THE YAQIN MC-10L MODIFIED

The Yaqin MC-10L is one of many tube integrated amplifiers to come out of China. It has an unusually good reputation and great reviews everywhere.  Given this and its price of under 500 dollars, one must be ordered.  Enter Canadian eBay seller tabnac located in Richmond Hill, Toronto, Ontario, who has excellent feedback.  The amp arrived inside of five business days, in crisp double box packing.  The tubes were shrink-wrapped in hard plastic protective cylinders.  The amp smelled of new fresh paint and had that crisp shiny gleam of new gear.  Not a single fingerprint or dust, wow. The effort put into the packing pays off as the amp arrives without a scratch even though the outer box receives a beating.  The amp also arrives with a set of clean and clear English instructions, some protective covers for the inputs, a nice shielded allen screwdriver to set bias, all tubes necessary, plus a grounded power cord.  The amp was run for three hours and it sounded great, but it was now time start the slight improvements planned.

This amplifier is available for purchase with all Russian upgraded tubes and caps and the work as listed for $999 USD.  Pay Pal add 3%.  Shipping cost can be arranged and will be packed in all original packing material.

WARNING:  Tube amplifiers operate at high voltages of up to 500 Volts and can cause injury or death.  The following modifications are only suggested for trained personnel.  Before working on your tube amplifier, ensure all capacitors are discharged.  If this poses a question, then STOP and let trained personnel work on your amp.

For those that follow, proceed at your own risk.  Your results may differ.

The intention is to make minor modifications to the amp, though I can attest with the other reviewers that it is quite good out of the box.  The modifications involve replacing the coupling caps, upgrading the tubes, replacing the “open to the elements” selector switch (Mouser part: 105-SR2611F-34-21RS), replacing the stock volume potentiometer with an Alps Japan 100K ohm (Mouser part 688-RK27112A00AK), and replacing the four 1K  ohm output tube coupling resistors with Takman resistors  Parts Connexion TAKMAN-71741) , replacing the knobs (Radio Shack part:  274-416) , and touching any areas that might be shy on solder. 

Special note concerning replacing the 20K ohm volume pot to a 100K ohm volume pot:  Some compensation will be necessary in order for the amplifier to not be too loud after the change.  I accomplished this by installing a 20K ohm resistor (Parts Connexion part TAKMAN-72104) and a .0015 uF polyester film cap (Mouser part 647-QYX2A152KTP) in line between the selector switch and the volume control. This approach will change the tone of the Yaqin slightly, so you may want to assess its risk. If you have dull speakers, it may be helpful.  Mallory 150 Series capacitors are also ideal use here.

The Yaqin has six very nice rubber feet, being that the amp is heavy, they provide the grip.  Find a hand towel to lay the amp on, it will make turning and sliding the amp much easier while working. 

The amp cover comes off by removing the six torx screws (leave the screws around the transformer covers alone), three in the front and three in the rear, and then you remove the eight Phillips screws, four on each side along with the trim piece.  Lift the cover straight up.  Next you remove the front PCB board by removing the two small nuts on each side, left and right, and then the Phillips screws that hold the PCB board to the nylon standoffs.  There might be a small spacer washer between the PCB board and the standoffs, these will fall out.  It is a good idea for Krazy glue these down to the standoffs or you are going to earn some frustration trying to hold these in place while you line up the PCB board to the holes.  Once all fasteners are off, ensure that wires can clear so that you can turn this board up in the front while the back stays down.  Be careful not to damage the LED lights.  I unsoldered the two red wires going to this board for ease, but this was not necessary.  You are now ready to take the six square blue caps out. Ensure their position, or confirm that it is written on the PCB board.  But first, put some resistance between two wires (light bulb or big resistor) and short out the big voltage electrolytic capacitor so they don't shock you.

Replace these blue coupling caps shown above at this point.  I used Russian K42Y-2 PIO caps from eBay seller ToneAudio.  These sound great but are not a very good fits for the tight space.  German WIMA MKP caps might be a better choice here or using the cap of your choice can pay off.  Keep in mind that you must allow for the standoffs to clear once that caps are installed.  SO, don’t cover up any PCB board screw holes with your new caps. 

There are four .1 uF caps and two .22 uF caps.  Make sure to use a voltage rating of 275 VDC or higher as the original caps.  Test your caps before installation.  Make sure their capacitance is at least 5-10% of spec and ensure that they are not shorted using an ohmmeter.  One of my caps was and it caused me an entire day of fumbling around until I discovered that the new cap installed was shorted out.

Next is a cathode resistor bypass with 220 uF / 63 volts electrolytic capacitors mod that Les Carpenter of the UK recommends.  I used axial caps (leads on each end) instead of radial caps for this and soldered each end to the BIAS setting brass posts.  If you do not have a higher wattage soldering iron, you might choose to solder these off the PCB board.  Make sure to tie negative side of the cap to the negative post.  If unsure, the cover tells you which post in negative and it is where you place your meter to measure BIAS.  BTW, the recommended BIAS setting is for 375 mV, so set your meter to the 2 Volts DC setting and measure .375  V.  The BIAS is set at the factory, so if it between 350 – 400 millivolts, you can choose to skip this for 6 months or do it if you upgrade the stock Chinese tubes.

What do these capacitors do? By placing a capacitor in parallel with the cathode bias resistor any instantaneous rise in cathode current will be diverted into charging the capacitor, and if cathode current falls, the capacitor will supply the deficit from its own charge. Another way of looking at it is to say that the capacitor shunts or ‘bypasses’ to ground any AC signals on the cathode so that signal current does not flow in the cathode resistor, while the DC bias voltage remains unchanged. With either explanation the result is the same: the cathode bypass capacitor ‘smoothes out’ changes in cathode voltage, helping to hold the cathode voltage constant, preventing cathode feedback and allowing full gain to be realised.  A capacitor will allow greater current flow at high frequencies than it will at low frequencies. If we want the stage to have maximum gain at all audible frequencies then the capacitor must be large enough* to smooth out the lowest frequencies of interest, and the stage could be described as being ‘fully bypassed’. If the capacitor is made relatively small then only high frequencies will be smoothed out while lower frequencies will not. Therefore the stage will have maximum gain at high frequencies and minimum gain at low frequencies, producing a treble boost, and the stage would be termed ‘partially bypassed’. To the designer, this is an extremely useful consequence of using cathode bias. If the stage has no cathode bypass capacitor it may be described as ‘unbypassed’ and will have minimum gain.   This paragraph is taken from MH-Audio Netherlands --Thanks.

Here are schematics sourced from the internet (click on schematic for full size):

We now move on to the selector switch.  I decided to replace the switch for the sole reason that the one provided with the amp has open sides and eventually, dust, moisture, and even smoker’s patina will affect the conductance of its contacts.  A sealed one is a top candidate and comes from Mouser Electronics.

It is a four position three pole type, but we will only use the 2 positions for L/R and then the ground is passed without any switching.  This part requires most concentration since you want to end up with the faceplate matching the back inputs.  Identify L/R cable and label them.  To make things confusing, there are two black wires per side, one is a ground and one is an input.  Identify the 5 wires in each L/R run, Red, Green, Black (ground) Yellow, and Black corresponding to:  Aux 1, Aux 2, Ground, Aux 3, and Aux 4.  Clearly identify all this before taking the assembly out.  Along with the selector, we replace the 20K ohm volume pot with a better quality unit made in Japan by Alps.  It is 100K ohms as opposed to the 20K ohm that we take out, so we will do some compensating for that later, or the unit will scream at you when you touch the volume control.  This might be a good time to take some photos of the wire configuration for posterity in case you make a mistake—or to avoid one.

Note the wires going to the volume pot and also remove it and them.  Get a 3/8” bit to enlarge the hole for the selector switch.  Please confirm this size BEFORE any drilling.  Old rule applies, measure twice, cut once.  Once drilled, blow all the metal shavings out of the amp.  Since the connector between the selector board and the volume pot board are no longer necessary, I decided to use my own 4-lead cable between these two and discard what was provided.  I used two twisted pair Category 6 plenum-rated cable--my favorite for signal wiring.  You can use whatever fancies you up to silver plated copper or even 99.999%. 

Above you see the two cables that go to the amp PCB board and provide the signal from the volume pot.  You can solder these on using the diagram below for guidance.  The longer wire provides signal to the right channel.  One word about these two wires, when you screw down the PCB board, make sure you can get to them to connect to their little white connectors. 

 Now solder the input wires in their proper order to the L and R input lugs of your new selector switch.  Take a piece of bare wire and tie all 4 lugs of the third pole together.  This will serve as the negative and ground tap for the wires coming from the input, going to chassis ground, and going to the volume control.  Meaning, the ground will skip being switched. 

Once you have all your wires soldered on, install the new selector switch and connect the resistors to the wires going to the Alps pot, the grounds to the ground tap and solder the other side on to the Alps volume control.  Below is a useful schematic provided by Arjen Helder on how an Alps pot is wired.  Install your Alps pot and ensure key fits in hope on faceplate and arrange the wires so that they do not cast a shadow on the Yaqin faceplate marquee by blocking the LEDs. 

Special note concerning replacing the 20K ohm volume pot to a 100K ohm volume pot:  Some compensation will be necessary in order for the amplifier to not be too loud after the change.  I accomplished this by installing a 20K ohm resistor and a .0015 uF polyester film cap in line between the selector switch and the volume control.   The picture below shows this modification.  Once again, proceed at your own risk and make your own decisions on how far your skills can take you and the Yaqin.

CAPACITOR BYPASSING:    

Some of the higher frequencies are allowed to go through the capacitor, which would otherwise be blocked by the resistor in the line as pictured here.  These pictured are Nichicon .0015 uF polyester film capacitors, Mouser part 647-QYX2A152KTP.

The chart below provides some guidance on what rating this capacitor needs to be in order to achieve a bit brighter top end (otherwise lost by the 20K ohm resistance to the signal).  Use your own ears as the last judge.

I was so impressed with the quality sound of the Takman resistors that I decided to replace the four 1K ohm coupling resistors to the output tubes.  These are from Parts Connexion in Canada.   I did not want to remove the circuit board, so I cut the lead of the old resistor as close to the body as possible and tied the new resistors to the remnant of the leads with silver wire and solder.  You can see them in pink in the picture below; if I recall correctly, these are 1 watt rated.

Once done, double check you work and assemble your amp and it is ready to play. You will no longer be able to use the knobs provided with the amp as these are for a different shaft type.  Install the ones from Radio Shack, these are good quality and fit well with the faceplate.  Before putting the cover on, ensure there are no wires in between the LED and the Yaqin marquee and that the LEDs are actually pointing at it.

Also, keep in mind that new caps take up to 200 hours to burn-in and will sound dull when new; be patient and do not despair! For slight tube upgrades, consider the Russian Mullard reissue of the EL34 and the Russian military spec 6N1P-EV (EB Russian for long life).  If money is tight, the Chinese tubes are quite good and need no replacing.  You may encounter some tube whistling from V1 and V4.  Tap on the tube, if this doesn't stop the whistling, swap them with the phase splitters in V2 and V3.  The Russian military tubes are hardened and do not experience whistling.  After a few hours of listening, this is one winning amplifier and worth every dollar and the effort invested.  If you proceed with this work, let me know of your outcome.

Modifications completed, these upgrades will take the Yaqin to another level. As you can see, the Yaqin has some generously sized transformers, just compare them to the size of the Mullard EL34 output tubes. In this photo, the Mallory 150 series capacitors (now white and made by Cornell Dubilier) are in use to bypass the Takman resistors.  These BTW, are an excellent and affordable choice metallized polyester capacitor for coupling the entire amplifier and are available at Mouser under the Cornell Dubilier 150 name, you can find the datasheet by clicking this link.  They are also the favorite choice of some gear head musicians.



The Yaqin has an unusually pretty glow in the dark, in part due to the intense blue LEDs that light up the brand name and bleed back to the ceramic tube bases.  The Russian 6N1P have a generous red glow to them.

 

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