• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Yaqin MC-100B - not as powerful as advertised, but still a good value.

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To avp1
no for matching the tubes..

Considering the design of the ampli:
the 6sn7 driver of right channel (v1) is for the left channel v3 or v4??
In other words the design of ampli for right and left channels are simmetric or specular???
Considering the 12ax7 it seems specular, and also from few posts I read..

But I must be sure

Thanks

Thanks
 
This is a third and final part of my analysis of Yaqin MC-100B, where I describe my attempt to improve its sound. <snip>

All the work you did here is really impressive. I'm wondering though, if you would provide a simple how-to, with photos and diagrams, for another person wishing to make these mods. I can solder, but I can't really read a schematic and correlate it to the circuit in front of me. Not easily, at least. A step-by-step how-to guide would be really, really useful.

Charles.
 
Ex-Moderator
Joined 2011
All the work you did here is really impressive. I'm wondering though, if you would provide a simple how-to, with photos and diagrams, for another person wishing to make these mods. I can solder, but I can't really read a schematic and correlate it to the circuit in front of me. Not easily, at least. A step-by-step how-to guide would be really, really useful.

Charles.

Here is well documented paper for the MC-10L which is similar to (but not the same) as the MC-100B which might be worth a look. The biggest problem with the MC-100B from reading the above posts, seems to be the low B+ voltage, the amplifier will never be able to put out its rated full power given the low B+ voltage.
 
Ringing

Now it was time to address ringing in amplifier after applying global feedback. From simulation I noticed that there is not enough of phase reserve at high frequencies. This is a common problem with tube amplifiers when they use output transformers. I verified that there is no phase problem in the first three stages. Thus all my attention was concentrated on the final stage with a pair of KT88. I did some research and found one possible solution – shunt primary winding of the output transformer with capacitor and resistor. One practical example from 1950th for KT88 based amplifiers included 1000 pF capacitor in series with 1 KOhm resistor between plate and screen grid connections of the output transformer (it has to be applied to both tubes in push-pull circuit). I decided to follow this example. The result was positive. It reduced ringing from 4-5 to 2 cycles. It also visibly reduced overshoot at fronts for at least 50%. I decided that it is enough of improvement, and didn’t try to get it any better.

Thanks avp1 for a concise description of the work you have done to improve the MC-100b.

I have 2 MC-100b amps (bi-amping a pair of home made OB`s) mine suffer from ringing as well. I would like to fit the caps and resistors as above but although my soldering is up to the task I am not an engineer so I have no idea where to put the components. Can anyone do a diagram or post photos of the work required.

Thanks
Alan
 
Hi Everyone, thought i would share my experience with this amplifier. I got it off Ebay delivered including taxes for £500. Ordered it thursday midnight and it was at my front door midday the following monday. Wow was quick.

Anyway i connected it up and had a listen. It sounded terrible to be honest. I reached for my soldering iron and got a circuit diagram. I removed the feedback loop completely. Got rid of the volume control and selector switch from the circuit. I then wired the front set of phono sockets to the grids and added a 47k grid stopper in series. I then noticed that the grid leak resistor was actually 510k and not 39k as per the drawing. I fitted 47k instead. I then removed the capacitor across the input stage cathode and made the two cathode resistors one value.

I threw away the ecc83s and 6sn7s that came with the amplifier and fitted mullard and sylvania respectively. I switched on and biased the output stage to 65ma having measured an anode voltage of approx 420Vdc.

Time to listen. I was gobsmacked by the sound I heard coming from my B&W683s. Sublime is the word. Amazing imaging and depth, with massive space around all the vocals and instruments. I played disc after disc and the hours flew by. I didn't want to switch it off. The sound continued to improve as the amplifier burnt in. Sounding good on CD and Vinyl.

6 months on and i'm still grinning. I have since replaced the ECC83s with Russian 6n2ps and modifed the PCB to take into account the different heater arrangement. I split the track between pins 4 and 5 with a junior hacksaw blade and moved the heater supply wire from pin 9 to the now spare pin 5. I've also replaced the 6 0.22uf caps with wima MKP 10s. This has added another dimension again. The amplifier sounds so fluid and natural and makes listening a real pleasure.

I use it with a Conrad Johnson PV10a and a heavily modded Mission PCM4000 and Technics 1210 with SME V.

My only wish is that the amplifier had more power, but for the money I can't complain. I punches well above it's weight.

I will post any further mods I make. Next mod is to use a bunch of Elna Cerafines leftover from another project in the power supply. Should spice things up a bit more.

Bye for now ;)

Feedback loop is only when SP are connected to 8ohms SP tap right?

1,2k+330R=1,5K you fitted right?

Thx
 
To avp1
no for matching the tubes..

Considering the design of the ampli:
the 6sn7 driver of right channel (v1) is for the left channel v3 or v4??
In other words the design of ampli for right and left channels are simmetric or specular???
Considering the 12ax7 it seems specular, and also from few posts I read..

But I must be sure

Thanks

Thanks

Viewing from the front, the left hand 12AX7 feeds V2 6SN7 which is the Left Channel phase splitter. This feeds V1 6SN7 which in turn drives the KT88's V1 and V2.

The right hand 12AX7 feeds V3 6SN7 which is the Right Channel phase splitter. This feeds V4 6SN7 which in turn drives the KT88's V3 and V4.

Hope this clarifies the tube signal paths.
 
Have anyone here tried to bypass/remove the UL/Triode buttons?

I want to connect my KT88 direct without any switches because i never listens to UL anyway. Triode sounds better in my ears!

I'm little unsure though how to do it. In triode-mode signal passes through the longest winding right and UL on two middle"taps"?

I ask before i do anything stupid ;)
 

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