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Old 10th November 2010, 05:14 PM   #1
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Default Yaqin MC100B Newbie Questions

Please forgive my ignorance. This amp has four KT-88, four 6SN7 and two 12AX7 tubes. The KT-88 tubes are "output." Which of the remaining tubes could be called "input?" The reason I am asking is because I wonder which tube can be "rolled" to yield the greatest sonic improvement.

The amp also has "caps," and apparently valve amp sound can benefit from upgrading these.

Basically, I am at the front edge of a steep learning curve regarding this amp, and tube amps in general. I am an intelligent person, and a ferocious tinkerer, but I have never dealt with electronics.

Finally, FWIW, this amp comes back from the shop today. I had bought it used for $366, appeared to be in good shape, listened to it, brought it home, listened to it some more (CD player only) then.......the next day, I brought home a solid state phono preamp, plugged it into the front inputs, switched the unit to ".6V" input, switched it on and loud, high voltage snapping occurred near one of the KT-88's. The shop now says 3.5 hours of repair were necessary to the PC board underneath. Sht!
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Old 11th November 2010, 10:40 AM   #2
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Best tubes to roll are the 12AX7 tubes then the 6SN7. Try Mullard for the 12AX7 and Sylvania for the 6SN7 - both of those tubes are getting pricey these days tho. The Chinese KT-88 that come with this unit are ok not too bad, not the best but you gotta spend some $$ to improve any sound if at all on the KT-88 tubes. The rest of that amp prolly isn't worth spending a lot on tubes, just get something other than those stock tubes on the front end, they are too bright for my taste.

On tinkering you probably haven't taken the lid off of this one yet. When you do you will find this is not your typical tube amp. The boards are difficult to take out and have CRAPPY soldering ta boot. You would probably be better off getting a beater mono amp and do a restore to learn on. Get this one going and get another point to point wired one to tinker with.

BTW - you do know there are lethal voltages under the hood right? Suggest reading up on working on tube gear before you dig in. Safety 1st, 2nd & 3rd.

Just my .02 - those Chinese amps are not worth the price paid for admission. Better to get a vintage unit like a Heathkit or the like to start out on, specially if you like to tinker. The vintage units will have to be re-capped and the like anyway to get it going for long term and there you get your initiation on sourcing your parts, schematics and help from those that have gone before you, not so much with the chinese gear.... Complicate the simple is what your amp looks like with the hood off.

Like I mentioned - better to listen to than to work on. Try another amp to get your feet wet. Heath W-4M ... good place to start, something like that. Not a ton of parts and the circuit is classic and MANY folks can help you if you need it. Learn to solder, and I mean REALLY learn to solder. Read up on it, practice it, practice again and get it perfect... then and only then Grasshopper, you will be ready.

My .02....

Cheers,
Bob
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Old 11th November 2010, 05:32 PM   #3
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Very cool, Bob. Thank you for your patience and advice, which appears spot-on.

The amp is back from the shop. As you say, the tech did not enjoy his experience repairing some fried wires which apparently required getting between the printed circuit board and the chassis. A little expensive. But.....it works. And frankly, as I knew, I strongly prefer tube sound.

The sound is not awesome. As you mentioned, there is a bit too much brightness up top. I'd also say it is also missing something in the mid-to-low range. Low end is fine, just a bit flabby. And there is no punch at the upper-low/lower-mid area.

Again, I appreciate the advice and the time you took. I have a couple of questions (I don't really run out of them). First, I want some definitive information about the input (RCA) connections at the front of the amp. This amp fried when I hooked a small, simple phono preamp to my turntable and plugged it into those inputs, having the switch on ".6 V" . Did I do the wrong thing? Aren't these inputs for the purpose of a phono preamp? I can tell you that plugging the turntable directly into those inputs results in the music coming out of the speakers, but very faintly.

Bob, I can see the wisdom in your advice to find another piece of equipment to modify and learn. And yes, I do understand there is dangerous voltage present. I want to learn more about that, as I am pretty trepidacious about even using my mutimeter on it. But in the meantime, would it not make sense to hotrod this thing just a little, with tubes as you say, and perhaps caps?

Again, I am hoping for less brightness up to, and more punch in the mid-to-low area.
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