[Patrick Turner]

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.