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Old 16th June 2010, 02:26 PM   #1
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Default mingda mc34ab mods?

does anybody have any experience with this fantastic monster of an amp i've just bought? i could've sworn i was glancing at a discussion about modding this amp on one of my countless nights of insomnia binge. would've bookmarked the page if i knew i was gonna end up spending the money i can't afford on it. the bass on this amp is more terrifying than the nad 2200 i used to own- no small feat when compared to a 400w per channel SS. and when the triode mode is flipped on, oh gosh, it's like wearing a weightless headphone- the music just emanates from inside my head. totally transparent. but eherm, excuse my rambling and please share with me your info.

Last edited by PreSapian; 16th June 2010 at 02:29 PM.
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Old 3rd September 2010, 06:33 AM   #2
idemo is offline idemo  United States
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Default Mods on MC34AB

Yes... I have owned this amp for several months now and have done quite a few mods to it...all with great success. First... throw out the 12AU7s that came with the unit and replace them with Tele 12ax7s or some good quality tube.
Next... Replace ALL the coupling and bypass caps in the unit with Vitamin Q or Russian K40, K42 units. Most of the caps in the amp are fakes....in fact even some of the names on them are spelled wrong! It's a pain to try to stuff in 8 1 or 2uf pio caps in the unit, but it can be done...and it comes out rather neatly if you take your time. I also eliminated my aux input and installed a preamp output for my sub, but as it turned out the unit is so good now I don't need it! Also....check to make sure no ChemiCon power supply caps were used in the unit. None were in mine, but if you see them....change them out for Nichocan or Panasonics from Digikey.
Next I swapped out the blue bottled EL34s for an octet of Gold Lion Reissue KT-66s. The difference was almost as dramatic as the PIO cap changes. ( The gold lions have the tapered bottoms so you don't have to remove the chrome beauty rings around the stock Ming Da sockets. If you want to run KT-88s, you can, but you'll need to purchase some inexpensive "socket savers" for octal tunes.....about $5 each on eBay to space the tubes up an inch or so to clear the beauty rings.)
The unit now sounds better than my original setup which was a Mac275 Reissue with an Audio Research LS25 Mk2 driving it. I'm using Altec Lansing model 19 speakers and alternate between them and Klipsch Cornwalls with the older motor run caps for crossovers.
This is an awesome unit!! The build quality is excellent. It's built like a tank and has the sweetest sound I've heard in years....with bottom end slam that almost equals my McShane modded Citation 2s. If you get a chance to get one of these....do it! Make the mods....leave the light bulb 6SN7s. I tried 16 or 17 other NOS units in their place and couldn't find an equal. For me...this is a keeper and at the $800 I got it for brand new on eBay...( plus $300 shipping), it can't be beat. It's outplaying a $10,000 system!
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Old 3rd September 2010, 08:29 PM   #3
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Quote:
Originally Posted by idemo View Post
throw out the 12AU7s that came with the unit and replace them with Tele 12ax7s or some good quality tube.
To do this properly the plate and cathode resistors would have to be replaced so that the 12AX7 will bias correctly and be properly loaded. To just replace the 12AU7 with a 12AX7 would load the 12AX7 with way to little plate resistance and will result in gross 2nd harmonic distortion. Further, the gain of the 12AU7 is way less than the 12AX7 so the following stage will have to be modified so as not to overdrive it. So what we are really talking about is an re-engineering of the first couple of stages. Many here can do that easily; if you aren't one of them, I would stick with a good 12AU7 variety, say an Amperex 7316.

That is not to say that anything will blow up or won't work with the tube type swap. It just means that the 12AX7 will badly distort and overdrive the next stage. Of course, you may LIKE extreme 2nd harmonic distortion in which case have at it....

Last edited by Palustris; 3rd September 2010 at 08:34 PM.
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Old 3rd September 2010, 09:34 PM   #4
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Quote:
Originally Posted by Palustris View Post
To do this properly the plate and cathode resistors would have to be replaced so that the 12AX7 will bias correctly and be properly loaded. To just replace the 12AU7 with a 12AX7 would load the 12AX7 with way to little plate resistance and will result in gross 2nd harmonic distortion. Further, the gain of the 12AU7 is way less than the 12AX7 so the following stage will have to be modified so as not to overdrive it.
You could also use the middle tube - the 12AT7 - a good tube IMO.
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Old 3rd September 2010, 10:10 PM   #5
idemo is offline idemo  United States
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I tried a number of other 12AU7s in it's place before the teles went in. The Chinese 12AU7 that they use is either NOT a 12AU7 or it is grossly out of range for that type tube. I tried Bugle Boys, 5963s and a number of like type tubes......all of which resulted in a much lower and noticeable volume loss.....hence my step up to a tube with more gain. I have not measured the Chinese version on a tube tester yet to see what it actually is, but it's NOT a run of the mill 12AU7.
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Old 3rd September 2010, 10:17 PM   #6
idemo is offline idemo  United States
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In answer to one of the above statements.... I've been working with tube gear and building my own amps since I was 14....and that was in 1960. I believe I could handle rewiring a single stage...and if the tube in question required that I would certainly have done it. Thank you for the heads up though.
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Old 4th September 2010, 05:05 AM   #7
idemo is offline idemo  United States
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Sorry... I think I mislead you or didn't state the situation as I should have. What I meant to indicate is that even though these Chinese tubes were marked as 12AU7s, they are, in fact, not. Every 12AU7 that was rolled into this amp cut the volume by more than 50%. I believe that the amp actually calls for a 12AX7 in it, regardless of what they tubes were marked as. I also did try them in another amp that actually needs 12AU7s and yes indeed....the distortion was obvious. I put them into another amp that wanted 12AX7s and they worked out just fine...despite the fact that they had the audio quaility of a $2 Chinese tube!
The whole point to this post was simply to point out to the buyer of the unit that it is indeed a wonderful piece of equipment and lends itself to simple mods quite easily. I'm not pursuing this amp to mod anymore than I have already...it's quite adequate for it's purpose and surpasses all other imports that I've tried in the past.
I'm not much into imports... I deal mainly with old Macs, Heathkit W6Ms, and primarily Citation 2s which once modded are quite possibly my favorite amp with the exception of the W6s which have also been Mcshane type modded and have mostly PIO caps in them. Huge Perless transformers with a power transformer the size of a Citation 2 power transformer for each unit lends itself to some extraordinary sound...provided you can find space for them on your shelf and with 5 or more times the power supply capacitance you can imagine that they never whimper at all even under extreme loads.
Again though....even though I prefer vintage gear which will always hold it's value and certainly increase in time, for everyday listening and if you're not concerned with trying to make a profit on the unit, the Ming Da is an excellent investment.
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Old 30th April 2011, 02:59 PM   #8
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Default New MC34-AB

Hi,
This has been a great forum and I really appreciate the suggestions.

I received the Ming Da MC34-AB several weeks ago from Cattylink. Their email response, customer service, and shipping was excellent.
So far below are changes I have made:
*tube changes: Genalex KT66, Mullard 12AT7, kept the stock onion shaped tubes.
*Cap changes: replaced all 12 of the "Ming Da" white caps with Jantsen silver.
I am very impressed with the sound. The mids and highs are very clean and extended. The soundstage is very deep and 3D (almost as good as my Welborne 300B Terraplanes).
The bass is well extended.

There is one concern the first two seconds when I turn the unit on. The two
12AT7 tubes light up like a light bulb. The tubes then dim back to a more normal brightness. I am afraid this will greatly reduce the life of the tubes. Is there anything I can do to add a "soft start".

Thanks,
Jim
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Old 30th April 2011, 07:10 PM   #9
DF96 is offline DF96  England
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The 'light bulb' trick on switch on is normal for Mullards. Don't worry about it.

I am puzzled that allegedly fake components, most of whom need to be swapped, and mismarked valves can be reconciled with "excellent build quality". Do you mean excellent metal bashing?
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Old 30th April 2011, 09:39 PM   #10
idemo is offline idemo  United States
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As stated above the bright light for the Mullards is common. It happens in Macs as well....not to worry. The build quality on the unit could be better....sure....it's typical Chinese with main busses and everything else attached to them, but it's laid out well and pretty solid....better than most. Rebias the KT66s though....they'll take quite a bit more than the EL34s and you'll hear the difference.
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