Another Yaqin 13s on the workbench for the usual upgrades: upgraded coupling capacitors, upgraded grid stopper resistors and fitting of cathode bypass capacitors. These mods worked their usual charms and gave a very decent performance, even with the stock tubes. This time tho, I tried the amp with negative feedback removed. The difference was dramatic, and so a small switch was installed on a plate to replace the mains voltage switch which was re-mounted inside the chassis. In my set up, I never switched the feedback back on and both myself and my regular ‘golden-eared’ mates considered it way better. Here’s what the owner had to say:
‘Just collected my MC13S following David’s mods and I’m stunned! In addition to the mods described above David also fitted a feedback bypass switch and with the negative feedback switched off the resolution of this amp is incredible. I keep wanting to try another track to test the improvements but find myself having to hear the whole track before switching, which is not something that usually happens when I buy new kit. All in all I’m very pleased with these mods….’
AD Audio Consultants 
Hi. Are there detailed instructions on the upgrading coupling capacitors and grid stopper resistors as well as how to fit cathode bypass capacitors? I am not technically capable of making these mods on my MC-13S and was going to pay a local repairman to make the changes. I don’t know where these components are located nor do I know what to replace them with. I haven’t been able to locate a schematic for this unit either. Your help is appreciated!
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Hi, most of the information you require is available on the Web but any competent repairman should be able to do these upgrades. Needless to say, you need someone with experience of tube amplifiers. The components I used were: Elna Silmic bypass capacitors,Mundorf ZN coupling capacitors, and Takman carbon film resistors. Hope that helps!
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Hello. I have the Yaqin MC13S, but I live in San Juan, Argentina, a place where TUBE amplifiers are very rare.
I’ve found a technician that worked with valves, but I am not sure if he will understand all the upgrades you have done.
Is it possible for you to send me by email your squematics?
I would be very greatful.
Best regards
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Any competent tech should be able to do the upgrades I’ve described. Just make sure they are experienced with tube amps
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Can’t I just mail you mine and pay you to upgrade it?
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Of course you can! That’s how 70% of amps arrive at my workshop!
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