Yaqin MC-100B Tube Amp Tune-Up Mods

April 27th 2014
Created by: Jeff Doucette E.Tech
jeff@hi-tronics.ca

Here is some information on modifying your Yaqin MC-100B vacuum tube amplifier for improved linearity and overall audio quality enhancement. There are some design flaws which affect not only the longevity of the tubes, audio quality and clean headroom in the stock design of the Yaqin MC-100b. If you would like any modifications performed and tested by a professional technician we can perform the mods at Hi-Tronics. Please contact us for a quote on rework or repair services for your Yaqin MC-100b.

Update – Nov. 7 2015: There is a design flaw in some Yaqin products that use this same 6SN7 phase inverter. I forgot to add this update with the original article and some re-investigation into the amplifier with the phase shift distortion removed and balanced will be needed. If you are skilled with electronics you can balance your PI manually by putting a pot in place of either R6 or R7. This is dangerous and you must understand the risks. If you feel uncomfortable or have any questions about what you are doing, don’t do it! Use the pot to find the optimal value for your tubes on the oscilloscope and then sub a resistor for the pot. This will balance the PI and fix the phase inverter distortion in the Yaqin MC-100B and various other Yaqin and Chinese designs where the PI load resistors were not chosen correctly.

 

Yaqin MC 100B schematic/wiring diagram with dc voltages around the tube sockets:

yaqin mc-100b schematic

If you are among many who have tried swapping tubes without much sonic improvement but large costs, it is perhaps time to start looking at making some changes to the circuit and the points of operation.
The first stage is a 12AX7 pre-amplifier SRPP (shunt regulated push-pull) type. It works with a high voltage of about 330V and a current of about 0.65 mA, which is well chosen.
The second stage of MC-100B is Q1 6SN7: Cathode follower with a double triode 6SN7, loads of plates 47k ohm and common cathode resistor of 68k ohm. The voltage is around 354V. There is a direct connection with the previous stage, the grids are at 164V, 170V and cathodes (2.5 mA at 68k cathode resistor) so bias is -6V. This stage works under very low current of 1.25 mA per triode, so right in the rounded area of the load line curves and thus it produces a large amount (too much) harmonic pairs.
Third stage is Q2 6SN7:  Driven by cathode follower with 100k cathode load resistor attached to the grid of the KT-88. at a quiescent current of 3 mA triode and 414VDC. There is a direct connection with the second stage so grid voltage is about 296V (the plate voltage on the phase inverter).  3 mA at Rk with 100k resistor puts the cathode at 300 V that the filament does not appreciate at all. The bias is -4V at this stage.
Output stage:  KT-88 push-pull configuration.  UL (ultra-linear) or triode mode operation is switchable, adjustable negative bias, high current about 60 mA idle with polarization to -57V.
The second stage (Q2) seems like a great place to review. It is working at about 3 mA, which is a minimum, and the Yaqin MC-100B already produces high levels 2nd order harmonic distortion.  The tube must still be linear and should not distort. A better option is the 6SN7 should work under 9-10 mA.  You do not need to change the high voltages of the other stages, which are just fine. The calculated optimal values are here:
R6 and R7:  Plate loads of two triodes of the phase splitter, change 47k  ohm to 18k ohm 2 watt resistor.
R9:  common Rk  68k ohm changed to 27k ohm 3 watt resistor.
R11:  The series resistor on the PSU goes from 20k to 9.5 k resistor (10k should suit) still 3W. This change keeps the power supply voltages identical, although the current consumption are higher.
R14 and R15:  Two R 5.1 k resistors in series to give 10.2 K,  use 7.5 k 3 watt resistor for the same reasons as above.
With all these resistor changes, the operating points of tube stages 1, 3 and 4 should not move, only the second is changed and now each triode sees about 3.15 mA, and its polarization -6V  to 4V; big change in linearity. If you find the MC-100B does not work better at this bias, simply return to the old values ​​ Note:  The voltages are those found with AC 220V model (planned for this amp).
Please check your heater voltages as many of the Yaqin MC-100B models I have seen are providing too much heater voltage; sometimes over 7 volts! If you want the tube retains its quality for thousands of hours, the heater voltage must be precise, with a tolerance down but not up. For calculating the heater supply resistor, you must measure the exact voltage across a power tube, and calculate the Resistor here: R = (V measured at tube heater – 6.3) / 4.7 amp.
I suggest change the coupling capacitors (C6 and C7) which 0.22μf  stock to 1μf audio grade capacitors.
 If you want to remove the harshness of the corner of the square wave change C10 which is 50pF to 72pF, it will round the corner of the square and remove the shrill and hardness of the overall sound.
The many of the capacitors in the Yaqin MC-100B are under-rated for the B+ developed across the HV rail and should be changed. Please contact us for recommendations or for any rework on your Yaqin tube amp.

We have found some additional non-linearities in the design of the Yaqin MC-100B and currently reworking the circuit to improve the headroom of Q1 and Q2. We are looking at some major design flaws that are present at the preamp section of this amplifier, for example lets take a look at the half volume/full volume oscilloscope screens of the Q2 6SN7 stage PIN 5 with a 2Vp-p 1Khz input signal applied at the RCA “CD in”:

Half volume:

Yaqin MC-100B 6SN7 Measurements

I will update this page when we have these distortions reduced further. It appears that stock MC-100B amplifiers are very much un-usable past half-volume due to the amount of distortion present within the preamp. It is a combination of problems; including inter-stage gain (+60db) too high and poor choice of operating point on the PI. If you have any questions or comments you can contact us.