• 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.

[n00b] Yaqin MC-50L re-cap and re-pot

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hello all!

I've had a Yaqin MC-50L tube amp for a while now and keep reading about how re-capping it and replacing its volume pot will take it to a whole different level.

I'm a complete n00b when it comes to electronics, but willing to learn:
  • I know about discharging caps (I built myself a tool to do it - clips, cables and resistors inbetween)
  • I own and understand how to use a multimeter
  • I own and understand how to use a capacimeter
  • I own a basic soldering station and I'm practicing on older components / educational kits
  • I have an analytical mind and can follow calculations / logical flows
  • I'm willing to read, read, read

Any suggestions on where to start are welcome, and here are my questions so far:
  1. Where would I get a service manual / schematic for said amplifier? (Yaqin MC-50L) - I googled different keywords, but can't find this specific model: it seems it's less popular than the 100B or the 13S
  2. How do I know which caps to change? My n00bness is telling me to go for the larger ones and get audiophile grade equivalents of same capacitance and higher voltage / temp rating
  3. What caps should I actually get? Most topics I've read on the subject recommend Mundorf ZNs

I'll keep the thread updated with pics and progress.

Thank you in advance for any pointers!

Raz
 

Attachments

  • IMG_9965.jpg
    IMG_9965.jpg
    853.3 KB · Views: 252
[*]Where would I get a service manual / schematic for said amplifier? (Yaqin MC-50L) - I googled different keywords, but can't find this specific model: it seems it's less popular than the 100B or the 13S

I doubt a service manual exists.

China-HiFi-Audio.ru - Схема электрическая лампового усилителя YAQIN MC-50L

[*]How do I know which caps to change? My n00bness is telling me to go for the larger ones and get audiophile grade equivalents of same capacitance and higher voltage / temp rating

I'd try some different coupling caps first (C104/C105) & the other channel, but I'd leave the PS section alone for the reasons stated above.

Before you dig into the amp, try rolling some different small signal tubes first, if you're still running the stock Chinese ones.

jeff
 
Start by finding someone local to you who can measure the amplifier. Some of these "100W" Chinese amplifiers are making less than 20W, and addressing this first will be more important than what kind of coupling capacitors you have.

Replacing the 30K/2W resistor with a current source (this will require using your DVM to measure the voltage across said resistor during operation) will provide improved AC balance from the phase inverter.

A whole lot of voltage is dropped before the LTP. I'd reduce the value of R20 A LOT and up the values of those LTP plate load resistors accordingly.

The grid stoppers (according to the schematic posted) are not implemented properly. You could fix that with a hobby knife and some patience.

The power transformer is either 110 or 220. If you have 120V or 240V at your socket, address that before heat kills your PT.
 
Thank you all for your valuable input!

DF96 - My understanding is that the ways they cut costs in these Chinese amps is by copying well known schematics and using cheaper components on them - so changing the cheap caps to audiophile grade ones with a tighter tolerance should make a difference. Same goes for the volume pot, which has crackling (DeoxIt Fader on the way).

vinylkid58 - Thank you so much for the schematic! I couldn't find anything googling either way. Thank you for pointing out the caps to change as well - as I said, I don't have any experience doing this. Right now I have JJ KT88s, Svetlana ECC81 and Tungsram ECC82 in it and have some Tesla ECC81 and Haltron (surprise!) ECC82 on the way.

audiowize - Not finding someone local is the actual reason I'm trying to do this myself. I'm less concerned by the power output: it's rated at 60W and I've never gone above 50% volume with it (for now it's either driving a pair of Heco Victa Prime 702 or one of B&W DM220i, but I keep looking for something with a higher SPL). I'll need to do some reading to fully understand your recommendations - so far the only one I get is concerning the mains voltage: I have 230V at the socket :D

Again, thank you all for taking the time to reply!
 
50% on the volume pot does not equate to how much power it is putting out. You could easily be clipping the amplifier at that position on the volume control depending on other factors.

You might consider buying a USB oscilloscope and an 8 Ohm 100W resistor. You can get a very good estimate of output power with those two tools.

You are correct that the Chinese cut corners where they can. An inexpensive pot will have bad channel balance at low volumes, but if this is an issue for you, it can be resolved with a couple of resistors.

The issues that cheap output transformers introduce are the ones you want to go after (they are usually the most measurable as well)
 
Last edited:
razvanbrates said:
My understanding is that the ways they cut costs in these Chinese amps is by copying well known schematics and using cheaper components on them - so changing the cheap caps to audiophile grade ones with a tighter tolerance should make a difference.
In most cases the difference in audio performance between a cheap component and an expensive audiophile component is zero. Newbies often find this hard to believe. The big exception, where cost usually matters, is output transformers. Volume pots may also benefit from being better quality.

The big issue with some Chinese amps is that they cut corners on safety. Some mains transformers may have poor insulation, while others may be near core saturation (and hence overheating) even when given just their rated mains voltage.
 
Member
Joined 2015
Paid Member
A picture of the inside of this amp would be useful. I have checked/modified several low-end China tube amp but not this one. My personal experience does match what has already been said in this thread. The weak spots are usually the transformers and electrical safety. There is little to gain by exchanging the capacitors, unlessy they are of the unmarked kind and faulty or out of spec. I will usually replace them with any good brand capacitor I have at hand. Vishay and Nichicon are my primary choice lately. Exchanging the volume potentiometer may be a good idea, some have totally mismatched sections and are very poorly built. I use Alps potentiometers if possible, but the shaft diameter should be taken into account if the original volume knob needs to be preserved.

Electrical safety should come first. Are the fuses UL or CE listed? Is the wiring on primary side of the transformer properly insulated and does the power switch bear the safety marks? Is the ground connected to the chassis with a dedicated yellow/green wire (for CE compliance) of the same size of the mains conductors?

Transformer saturation and overheating could be mitigated by changing the working point of the tubes or exchanging them with smaller ones when appropriate. I've come across several SE EL34 amplifiers that are really a 6v6 design that has been poorly stretched.

If the goal is merely to change the sound a little, I subscribe the idea to try a different preamp tube first. The phase inverter tube is the one that should give the most difference. Factory tubes rarely are the best sounding ones.
 
With any cheaper Chinese amp you need to test on an oscilloscope for oscillation; most i have bough are oscillating one was so bad AM radios were picking up the oscillations 50 meters away : I reduced that problem but am still working on getting the amp free of oscillations will affect the sound much more than changing caps. tuning a Am radio near the amp will give you an idea if it is oscillating you will hear whistles. I find caps and resisters are good quality in most Chinese amps i use the same caps and resistors in my own built amps. Of course all amps should be tested for oscillation.
 
Thank you for the extra info, everyone :)

I've actually re-checked the tubes and both the 12AU7 and 12AT7 are WA tubes (military grade) so I should be ok on those for now.

I have an oscilloscope DIY kit, a signal generator kit and some 8ohm 100W resistors coming over in a few days (thank you for the suggestion, audiowize), so I'll be in a better position to test (and I'll also use those as soldering practice). At that point, I'll open it up and start taking pictures. In the meantime, I'll try multi's test.

Thank you for the (re)clarification, DF96. Once I have it opened up, I'll go through a thorough check of the transformers (I'll need to read up on what to look for, though :p ).

Bonus riddle: I know that you're not supposed to power up tube amps without speakers (or a dummy load) connected. But I was playing around with my speakers, switching from one amp to another.. and I did. Twice (once for less than a minute, the second time for about 20 minutes). Some time after the first time, one of the power tubes went out (it wouldn't bias over 400mV with the pot all the way up, and had a silver coating on the inside, where the others didn't). The second time around, with a fresh set of new tubes, nothing seems to have been affected (and that was the time I left it disconnected for 20 minutes!).
How come? Could it have some kind of protection built-in? Is it something one of you can figure out by looking at the schematic?

Thank you for your ongoing support, great community - there's a lot of knowledge concentrated in one forum!

Raz
 
You might get better sound (better is subjective), but in my opinion it's not worth it. I've listened to a few of these amplifiers and to be honest, these only add to the bad reputation of tubes and the so called "tube sound". Distorting, lack of detail and dynamics, mushy, rolled off frequency response.

So I recommend you.
1. Leave it unharmed and enjoy it as it is.
2. If you plan to upgrade it, scrape everything and leave the chassis only. :D
3. Build a new amplifier from scratch with the high quality parts you intend. Remember - good sounding parts don't have to be audiophile grade. For example as capacitors go, there are many industrial, normal grade ones that are excellent.
 
Let's see a photo of your destroyed tube! The amp could oscillate without a load, and hot swapping the speaker cables on a running amplifier can also cause some problems.



Hi audiowize, here it is: IMG_0091.JPG

The other ones were still clear, and this one wouldn't go over 400mV. The new JJ tubes bias just fine and are very stable over time.

What should I check for?
 
Using your oscilloscope, see what you get out of the speaker terminals (using an 8 ohm resistor as dummy load) when playing a 1 kHz square wave: That will give you a pretty good idea as to possible oscillation or frequency response anomalies and can very quickly tell you what needs to be done.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.