Just want to share with fellow audiophiles a schematic of the Ming Da MC-2A3 preamp and my experience as a curious owner.
I have always been a believer of preamps being responsible for setting the tonal quality and power amps just delivering power with transparency and stability without messing with the tonality.
I have been a fan of 6SN7-based tubed preamps such as the MFA Luminescence, Melody 1688 Signature, and the Canary CA906. Violin music just sounds magical with those 3 preamps. I also briefly owned a Cary SLP98P which I was not too impressed with due to the blurry bass and somewhat AM radio-like mid-highs.
The description of the MC-2A3 preamp at the manufacturer's website sucks.
There has not been a review of this unit on the several major audio-rags or sites like 6moons or enjoythemusic. The unit looks somewhat of a monstrosity.
Based on a discussion thread on Audioasylum, the Ming Da MC-2A3 initially struck me as an interesting design (tube rectification and regulation, dual stage chokes, and of course… 6SN7) and I had the opportunity to pick up one at a bargain price. The preamp arrived with the cage detached from shipping and created a ding on one of the front support “pillars”. No big deal, just cosmetics.
Stock tubes included Shuguang 2A3 rectifiers, and the rest of the tubes are EH.
Out of the box and as expected, there is a sweetness to the violins that makes this preamp very desirable. However, careful listening reveals the same weaknesses as the SLP98P I owned previously. Flabby and lethargic bass, a bit veiled mid-highs, no top end extension. The somewhat narrow soundstage destabilizes when playing complex passages. Still the musicality is there that makes it an enjoyable preamp.
Off comes the bottom cover
OK let’s take a look at what’s inside the unit. Wait… where are the Sprague gumdrops and the Mundorf M-caps in the advertisements? All I see is run-of-the-mill SCR-MKP caps. Oh well no problem, I was gonna swap them out with my collection of Mundorf Silver Oil, Dynamicaps, Jensen Copper-oil, or Hovland Musicaps anyway.
The B+ power supply sure looks impressive. L-C-L-C-[series-pass-regulator]-C-R-C+bypass. Wow… but wait… how’s the filament power supply hooked up?
There seems to be 2?! Nice bridge-C-R-C to the 6SN7s but wait… it supplies it with an unregulated 13.8V to be share serially among 2 6SN7s! That means my left channel 6SN7 has filament voltage of 6.5 on a good day at 0-6.9V and the other tube has it raised to 6.9-13.8V! That’s not a good design The 6922 side is even worse, 20+ volts shared between 3 6922s, that means one channel would have its filament at absolute potential starting as high as 13.5V.
Looking at the circuitry, the 6SN7s are in SRPP config, no surprises there. The middle 6922 is the cathode follower, sounds logical. The 2 6922s on the side are a bit idiosyncratic. ½ of each “side” 6922 serve as the constant current source to each half of the the middle 6922. the other ½ of each of the “side” 6922s is the top tube of the bootstrapped cathode follower for the middle 6922. Seems convoluted enough but sure convinced me that a lot of thought has been put into the circuit (well… read on).
It's ironic that someone wrote on a vendor's website that this preamp is sensitive to microphonics of the 6922 tubes so Teslas might be a problem.
Well, the 6922s are either used as constant current source or follower so how the heck can microphonics be an issue? (read on... it's OSCILLATION due to inter-tube-capacitance mismatch!)
I decided to start with a shotgun approach by taking out the SCR MKP output caps ( was 2.2uF paralleled with some PIO-like 0.01uF) and replace them with Mundorf 2.2uF Silver Oils. The 0.22uF caps between the cathode follower and the top 6922 was also replaced with Jensen 0.22uF Copper Oil. The sound opened up quite a bit and every sounds more well defined. However, it came at a price of a pronounced mid-high and the bass is equally as flabby. I believe the change has more to do with the output cap than the cap between the cathode follower and the upper follower tube’s grid.
Thanks to a fellow audiophile who graciously shared with me a copy of the schematics.
I looked at the 2 High Voltage bypass caps closest to the 6SN7s and right at the regulator outputs and according to the schematic they are 100uF 400V rated (my unit has 1 Philips and 1 BC, both looked identical and marked 150uF). So in went to Blackgate 100uF caps in place of the blue 150uF caps marked Philips and BC (they looked identical and based on the markings on the cap I have reasonable suspicion that they are some Kung Pao Mushu counterfeit caps). As expected the bass got deeper and fuller but it still sounds slow and muddy. There is a slight incoherence in the mid-high that I associated with the 0.1uF bypass cap across strapped from B+ to ground. I came across several writeups that Blackgate lytics do not like to be bypassed so I took it out. Marginal improvement I called it.
Colored sound and lethargic bass
“So why didn’t you wait a few weeks to let everything run in?” You might ask.
While I believe in running in I refuse to believe in the 400+ hours of break in time. If something does not sound decisively better than the old component a after few hours of run-in. It’s likely your mind playing games on you cos your ears are getting used to being pierced. Anyway, the preamp still sounds lethargic and presents a somewhat colored midrange after weeks of running in with the Mundorfs and Blackgates.
Why so finicky about tubes?
The same audiophile who provided me with the schematics was very kind and generous to loan me some of his prized collection of tubes. So the MC-2A3 is now equipped with RCA 2A3s, an uber expensive Philips Miniwatt EL34 as regulator series pass (yes!), my trusty RCA 6SN7GT smoke as comparator, and mega-expensive pinched waist 6922s as followers and constant current source (yes!).
For the 6922 in the input/gain stage I tube-rolled among aged Sovtek (don’t laugh, they ARE competent), Ken Rad, RCA-smoke, Sylvania stubby chrome top, Raytheons, RCA redbase, and CBS 6SN7GT and CBS 5692.
Each set of 6SN7s gave it a different sound as expected, and I won’t go into the details. But what I found was that with certain 6SN7s the preamp sounds WEIRD. Also with some known good, known quiet 6SN7s the preamp started to hum as if grounding is a problem. The midrange gets additional harmonic coloration which I know from using the tubes in other applications is definitely NOT a characteristic of that tube. So onto the bench it goes and we checked for gremlins.
We looked at the output of the 6SN7 to the 6922 middle tube and found there is SERIOUS oscillation when RCA smoke-glass 6SN7 gain stage mates with my (couldn’t resist so paid the big bucks) pinch-waist 6922 follower. A quick look at the schematics reveal that Ming Da uses a 220Ohm grid stop resistor between the 2 stages, that’s about 1/10th of what we’d expect in a circuit like this. Out goes the stock 220Ohm and in comes a 2kOhm grid stop. No observable high-end roll off there but the strange noise is gone and the voltage measures normal. Now I can at least seriously evaluate the tube combos without this tube-capacitance-matching problem.
Is that the output caps?
So back to the pronounced mid-highs. After running the Mundorfs for a few weeks the preamp still shows this tonal imbalance so I swapped in 2 2uF Dynamicaps. Initially it sounded more buttoned down but the air and decay went away. Not my cup of tea.
Faulty Jensen, a blessing in disguise
At around the same time the preamp started to emit a “puff puff putt putt” sound and it was traced to a faulty Jensen cap in the boot-strap-follower upper tube. Out goes that cap and the preamp actually sounds cleaner. Swapped in an Auricap 0.33uF, and then an Obligato 0.1uF with no success. Everytime that cap (actually it’s 2, one per 6922 on each side) is in place the coloration comes back.
Looking at page 168 of “Valve Amplifiers, Third Edition by Morgan Jones” the upper tube of that bootstrapped cathode follower is for distortion cancellation but even Morgan Jones commented that it “invites further problems” and “requires a separate heater supply” which the MC-2A3 certainly does not have. (6922 tube specs suggests that the
max cathode to heater voltage be +/- 100V so the voltage for the top triode of each of the left and right 6922s would violate that)
So I decided to leave that cap OUT.
Experiments with NFB
It’s common assumption that preamps with NFB gives a punchier sound. So I proceeded to investigate whether a small amount of NFB, implemented using a cap and a resistor looping either at the output cap or before the output cap might serve to tighten up bass.
I tried resistor values ranging from 25 to 80kOhms at 10k steps and various capacitor sizes without success. Too high a resistor value (low NFB) gives no effect, and once the value is small enough (50k) to tighten the bass it makes the preamp sound nervous and constricted. No go with NFB.
Scrutinize the schematic
Running out of ideas, I decided to spend some time looking at the schematic. The unit that I got as well as the units depicted in THIS picture
do not show the 5.1kOhm resistor per the schematic on the B+ rail separating the 6SN7 stage and the 6922 follower stage. The way my MC-2A3 was wired the resulting B+ to the 6SN7 be the same as the 6922s top triode.
According to “Beginner's Guide to Tube Audio Design by Bruce Rozenblit”, this kind of an arrangement could even result in coupling between the 2 anodes.
Perhaps I should put in a 5.1k resistor as the schematic suggests.
The schematic also agrees with European units shown here:
My unit has the green 300Ohm resistor just as illustrated here:
but the unit depicted here
does not have that resistor.
The 300Ohm resistor in my unit (and possibly all units coming into the US) connects the post-regulation B+ to the main rail going to the anode of the top 6922 triodes. This resistor drops the 220V regulated output to about 202V or so. No problems here. BUT if you take a closer look, this means the 100uF filtering capacitor (now a blackgate) at the output of the regulator has to go through the 300Ohm before getting to the plates of 2 6922 and 2 6SN7. With this arrangement, the only bypass cap directly serving those tubes would be one single 100uF lytic ( I am now using Blackgate WKZ 100uF 500V in place of that mysterious "Philips" blue cap).
The photo of the European version (well, downloaded from a European site) looks different. There is no 300Ohm resistor separating the output of the regulator and the 6922 anodes. This means it assumes the regulator provides clean flat 220V to the main rail. A 5.1kOhm resistor separates the 6922 anode rail from the 6SN7 anode rail. This means (and it agrees with the schematic) that the 6922 sees 220V B+ and the 6SN7 sees 190V. This 5.1k Ohm resistor prevents anode coupling between the gain stage and the cathode follower. The 6SN7s also get full access to 100uF of bypass and the 6922s get their share of a full 100uF of bypass. This all makes a lot more sense. It’s time to follow the schematic and start making corrections.
With this change to make the circuit a big step closer to the schematic, the midrange coloration (an occasional overtone noticeable in complex passages) is completely gone.
I’m sure everyone has come across postings bashing the lytic caps. Sure I’d like to use film caps for bypass but how do I fit a total of 200uF or more into that chassis? Also since Blackgates are known for their deep bass will I be going the wrong direction by attempting to replace the 2 blackgates with film caps? There is definitely enough bass from the MC-2A3 now but the problem is with the quality of bass I am getting. I am looking for tight bass with slam, and a clean decay after the initial attack, not the “farty” bass I am getting. I have a handful f Solen 33uF caps but it would be a challenge to get the to fit.
Fix the obvious and snub them Zeners!
While trying to figure out how to squeeze 4 33uF film caps into the chassis, I notice that there are 2 (well technically 3) places where zener diodes are used. Zeners are known for their diode noises but they are a necessary evil for voltage reference to the constant current source and the main B+ voltage regulator (no gas tubes here). Zener noises are usually snubbed by placing a cap across the zener. The zener used in the 6922 constant current source for each channel has 2 stage snubbing. But 70V 220uF lytics are used so I figure that low frequency ringing should not be a problem. But what about high frequency ringing? I duct-taped it with a cascade of film caps so as to ensure no high frequency zener noise gets into the CCS. Also I noticed the zener snubber in my unit is wired across 1 diode only and not both as depicted in the schematic. I ADDED another cap just to make very sure there is no zener noise getting into the EL34 series pass. The result was not dramatic but noticeable. The noise floor was lowered and I can hear more details.
Film caps for bypass
After I figured out how to physically squeeze in 3 Solen 33uF caps, I used them in parallel for post-regulation B+ on the 6922 rail before hitting getting to the 6SN7s which are still bypassed by a Blackgate. The net capacitance there is not lowered so I did not expect any drop in bass response.
Even without any warm up I notice that there is an improvement that is not subtle. The lethargic sound is gone. Dynamic contrast is outright scary with absolutely no sacrifice in smoothness or coherence. This is a much bigger improvement than swapping in Mundorfs and Blackgates in place of the stock caps.
1 More RC filter stage may not improve sound
Time for another fancy idea. Why not put back the 300Ohm resistor and put back the Blackgate 100uF lytic to ADD another C-R filtering stage before the 6922 rail? Due to space constraints, I’d end up with only 66uF servicing the 6922 (2 Solens instead of 3).
The result? A slight but noticeable incoherence and roughess in the mid-highs. That’s exactly the opposite of what I’d expect from an additional C-R filter.
I want to keep a good 100uF (3x 33uF instead of 2x 33uF) bypassing the 6922s anyway so I ended up taking out the 300 Ohm resistor which is not in the schematic anyway. Out goes the Blackgate again and now regulated power goes direct to the 6922's B+.
The filament power supply is completely inferior compared to how they executed on the B+.
CM250X bridges are used and they are known to be noisy.
The current rating of those bridges are way overkill and I am not sure why (25amps!?!).
It would be a hassle to put stealth bridges in so I duct-taped the caps with some Nichicon Muse 2200uF 100V caps hoping to get better filtering of the filament power.
Soon I might consider swapping in IXYS but I am not sure if they will make a difference.
The overvoltage of the filaments (13.8V shared among the 6SN7s and 20.5V shared among the 3 6922s) might be a blessing in disguise. I can probably add more RC filtering with 1-2Ohm power resistors and 2000-3000uF caps to smooth out the remaining AC ripple which probably is the cause of the faint hum.
I was also surprised at how the preamp reacted to different EL34s as regulators. A good regulator should dissipate incoming power and provide smooth constant voltage to the B+. The EL34 is not “playing music” so why is the preamp sensitive to that tube. Also why did a Groove Tube E34LS high current EL34 which is supposed to have great heat dissipation sound bad as a regulator in my preamp?
I was suspicious with the quality of the already-choke-filtered high voltage going into the regulator and I came across this posting about bypassing lytic in the main power supply with film caps:
(This trick sure did not work with Blackgates. I am bypassing pre-regulation high voltage here only)
So I decided to squeeze a small film cap (0.47uF Auricap) right between the EL34’s plate and ground. In theory this cap should serve as a fast reacting capacitor filtering out any high frequency noise or small ripple going into the regulator.
Another problem I found was that the voltage across the serially connected filter caps are not even. One cap would have 240V across it and the other 160 some Volts. Putting a 270kOhm resistor across each of them served to even out the voltage distribution across the caps (aren't they supposed to be installed for safe bleed-out of the capacitance after power-off?).
Voila! Another MAJOR improvement. Now I get copious amounts of detail, air, decay, limitless dynamic headroom and soundstage stability. Never had I thought that DC supply pre-regulation is THAT important. (Are those “BC lytic caps” fake). The preamp us now uncolored and coherent.
I guess what I really should have done is replace the 2 blue 330uF 400V lytic caps with Nichicon. Duct tape approach works… at least for now.
This was indeed an interesting journey in audio. The fact that the MC-2A3 uses point-to-point wiring gave me much confidence in trying a variety of things. I absolutely owe credit to the authors of these 2 books
Beginner's Guide to Tube Audio Design by Bruce Rozenblit
Valve Amplifiers, Third Edition by Morgan Jones
Chinese gear sure seem to be a great bargain but IMO in a sense you get what you pay for... I was a bit dismayed by the departure of the circuitry from the schematic and it's still a mystery why my unit does not have the MCaps and Gumdrops shown on various vendors' websites.
Edits: 06/13/08 06/13/08 06/13/08 06/13/08 08/10/08 08/17/08 08/17/08 08/17/08 08/17/08 08/17/08 08/17/08 08/18/08 08/19/08 08/19/08 08/20/08 08/28/08 08/29/08 08/29/08Follow Ups:
... > Seems convoluted enough but sure convinced me that a lot of thought has been put into the circuit <
This is in referenc eto the bootstrapped cathode follower output stage.
Yes a lot of thought has gone into that, but not by those Chinese thiefs. That circuit was designed & developed by myself, in around 1983 as documented in my 1994 printed Tube Preamp CookBook. It was first used in the RTP3 preamp of 1983/84 and can be seen on Page 9 of the RTP chapter.
Since it's success there, it has been used as our default output stage for preamps and poweramp driver stages, and has gained the marketing name of SLCF (Super Linear Cathode Follower). By our tests, it's by far the most linear and best sounding implementation of a low output impedance output circuit. It is used in our two current preamps, the RTP3D and the SVP-2.
We even use it with jets and bipolars.
Did the Chinese ask permission to use this circuit? What do you think...
Oscillation? It will oscillate very easily, but solved by putting the ESSENTIAL gate stoppers RIGHT at the tube soket pins.
And why oh why do they use 2A3s as rectifiers?
So one of the importers of this preamp has indicated in several forums that my findings are based on a "hacked" unit and not theirs.
Well what d'ya know? An owner who bought a so called x8 recently gave his unit for me to apply some mods and here's a pic of the filament caps.
...but I see Rudoycon on those caps.
More Chinese fakery?
Shameless Ming Da... I just got the chance to open up a so called MC-2A3 X8.
The filament caps read Rudycon instead of Rubycon.
Dammit Ming Da you give Chinese audio mfrs a BAD BAD name!
Excellent write up jwu - snoopy. I have been working with Ming-Da products for about 7 years now and think I can help answer some of the questions you bring up.
One of the reasons why your unit does not look like others you see is because they were not all manufactured for the same distributor. Importers/distributors can determine what level of parts are in the products they bring in from Ming-Da from the best to bottom of the barrell.
Another reason is because what you have is an older version and not the 2A3-X8 currently being sold in the US (exclusive to the US to my understanding). The X8 does differ from other versions out there.
In regard to your upgrades (changes) to the circuit, you are right on track. After looking over the circuits, I too would do most of what you mentioned. I think one of the biggest issues to address is the power supply.
I have done similar changes to many of the Ming-Da products and have modified over 50 of the MC-7 preamps. Good products that can be made into great products and still have a reasonable amount of total investment.
"Build your system for yourself not the critics"
This upgrade was exactly what I needed!
have had this preamplifier for about 1.5 years
has replaced the 6922 to some pc88 7V with major upgrade in sound
but 6SN7 tubes I have always had problems, also the original
they have had to be moved regularly around to stop grumbling sound.
The old NOS tubes I have purchased has not been a success!
Some tubes have only made some nasty noises that almost killed my speakers.
no mather what i tried, the sound didnt get there!!
after this brilliant upgrade, the old tubes work!
and the fog has cleared up, it played even ok with the original tubes!
Moderator, why does this post show 8 replies but none are visable. And the last marked poster is said to be unknown.
What's going on?
Regards, Allen (Vacuum State)
...The "TubePreamp CookBook", you'll see where they stole the current sourced, voltage buffered cathode follower circuit from.
This is the second Chinese company to steal my circuits without even an acknowledgement, thieving bastards!
Regards, Allen (Vacuum State)
Steel from the rich and give to the poor.
The Pop Group
"Steel from the rich and give to the poor."
The rich in the story in question had not earned their riches, the riches were obtained by force of taxation.
In this situation it is outright theft of IP.
Seems like you have referenced a few tube amp design books, but not my one. My "TubePreamp CookBook" describes the three tube bootstrapped constant current sourced cathode follower output stage of this Ming preamp - meaning they ripped off my design!
The book clearly states if you want to use any of the designs in the book, you need to contact me for a license, but they sure never have. The link below is of my FVP-5A circuit, available on my website - for hobbiest interest - not Chinese commercialism!
But they have not followed my other design ideas, to their detriment.
Regards, Allen (Vacuum State0
Thanks Allen! Sorry I was not aware of your publication.
But I enjoy listening to one of your creations the SVP1 (own by a friend)
To all MC-2A3 owners,
Several days ago I completed the mods recommended by Johnson and it has turned a very good preamp into a fantastic unit. The sound is much cleaner across the board. The bass is a bit lighter but much more natural. I can clearly hear all the bass notes. Before the bass was stronger but somewhat distorted. I compensate for the bass by turning up my woofer plate amps a bit. The best way to describe the changes are now a good vinyl recording being played sounds much more like live music.
Below are the changes I did:
*Replace all the tubes except for the 2A3C.
*Do the small circuit change.
*Replace the output caps with 3.3uF Mundorf silver oil.
*Use Two 47uF VALAB caps for the 6922 section.
*Use one Solen 47uF cap for the 6SN7 section (not enough room for more).
Thanks again Johnson for the awesome work on this preamp.
I give this mod my highest recommendation.
The MC-2A3 replaces a Welborne Ultrapath BP battery powered preamp (very good unit but the MC-2A3 is another league better).
For reference my system is:
Origin Aurora turntable, DIY Bottlehead seduction phono preamp, DIY K&K step up transformer, MC-2A3, DIY Welborne crossover, DIY Welborne Terraplane 300B monoblocks, Supravox 215EXC Field Coil drivers in open baffle (receives a full range signal, Fountek ribbon supertweeters crossover at > 10,000Hz, four 15" Hawthorne "Augie" woofers in open baffle crossed over at <70Hz 2nd order.
Last night I was able to squeeze a Solen 36uF cap in parallel to the Solen 47uF cap powering the 6SN7 section. This bass is now just as strong as before the mods but now you can clearly hear all the low notes. I hear bass details that were not produced on my system before. This preamp along with the mods produces the most significant improvements to my system other than switching to field coil drivers!
Found 2 major design flaws of the Ming Da:
1. Filament power supply in series, 2 supplies, one shared between 2 6SN7 and the other among 3 6922, non regulated.
Be prepared for it to sound good one day and bad on another.
2. Grid resistor from 6SN7 stage to 6922 stage is 1/10 of what it should be. In the unit and the schematic there is a 220Ohm resistor but with that you'd have to have a 6SN7 and the middle 6922 to perfectly jive with each other to avoid a high frequency oscillation which sometimes has the side effect of inducing a pronounced hum as if grounding is not proper.
This is why this preamp is both a tube-roller's dream AND nightmare.
Took me a while to figure it out. Swap in a 2-5k resistor and all is well.
Can someone help me to undestad witch are the resistior to be change on the schematics and the other mod? I can not udestand well i'm not so expert and i have a per mc2a3 with horrible sound!!! Please help me
Eversince I wrote this I have been getting one email every few weeks asking me for details on how to do the mods. And recently I am seeing that Ming Da has decided to introduce blue-tinted EL34s and balloon CV181a as stock tubes in replacement of 6SN7EH. Gimmicky I think, but I dont think they ever addressed the problems, some more serious than others, in the design of the MC-2A3. I also see some units using black colored lytics instead of the blue "B-C" lytics after the choke... all I can say is "sigh"...
As I mentioned, you will need to do all these to get the sound fixed:
1. get 2 of the largest value Solen FastCap you can fit into the Chassis. 33uF 450V+ would be my choice
I was able to fit 4 33uF into the box, 2 in parallel for each section. That size is governed by the diameter of the cap whether you can fit it into the chassis. Those 150uF small blue caps sound bad. With this mod you essentially fix the muddy bass and somewhat veiled highs which is an obvious sonic signature.
2. get 2 x 3.3uF or 4.7uF Mundorf MSilverOil or maybe dynamicap or VCap for the output cap. (too expensive..... MSUpreme or Jensen copper is more reasonable)
3. get a pair of 1MOhm resistors and replace the 0.22uF + 1MOhm resistor combo next to each side-6922 tube. To rephrase: Keep the 1MOhm resistor but get rid of the 0.22uF cap.
4. take away (bypass) the 333Ohm green resistor and put 5.1k 3W resistor where the diagram shows 6922 B+ rail goes to 6SN7 side. The B+ going to the 6922 rail should be 220V.
Another way to put it: bypass the 300 Ohm resistor so that the regulator output
goes direct to the 6922 section. But disconnect the 6922 sections rail to the 6SN7 section and put a 5kOhm resistor as the connection. This will lower B+ of the 6SN7 section to approx. 200V.
5. Check filament voltage... the Ming Da is known to KILL tubes due to over voltage on the filament.
It pumps 7.3Vs into each filament instead of 6.3. Use an RC stage 0.25Ohm + 10000uF R-C filter in addition to what is there. Filament chokes need not apply due to size and filament regulators need not apply due to relatively small drop needed (1-2Volts).
6. Increase grid stop resistor to middle 6922 (the follower) from 200Ohm to 2k Ohm. This is necessary to avoid oscillation when you match NOS 6SN7 tubes with premium 6922s. Based on my experience anything other than the stock 6922EH will result in oscillation.
7. apply voltage balancing resistors to the pre regulation lytics (the 400uF and 330uF so called B-C electrolytics). This is needed to balance the voltage as you will find that each pair is wired in SERIES
and one of each pair will have a higher potential than the other, that's not good. Furthermore these resistors act as bleeders for safety in the event the regulator series pass tube (the EL34 goes bad).
So in order to accomplish the above these are the parts.
5k, 5-10watt resistor to fix B+ rail
2k, 1/4 to 1/2 watt resistor x 2 to fix oscillation
2 x 2.5Ohm 5W - 10W resistor x to fix filament
4 x 470KOhm 1/2 watt for bleeder (CAREFUL when working on those 2 pairs of lytics!!! Dont get electrocuted!)
4 Solen 33uF 400V or 4 x VALAB 47uF 500V from thetubestore or ebay to fix bypass (2 for 6922 section 2 for 6SN7 section, throw away those small blue lytics)
2 x 25V 10000uF electrolytic caps to fix filament supply.
2 x 3.3 - 4.7 uF Dynamicap or Mundorf M Supreme or something like that.
Hot glue to hold the output caps (larger than the stock fake Solen SCRs)
hot glue to hold the new 10000 uF lytics for the filament supplies.
Alternatively use tie wrap fastener blocks form hardware store or single terminal solder strips and use the screw / nut from the transformer to fasten the strip to chassis.
That's all I was able to do on my Ming Da before I sold it. My intention is to share my experience with this somewhat interesting preamp with fellow audiophiles. I assume no responsibility whatsoever for damage/injury that a DIY-er may inflict upon his equipment or himself as a result of applying these mods or any variation thereof.
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