Ming Da MC34B from 2003

[kwingylee]

All:
I recently acquired one of these Ming Da MC34B amps that have a different set of small signal tubes than what I have been able to find in various forums. It uses two 6H1N, one 6H2N and one 6TK7. The previous owner randomly plugged in the tubes and I have no idea which tube is supposed to be in each socket.
Can someone post what they have off a working unit please?
This amp was purchased directly from the Ming Da factory in China more than 20 years ago. I suspect there have been changes since then.

 

[vinylkid58]

It uses two 6H1N, one 6H2N and one 6TK7.

So 6N1P & 6N2P are listed as tubes in the MC34-B, but what's a 6TK7?

Does the amp look like this:

 

[kwingylee]

Hi Vinylkid, looks like I need to send those guys an email or two. The amp appears to be a 2000 special series. It uses four 6L6 power tubes.

 

[IBX]

kwingylee, the 6TK7 marking is OTK7 which I believe is a quality control marking so that "6TK7" is likely a 6H2N tube. The tube compliment is likely 4 x 6L6 (or 6PS), 2 x 6H1N and 2 x 6H2N. If you take a pic of the underneath of those 4 small tube sockets with associated circuitry, might be able to guess what goes where.

 

[IBX]

If you can trace the the input signal from the rca's on the back, thought the selector switch, then the volume pot, it will lead you to the first valve of each channel, this is likely the 6H2N spot, the other spot would be for the 6H1N. A pic of underneath would help clarify.

 

[spantou01]

@IBX provided good advice. I would add one caution - check power supply hookup to ensure your U.S. supply produces correct B+ ( and other supply) voltages, given the expected 240V line voltage. Are you using a step-down transformer?

I found the following schematic which may be helpful. As @IBX suggested, you have to trace out which small tubes are connected to the volume control; that would be the 6H2N spot. The small tubes you showed in the photographs are Russian military equivalents that might be upgrades to the original OEM tubes

.https://www.diyaudio.com/community/attachments/ming-da-mc34b-schematic-mod-1-gif.994913/

 

[kwingylee]

Thank you IBX for the great ideas. I opened the bottom plate and was able to trace the wiring. the two 6HN2 (12AX7 after suitable modification) are on the inside and flanked by the 6HN1 (12AU7 after suitable modification) on the outside.

 

[kwingylee]

@IBX provided good advice. I would add one caution - check power supply hookup to ensure your U.S. supply produces correct B+ ( and other supply) voltages, given the expected 240V line voltage. Are you using a step-down transformer?

I found the following schematic which may be helpful. As @IBX suggested, you have to trace out which small tubes are connected to the volume control; that would be the 6H2N spot. The small tubes you showed in the photographs are Russian military equivalents that might be upgrades to the original OEM tubes

.https://www.diyaudio.com/community/attachments/ming-da-mc34b-schematic-mod-1-gif.994913/View attachment 3203082

Thank you spantou for the beautiful schematics. I liked that yours showed 0.33uF caps which is what I have instead of 0.22uF or 0.47uF. On my unit there is a switch in the back that allows you to select between 240V or 115Vac. Its set to 115Vac to work in the US. No need for another transformer. OTOH, my amp has 470uF on the power supply instead of 220uF and one of them is bulging and will need to be replaced.

 

[spantou01]

@kwingylee ,
Sounds like all will be good after your maintenance rework of the PS caps. Good luck. Enjoy the music!

Let us know what you hear if you make the necessary modifications to use 12AX7/12AU7 tubes in stead of the current tube complement. (Noting that the parameters of 6H1N differ from 12AU7).

 

[kwingylee]

@kwingylee ,
Sounds like all will be good after your maintenance rework of the PS caps. Good luck. Enjoy the music!

Let us know what you hear if you make the necessary modifications to use 12AX7/12AU7 tubes in stead of the current tube complement. (Noting that the parameters of 6H1N differ from 12AU7).

Yea, I want to get it to work right first. Replace the rectifiers and the lytics and make sure its stable. I have orange drops I can use as bypass caps to replace the MING DA parts.
Have you modded yours? I see that from AK and DIYaudio forums that a lot of people have major circuit mods to theirs. Have you done any to yours. In your opinion what changes are the biggest bang for the buck?
I don't want to spend too much $$ on this. Because after awhile, you might as well buy a Primaluna, Willsenton or Dynaco.

 

[spantou01]

@kwingylee,
I do not own a Ming Da MC34B amplifier (or any Chinese amp) so I have no experience with modifications for it. If you want to use it soon as a trade-in on something “better” I would suggest keeping it stock. Modify as you desire only if you intend to keep it for a long time.

 

[crispycircuit]

Does anybody else have hum coming from preamp tube? I have a newer version but your schematic fits this amp. Someone changed the preamp tubes and wonder if this is the problem? Thx for the schematic!

 

[IBX]

I bought a few Chinese amps over the years, including an MC34B. The MC34B has good-sized transformers and a solid well-ventilated chassis. I mostlly used these amps for learning about modifications, how valve amps work, loadlines, gain stages and phase splitters (valve wizard helped a lot). I don't think I'll ever sell the MC34B, I've become quite attached to it

Have you modded yours? I see that from AK and DIYaudio forums that a lot of people have major circuit mods to theirs. Have you done any to yours. In your opinion what changes are the biggest bang for the buck?

When I bought mine nearly 20 years ago, I didn't know what a good tube amp should sound like, so can't really say if it needed mods or not. Mine has been through many circuit changes and now uses a long-tailed pair phase splitter driving 2 x 6P3S for each channel so only 6 tubes all up. I would be inclined to just listen to it for a few months, try some tube-rolling, and see if you like it as is or not.

The most common problem in all the Chinese amps I've had is getting the heater voltages within spec. I have 240V here and most Chinese amp power transformers are designed for 220V or 230V.
.

 

[kwingylee]

Finally done with swapping out the four big electrolytic caps and the amp sounds good. B+ is about 15% higher than spec. i increased the main caps from 220uF to 560uF. I may try to address that in the future.

 

[crispycircuit]

I have the same amp in for repair. It had hum problems which was takin care of with new power supply caps. The same ones you replaced. I've never seen wiring done in that style? I also replaced the big diodes (no improvement)...It's a nice amp with plenty of Bling... Did yours have hum problems? No DC for input/driver tubes.

 

[kwingylee]

I hope you are not referring to the clip leads and the speaker wires.... I didn't change any wiring. Just 4 caps. Yes I did it to cure the hum issue.
Since the recapped, the B+ is running high. Possibly due to the increase in main cap from 220uF to 560uF. I might need to add some resistance to decrease B+.

 

[crispycircuit]

I wouldn't worrry about it. If your heaters are above 6.7vac, I'd consider a Variac to lower input voltage.

 

[kwingylee]

Want to give a final update on the MC34B. Instead of a 2003 it is actually a 2,000 anniversary gold model.
All MING DA coupling caps (0.1uF and 0.33uF) have all been replaced with Mundorf MCAPs.
All electrolytic capacitors in the audio circuit have been replaced with Nichicons from Digikey. Voltage ratings have all been increased.
All Power Supply caps were replaced previously, including the addition of bypass caps across them.
The Full Wave Bridge rectifier was also replaced with one that have a higher voltage rating. Its physically larger but not by much.