Ming Da MC34-AB

[mcb]

Jeez thats nice work...very neat, bet it sounds a treat

Thanks gamve, you will notice there was no close-ups of the soldering .  As to sound, I have only had it on for a couple of hours, being fed by the duet, whilst doing other things.  I suspect that by the time they settle down, I will have forgotten what it sounded like before.  No audio memory.

Once I have finished the changes, Braddles may bring his amp (also a MC34-AB) around.  We can then get a good comparison.

[tuyen]

Wow, talk about neatness! Cable tied, heatshrink wrapped connections, caps firmly secured on sticky pads and all!

Nice work mate     Better job than I could of done thats for sure.


Resistors are an even easier job to replace.  Just snip off the stock ones and directly solder on the new ones.  No need to fit flyleads, sticky pads, cable ties, etc. 

Keen to have a listen to ya setup again, especially now that the mods on the Duet would have been run in (oscon caps, resistors in spdif out etc).

Tuyen

[mcb]

Thanks for your comments Tuyen.

I look forward to catching up with you again and getting your feedback.  I will let you know once the Jensens are in and I have a few hours on them.

Murray

[flemo]

Yepp I agree with Tuyen, quality work mate. 

Keep the pic's going as you continue with the mod's.

[Braddles]

Jeez thats nice work...very neat, bet it sounds a treat

Thanks gamve, you will notice there was no close-ups of the soldering .  As to sound, I have only had it on for a couple of hours, being fed by the duet, whilst doing other things.  I suspect that by the time they settle down, I will have forgotten what it sounded like before.  No audio memory.

Once I have finished the changes, Braddles may bring his amp (also a MC34-AB) around.  We can then get a good comparison.

No worrries Murray. Just give me a call when your ready.

[mcb]

Will do Brad.

I have just added six of the elecytrolytics today, two (the easiest two, I hope) still to go.  I have not been brave enough to turn it on.  Will leave that until the weekend.

As usual, my soldering has been rather sad.  The best join I did all do had to be re-done, as di the run wrong.

One thing with these amps, you need a very hot iron to undo the existing solder connections.

[mcb]

Following on from my previous posts (albeit, somewhat after the fact), I added the last two electrolytic and then went to start the amp.

Nothing!

After some checking, I established that it had eaten the fuse, so I put the spare in.  Same result.

Using a slow blow fuse did not help, so I borrowed flemo’s amp and did a line by line trace.  All looked good, so I re did a couple of joints.  More blown fuses.

In the end, I gave up and passed my amp onto a local specialist tech.  When he was picking the amp up (yes, he picks the gear up and drops it off), I had a good chat with him and learnt a few interesting things.  More on this later.

Anyway, once he had seen the amp, he quickly established what was wrong.  I had the rectifier (diode array between the two of the caps right at the back of the amp, when in stock form) back to front . 

Yes people, I now know that diodes, unlike resistors, are directional. 

The tech also expressed strong reservations about the various long power cable runs, as these can lead to the voltages becoming unstable.  Anyway, he rebuilt the rectifier for me, rearranged the layout a bit with in the belly box and got it going for me again.

He then dropped the amp back the next day.  All was good!  When he first arrived, he advised that he could not stay, however he then proceeded to take the amp apart, show me what he had done, explain what the thought the shortfalls of the arrangement are and suggested some ideas for improvements.
Following is a picture of the amp as it currently is.



From a purist perspective using tabs to connect to the screw terminals adds another step in the chain, however when multiple connections are required, such tabs are useful.  The tabs that were used are Keystone and have part numbers like K906, K918 or similar.



So was it all worthwhile.  Well, when I could not get the amp to start, I was deeply regretting embarking down this road.  However, now that it is all finished, the answer is a definite yes  .  I make no secret about having less than well developed hearing, however the mods have delivered more control, especially in the bass.  Braddles may be able to add a little more intelligent feedback, as he bought his unmodded amp around, so that we could compare the two.

I will and a further post, with what caps I used, what I have learnt, and what I would do different if I was to do it again (primarily relates to box construction).

[mcb]

Following from the earlier post:

What have I learnt (includes comments from the tech)

i)   Don’t wire up the rectifier backwards
ii)   Fuses do their job.  Don’t be tempted to bypass them.
iii)   Do not switch between triode and ultralinear on the fly (i.e. turn the amp off, to switch modes)
iv)   Do not run the amp without a load on it, including when biasing (i.e. either have the speakers connected up, or dummy loads e.g. 50 watt 8ohm resistors)
v)   Use dummy loads or sacrificial speakers, when restarting the amp after mods.  Not desirable to destroy expensive speakers (or for those of use with ML1 ultra’s speakers with lots of expensive caps in the crossovers). 
vi)   Keep cap wiring runs as short a possible, especially in the power caps.
vii)   Make sure all caps are securely clamped.
viii)   Try to avoid having wiring that moves when the amp is opened up.  When the tech took the  bottom of my amp to show me what he had done, one of the wires, broke at the solder point.  Vigilance is needed when opening and closing amps which have moving wires.
ix)   Connecting tabs make life easier.

Other tips

i)   Take lots of photo’s before you start pulling things apart.
ii)   Make sure you trace all the wiring and record it.  When you are trying to back trace, this is invaluable (unless you have a photographic memory).

What would I do differently.

Based on the comments from the tech, I would look to use a sub chassis across the bottom of the amp to mount the electrolytics. Possibly two straps (metal or timber) running from side to side of the bottom of the amp, with the electrolytics clamped directly to them.  This means that they are permanently mounted and the wiring does not need to move.  Also, the existing wiring probably will not need extending, at least not for the 4 big caps.

Add extension skirts to the sides of the amp (have not figured out the mechanics of this).  The four front .22 caps for the front row of valves could then be mounted to the front skirts, once again removing the need for wiring that has to move.

Then a new removable bottom, which fits onto the extended skirts.  See rough concept below.

[mcb]

One more post in this series.

The electrolytics that I have used are Jensens  https://audio.jensencapacitors.com/products/capelyt/elytradscr/

Item no.    Description    Tol.    Voltage    Dimensions    Quantity
020386    47µF    -10/+30    500 DC    035x054       1

020391    150µF    -10/+30    500 DC    035x070    3
020397    500µF    -10/+30    500 DC    050x080    4

edit:  these can be bought direct from Jensen or Partsconnexion.

[donberry]

thanks for all of the information and tips. Great posts and pics......

[flemo]

Great write up Murray.

 DIY is great when it works but so frustrating when things go "POP!" 

[tuyen]

Murray, your last few posts great.  Really appreciate the effort you put into the well thought-out writeup (loved the dot points)

It's the right attitude to have and show in a great DIY forum like we have here.

Many thanks again and enjoy your modded MingDa (can I come listen to it?? )

[mcb]

Thanks for your comments flemo and tuyen.

I have learnt a whole heap from yourselves and others who have been very generous with their time and advice.

Whilst I don't have the depth of knowledge that you guys have, I saw this as an opportunity for me to pass a little bit on.

Tuyen, it would be great to catch up again for a chat and for you to have a listen. 

[ozcal]

Great read Murray.

[audiophool]

All good stuff, a good read and glad it is sorted now.
Ming Da do a pretty neato job of their layouts - I had a 34B that I tricked up a little - not as grunty as your beast but pretty sweet sounding and a pretty good headphone out on that too.
Seem to recall a heart stopping moment of blown fuse with that too when I was fooling with it, that was down to a crappy solder joint of my making that I found and sorted.  Looked a trifle spectacular before the fuse blew ;P - I'm with you on leaving the fuses alone.
Good write up.

[mcb]

Thanks for your comment audiophool.

Two more bits of info regarding this amp.

i) The 4 caps used for the front row of valves are Ampohm, Paper in Oil, Aluminium Foil, 630Volt, 0.22uf - CA-PF-XAL-AL-0.22UF-630V.  I am not sure if these are available any more.

ii) About a month ago I changed the stock blue EL34 tubes for Gold Lion KT77's.  When I first put these in they where a bit harsh on the top end, however they have now settled in nicely and also added control to the bottom end.  For those who do not want to change caps, then try changing the main tubes as it is another step in the right direction.  Based on the advice of others, I changed the bias point from 0.3v to 0.37v when I changed the valves. 

Note:  just for clarity the first valves to change are the 12AT7's, then the 6SN7's and then the EL34's.

[Braddles]

Interesting find on the change of power valves murray.

Im a little late but for what its worth murray and i had an interesting afternoon a while back where i bought over my stock mingda (except for upgraded front valves) and my ML1's to compare to murrays modded mingda and upgraded ML1 ultras. I must admit i went over thinking i was a hiding to nothing compareing the two sets but i thought at least i will find out what the next level sounds like. We had a good time going back and forth and at one time or another tried every combo of the 4 components. To cut a long story short i found more difference between the amps than the speakers. The ultras where better than the std ml1's but not 2k worth imo. For a lot less outlay but alot more work the improvement in the amp was more enjoyable and i dare say rewarding for all murrays hard work and patients. I came away from the afternoon with a better understanding of my future upgrade path.

Thanks for the afternoon Murray and for one of the best cups of coffee ive had in a long time.

[mcb]

Thanks Brad, yes it was an interesting afternoon.  I agree, to my ears, the differences between the amps was more significant than the difference between the speakers.

I have noticed over the last month, since the KT77's where added, that the whole system has jelled very nicely.  I am thinking that: i) the KT77's have been very worthwhile; and ii) the upgraded crossovers must have also hit the magic number of hours (whatever they may be).

One thing is for certain, the KT77 upgrade is easy, unlike the caps.

[kajak12]

fellow mingda owners its official you can use 5691/6sl7 in your amps,benefit of a larger sound stage 

[mcb]

Mario, are any specific 6SL7's that you recommend.