Tube amp repair.

Hi, my name is Brian and I'm new to to the site and forums in general so let me know if there is somewhere else I should be asking this question.... My 1st tube amp was a Chinese Yaquin MC-100SE (ya I know, that's my problem right there, lol) that sounded pretty decent at first then started occasionally red plating the kt88 tubes. At the time I also did not know about using a Variac so this could could have started the problem or it it could just be a 50 lb paperweight. Anyway I'm wondering if anyone knows of someone in Minnesota (preferably SE Mn) who does tube amp repair. The thing has been taking up space for a few years and I'm trying to figure out what to do with it. Not sure it's worth the cost of sending it anywhere but thought if there was someone within drivin distance that would take a look I might do that.


New Chinese amp will be cheaper. That's how it usually works with Chinese gear pretty much anywhere.

Two questions. Why do you need a variac and why don't you think it just needs new tubes? 

I had a Yaqin MC-30L amplifier that world flawlessly for me for several years and a friend of mine has it now.  I think Yaqin is above the Chinese "junk" stereotype, but repairs can still be expensive.  My local shop (across the country) charges around $200 per hour for repairs I believe.

I'd check with some of the local hi fi shops as they may have a person that does their repairs.

Is there a bias adjustments on each side. The desired voltage is written on the side 2V. There should be pots for the 6SN7 too. They are under the plate. Check the bias first, then it's most likely a bad valve. They have a soft start circuit, no need for a variac. Few newer valve amps require a variac. Most of the older ones if they have been worked on at all, something has been added to aid in a softer start.


I replaced the 4 power KT88's with a set of Gold Lions and of course I checked the bias. As far as the variac is concerned, many Chinese amps and also older designed US amps are designed to run on 110v. Most Wall sockets today are 120v, tho this may be vary some depending on where you are (mine runs from 120v to as high as 125v. I have talked to several techs over the years who say these types of amps should always be run with a variac to keep the voltage under at least 115v. 


Ralph Karsten (Atmasphere) does repairs. He is one of the nicest individuals in audio. I am sure it will be a piece of cake for him. His office is on Selby Ave. in St. Paul. Also, Hi Fi Sound in Minneapolis (near farmer's market) have a technician who repairs tube equipment.

They both have web sites.

Good luck,