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    Building, troubleshooting and testing of these amplifiers should only be
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MingDa MC-7R noise and gain

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johnm

johnm

Member
Joined 2003
Hi there - been a while!

Hope your Ming da is still singing beatifully! Have you performed amny more mods on it?

Would you be able to post any more pictures of the mods you've performed please? I want to try using my ming da with dynaco ST-70 I'm currently building and I'd like to use the two together if possible.

Cheers,

- John
 
johnm

johnm

Member
Joined 2003
Just to make sure I have got the total list of changes correct:

1) Remove 100uF cathode bypass cap from first 12AX7.

2) Replace 270k plate load resistor to first 12AX7 with 100K.

3) Replace 100K dropping resistor to same with 270K.

4) Remove 2M2 resistor off first 0.022uF cap.

5) replace 1M resistor (adjacent to the 2M2 resistor) with 270K.

6) Remove 330K positive feedback resistor.

I assume the 22k resistor stays unchanged?

I have attached a pic of changes to make sure I have all this correct - I'd be grateful for a confirmation before I bugger up the Ming da ;)

Many thanks!

- John
 

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johnm

johnm

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Joined 2003
Not to put too finer point on it, I think the Ming da's design is a little lacking in certain areas - most obviously in it's ability to drive power amps. Too much gain, noise and hum are usually the sign something isn't right. Rather than just use a temp. solution like the in-line attenuators I want to fix the problem properly/permanently.

It's a mystery why Ming da designed the MC-7R with a gain of 23.
 
kevinkr

kevinkr

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johnm said:
Just to make sure I have got the total list of changes correct:

1) Remove 100uF cathode bypass cap from first 12AX7.

2) Replace 270k plate load resistor to first 12AX7 with 100K.

3) Replace 100K dropping resistor to same with 270K.

4) Remove 2M2 resistor off first 0.022uF cap.

5) replace 1M resistor (adjacent to the 2M2 resistor) with 270K.

6) Remove 330K positive feedback resistor.

I assume the 22k resistor stays unchanged?

I have attached a pic of changes to make sure I have all this correct - I'd be grateful for a confirmation before I bugger up the Ming da ;)

Many thanks!

- John
Click to expand...

Hi John,
Yeah that's correct, note that the capacitor in some units is 1000uF, not the 100uF you mentioned. Such may/may not be the case with yours. There are slight variations in later units, but if you end up with it configured as drawn you'll be on the right track.

The excessive gain is the result of trying to emulate pre-amplifiers like the Marantz 7 which had a line stage gain around 26dB (I've forgotten the exact amount unfortunately) which was a reasonable choice during the tube era for use with 40dB phono stages, and line level sources that had outputs from 250mVrms - 775mVrms typical.
They obviously did not think about current sources likely to be used with this pre-amp. (CD players 2V out)

Kevin
 
johnm

johnm

Member
Joined 2003
Hi!

Thanks for confirming all that Kevin :D

I think my unit has the 100uF cap in there as per the posted schematic but I'll double check. In any case this is removed anyway.

Could I just confirm also that that 22K feedback resistor is unchanged as well?

Lastly is it both 12AXZs that should be replaced by 5751s?

Thanks mate!

- John
 

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kevinkr

kevinkr

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johnm said:
Hi!

Thanks for confirming all that Kevin :D

I think my unit has the 100uF cap in there as per the posted schematic but I'll double check. In any case this is removed anyway.

Could I just confirm also that that 22K feedback resistor is unchanged as well?

Lastly is it both 12AXZs that should be replaced by 5751s?

Thanks mate!

- John
Click to expand...

Confirming all above..
 
kevinkr

kevinkr

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f1802llk said:
Hi. A novice here - just to confirm, removing a cap means "cut the wire" and removing a resistor means "replace it with a piece of wire". Right?

Thanks,

Frank
Click to expand...

If you are talking about my modification instructions "removing" means remove without replacement with anything unless subsequent instructions indicate otherwise. Do not replace removed resistors with a piece of wire unless instructed to do so.
 
F

f1802llk

Member
Joined 2007
Hi Kevin,

I have a question about adding more capacitors to my MingDa (MC368-b90) amplifier. In the power supply, it uses two 330uf/450v caps in series in the output of rectifiers. I'd like to parallel this rail of caps with a rail of two 470uf/450v caps in series to increase the capacitance of the power supply. Is this a safe mod? For instance, would it create more current/voltage that may blow out the rectifiers or other things in the amp?

I understand this is not exactly relevant to this thread here, but it seems to me a simple question that does not really worth a new thread for. As you can see, I am a beginner in DIY.

Thank you,

Frank
 
kevinkr

kevinkr

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Joined 2004
kta-hifi.net
Paid Member
f1802llk said:
Hi Kevin,

I have a question about adding more capacitors to my MingDa (MC368-b90) amplifier. In the power supply, it uses two 330uf/450v caps in series in the output of rectifiers. I'd like to parallel this rail of caps with a rail of two 470uf/450v caps in series to increase the capacitance of the power supply. Is this a safe mod? For instance, would it create more current/voltage that may blow out the rectifiers or other things in the amp?

I understand this is not exactly relevant to this thread here, but it seems to me a simple question that does not really worth a new thread for. As you can see, I am a beginner in DIY.

Thank you,

Frank
Click to expand...

I'm not familiar with the amplifier you mention above so I can't give you a safe answer. You may or may not need to upgrade the rectifiers to prevent their destruction due to inrush current.
 
johnm

johnm

Member
Joined 2003
Hi Kevin!

Wee question if I may: Is it OK to run ECC83 instead of yoru suggested 5751? I believe the ECC83 are equivalents? I have a pair of Mullards here and would like to use them if I can rather than order a new pair of 5751s?

Just as an aside I'll be using the Ming Da MC-7R with a pair of Quad IIs, and also Dynaco ST-70 once I've got the darned things finished ;)

Many thanks.

- John
 
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