W.   
    It seems to me this will allow for good class AB sound. I do not know if   
   there is just ONE bias adjustment for all 4 output tubes, which IMHO would be   
   BAD engineering, because tubes would have different Ia and thus different   
   temperature. THERE SHOULD be    
   one adjust pot for bias on each KT90.    
   If so, then each bias pot may need to be adjusted 4 times over a 30 minute   
   warm up period to make sure bias Iadc is the same when all tubes have warmed   
   up properly.   
      
   In 2011 I worked on a pair of 85W a Ming Da mono amps made in 1996, each with   
   a pair pf 845 in PP at output. I needed to completely gut them and re-wire   
   them properly.   
   I hope your Ming Da have much better quality than was normally awful in 1996.   
   I also hope your amps have active protection to prevent an overheating output   
   tube from destroying an OPT which may be fragile, and now perhaps difficult to   
   obtain as a spare part. Don't say I didn't warn you.   
   Patrick Turner.    
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)