Hello, everybody!
I have 2 similar Chinese amps from manufacturer xDuoo, and the first is MT-602, and its circuit diagram I found here, loads of thanks to Sir Kit!
I'm also very grateful once again to Ian0 , KeithWalker and Audioguru again for their comments on the thread!
The second amp is MT-604, which is a balanced version of the MT-602.
I tried to find its circuit diagram, but so far without success.
Of course, the MT-602 circuit diagram is also a great help in understanding the MT-604 circuit, however, some details are still necessary.
Is there anybody able to help me?
Thank You in advance!

 

A question asking what music and listening experience you want to achieve that the MK-604 could improve on.
Some on here might suggest operating points based on experience. Others are anticipating emphasis on effects that
their ears want to hear involving distortion, color and vibrance that tube amps give that op amps might lack.
Vacuume tube 6j1 (caillotantoine.github.io)

 

A question asking what music and listening experience you want to achieve that the MK-604 could improve on.
Some on here might suggest operating points based on experience. Others are anticipating emphasis on effects that
their ears want to hear involving distortion, color and vibrance that tube amps give that op amps might lack.
Vacuume tube 6j1 (caillotantoine.github.io)

I love experimenting with vacuum tubes, and I have at least three dozen varieties of them, triodes, tetrodes, pentodes, and they all work wonderfully with these amplifiers. Most of them are Russian made, between 1954-1976. I have other amplifiers too, both pure tube and transistor ones, all class A. I even have a Dynaco ST -70, produced around 1974.
And regarding the MT-604, I have plans to modify its the volume control, but circuit diagram is required.

 

BTW Chinese 6J1 are almost the worst ones by the sound and amplification.

 

Very interesting! A single tube, possibly similar to a 6AU6, at least in pinout. Not a great audio tube.
And now I am wondering about a "balanced" amplifier. Is that the alternative to single-ended?
Certainly tube circuits are not currently in vogue, but certainly still interesting to some of us who are not entranced by only what is current. There is a certain fascination about circuits that do not require 2000x magnification to see clearly.
In addition, the ability to understand and service vacuum tube circuits is a skill not available in many circles.