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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!