Yaqin MC-100b schematic diagram

[munosmario]

I am at fault of tagging along with an error (most cerainly, a typo) in Gonzalo's post. He recommends 725 mA for bias. I am sure he meant 725 mV or 72.5 mA. Accordingly, my quotes of other posters' recommendations should have been 500 to 600 mV or 50 to 60 mA. In any event, my previous post is irrelevant (no response needed, Gonzalo) as I bought a plate-voltage measuring gauge that came with all required formulas and charts.

Cheers.....................munosmario

[ccschua]

I am interested to get 1 unit of MC-100B but was told of the quality issues and the capacitors being exposed.

I hope someone can show the photo of the underside (i.e. the pcb with the base cover open)

1. Are the components used are of good quality ? Which brand of coupling cap used ? WIMA ?

2. Can you tap the voltage to 230V

3. Is this a dual mono design ?

4. Are there enough space to modify the volume pot to step attenuator ?

5. Filtering capacitors using which brand ? Rubycon ? Nichicon?

[Gonzalo]

Dear audioguya munsomario, I have a good friend twaker mod guru extraordinnaire (wheezer) he has a small lab at homr where he measures output, voltage distortion freq. response etc. etc. etc.
One day nwe tood back cover out measured all data, output input impedances etc. and concluded that to achieve a very excellent voltage of 425 plate volts for the output on triode we needed to feed the amp not more than 106 volts fo so, so I finally settled on 105 off about 104 out of the varias when amplifier is fully loaded and on.
Then with only 425 volts on the plate he established that a good NOS like Tung Sol willl easy do 70 to 75 mA at that voltage without a problem.
This consistently brings the sound qualit of the amp to a whole new level of excellence.
you do neeed to bring voltage down . . if you adjust output bias to these values with the very dangerous 575 to 600V plate voltage with no varias inline straight form grid at 120 to 124 Volts as it is here, the output tubes will be destroyed in weeks . . .

So decrease teh voltage for the plate and increase the bias current, sit back enjoy!!

One more thing I found out by experimentarion.
Best power tubes 6550 Tung-Sol NOS bar none.
Second best SHuguang GEK-KT88 (Genalex copies form SHuguang in China), a little glary but definetely much beter than anything else out there.

Then 6SN7 either Tung-Sol NOS 6sn7-GT or GTA or GTB circa 1960's, then also more rare more $$ RCA gray glass 6SN7 GT circa 1940's are simply wonderful and also Ken Rad 6SN7 black plates clear envelope, or a ocmbination thereof of the ones mentiones, remember when combining to go form outside to insid eof the amp in mirror image to match the channel (dual mono)
Then for the pre driver te best tube I have found for this amp is in this order:

1- GE 5751 black plates or gray plates
2- Sylvania 5751 W1 gray plates
3- Tungsram ECC-83 (hungarian tubes)
4- Telefunken ECC83 gray plates ribbed or smooth both very good


Definetely the 5751's providing a bit less gain a A LOT better tone, structure better top to bottom balance, etc. etc.

It goes very well with my California Audio Labs Alpha tubed DAC sporting very excellent Tungsram ECC-83 (12AX7 equivalent)
fed form the front connectors of the amp with silver cables, either PSC Australian made or Kimber silver KCAG or the KS series (not hybrid!! just silver only)
By now you realize i have spend a fortune playing with tubes and selecting the ebst tubes for this amp (in my taste in my system, room, speakers etc. etc.)

I am very pleased the way this amp performs bass in triode mode form the 6550 tubes simply amazing!!
The vocals, harmonic structure, top end, sense of space, rithm, pace, slam, and detail are simply outstanding.
you can further teak the amp by cnahging soem of the amps resistors and cpacitors by superior quality i am sure it will greatly still improve sound quality to the likes of the $5K and over likes.
Just now with different tubes sound simply stunning.
Worth every penny!!!

Hope this helps.

Gonzalo

[munosmario]

Thanks, gonzalo. The results of your trials with your "wizzard" friend, namely, 104 volts AC power (variac reduced), 425 volts DC plate voltage, and 72.5 mA bias, do they apply basically the same to the Shuguangs (KT88-98 and GEK-KT88) as to the Tungsol 6550 NOS? Or some agjustments may be necessary? Have you tried he Shuguand KT-88 solid plates (Penta labs and perhaps sold under some other private label)? I have a set of both the KT88-98's (three hole plates) and the Penta solid plate variety...I learned about the Sophia from your post.

Thanks again..........munosmario

[munosmario]

Gonzalo, one more question. Do you happen to know or can you somehow acertain if the Yaqin MC-100B is Class A or Class AB? I have seen it advertized as one or the other by different vendors. Canada's Grant Fidelity and the Hong-Kong crowd refer to it as Class A (but I am a bit aprehensive as these vendors appear to be using hyped up specs like claiming power at 65W ultralinear and 32W triode (from the Yaqin factory's 60W and 30W respectively). Also Grant Fidelity* states that you can by-pass the preamp section using front input when that is not the case--anyone able to read the schematics can see that only the input switching stage is by-passed and the front stage is still in the amp circuit (reason why the quality of the 12AX7 tubes is still important even if you use a separate audiophile preamp). As you know, knowing the right amp class is important as power tubes can run at significantly higher bias when used in a Class A amp circuit.

Thanks & cheers......................munosmario
* Grant Fidelity's claims appear under the discontinued products in GF's website (it seems they stop selling their private label version a few months ago but still recommend it strongly).

[mastajax]

Originally posted by Koifarm
Boboo i just read your other thread. And i must say that you have bought the wrong amp for your speakers.

You can do two things:

1- Sell the yaqin on ebay or so and buy a stable amp like Hypex UCD with tube preamp for the nice sound.

or

2- Modifie the yaqin to you wishes like more bass. I can recommend you the following mods.

- Change tubes, for more and firmer bass i use always this tube setup. 4x EH KT90, 4 EH 6SN7 and 2 x EH 5751 Goldpin. if this is not enough,

- I change Capacitors C6 and C7 for Clarity Caps SA 0,47uF.

The input impedance of this amp is quit low, about 20K. So your sources must have at least 1 uF or higher Capacitors at the output.

I have read that the 6SN7 stage, Q2 & Q4 on the schematic might be running too low of current under 2ma when it should be around 9ma according to the datasheet to keep it linear. Here's an interesting observation article on the Yaqin translated from french:

http://forums.tabcrawler.com/index.php?topic=4903.0

What did you notice when you changed those caps? What value did you go from and to? I think mine has 220nf caps I was thinking of going to 1uf. Are these the only caps you changed?

What is the difference bewteen the KT-90 vs KT-88? Is it possible to use 6550 in this amp?

[Koifarm]

The Caps change give a deeper bass.
If you choose too high capacity for C6 and C7 then the output transformer got in saturation. 0.47 uf worked for me great and the Clarity Caps SA type gives then a bass i like most.

KT90 has more power and more control. 6550 runs also good in the yaqin MC100b.

Ronny

[mastajax]

Thanks Ronny.

I noticed the Macintosh MC30 monoblocks have .47uf coupling caps off the power amp stage also. I will defintely give it a try.

Have you found that 6sn7 might be working too low of operating point?

I think there is about 515v to the plates of my 120v model to that guy that posted about the plate voltage. Isn't this well within the spec of the tube < 800 volts?

[seadiver66]

Originally Posted by halx00

You might find this useful.
I have one of these amps, a most excellent sound.
Using one in the UK means I need to step down the mains 240volt supply or the heaters run at 7.4v and every thing is a bit stressed. I use a buck transformer this works very well.

Duncan

sorry about the quality, email me if you want the original jpeg. 1.24Mb

Hi Duncan, I know long time is passed since you published this post, but I would like to know if full image is still available.
If yes, could you plese send me a copy to eddi at frigos dot it ?
Thank you in advance and best regards!

[ChopperCharles]

Originally Posted by ChopperCharles

I just got a 100B Yaqin/Yaoin amp as well, and I'm really really pleased with it so far. I can;t believe how much better this thing is compared to my old sony tuner. i'm hearing details i did not know existed. Very, very pleased. But... it brought to light how inadequate my speakers are...

Charles

Well, I bouhgt my MC-100B in Feb 08, and it's now August 2010 and I've not had one iota of trouble with it. I've made no modifications, except for a little liquid electrical tape to the top of the exposed capacitors, just in case. I've run through a bunch of tubes too... had a set of RCA 12AX7's go microphonic on me, then switched to EH and then to repro mullards. I actually like the EH 12AX7's better in this amp, there's a little more bass extension. I put the mullards in my phono stage, and they sound really sweet there.

I had JJ Tesla KT88's and the bass response was stunning... but I had the bias set to what the manual recommended... and burned them up pretty quickly. Less than a year. For a while I was hearing a weird noise whenever the amp was turned on from cold and was warming up... thought nothing of it... eventually I was rocking out to something in the kitchen and I suddenly heard horribly loud crackling coming from my right speaker. Rushed into the room and the far right tube looked like a fireworks display. hit the power switch and let everything cool down.... tube was fried (literally, all the labeling turned scorchy brown on that tube).... but I put the original Chinese tubes back in and she fired right back up, no problems, no damage. I biased to a little lower than recommended, and I've been running them since.

In the market for some Gold Lion repros next I think. I liked this amp so much I bought the MS-12B phono stage from Yaoin as well. If anyone is wondering and worried about the long-term reliability of these units... don't be. Except for tubes, I've had zero problems. And seeing as I listen to music at least 2 hours a day every day (sometimes up to 8 hours), and I use the tube amp to power my front speakers in my HT setup, I think going through one set of kt88's and one set of (used) 12ax7's is normal wear.

In short, I'm extremely pleased. The tubes that come with it aren't great and should be replaced with something better, but other than that this thing sounds spectacular and has given me zero problems.

Charles.