MingDa MC-7R noise and gain

[ali8a16]

I just bought one couple days ago and would love to have these mods done. But i dont have any experiences or skills to do it by myself. I live in Northwest Arkansas. There is only BestBuy around my area. Any solutions or suggestions for my case?

Best....
Alex

 

[Scottreeder]

I think MingDa reads this post

I opened my mingda up and wanted to do a few of these update, noticed the 100uf 16v cap on the first gain stage was missing and the resistors for the plate had changed values, plus additional changes. I wanted to send an updated schematic to this post and see if anyone can review it, recommend changes to lower the gain/ noise. Anyone out there after all these years?

 

[chuongbeats]

I opened my mingda up and wanted to do a few of these update, noticed the 100uf 16v cap on the first gain stage was missing and the resistors for the plate had changed values, plus additional changes. I wanted to send an updated schematic to this post and see if anyone can review it, recommend changes to lower the gain/ noise. Anyone out there after all these years?

Yep, I'm still using this preamp.
This is the original schematic of Hetian Mao, the Marantz 7 circuit that the MC7-R based on.

Note that the there's many changes in the MC7-R circuit compared to this original circuit. Is it the reason why the MC7-R gain is too much?

 

[Tweeky1]

Just to confirm. I assume that when the 330k resistor is removed the 3.3uF capacitor that is in series with it is removed as well as it is effectively out of circuit?

 

[wag12]

Can I use 12AY7 to Replace the 12AX7 Tube

Both channels need to be changed as noted. Go back and read my old posts very carefully.

Kevin could I use 12AY7 tube to replace the 12AX7 tube instead of using 5751?
The 12AY7 has gin of 44 compared to 70 for the 5751.

 

[wag12]

Can I Use 12Y7 Tubes to Replace the 12AX7 Tubes?

Removing that 1000uF cap in the first stage will both reduce local stage gain and the overall closed loop gain to about 6. Verify that it is stable under these conditions (with a scope) before connecting amplifier or speakers to it.

If you do this mod use a 5751, the lower mu should help with stability.

I just realized there is another sneaky thing you can do to reduce loop gain further without changing the quiescent operating point. Replace the 270K plate load resistor with 100K and the 100K dropping resistor with 270K.

Do you think it would be ok to use 12AY7 tubes to replace the 12AX7 tubes?
They have gain of 44.

 

[wag12]

Just to confirm. I assume that when the 330k resistor is removed the 3.3uF capacitor that is in series with it is removed as well as it is effectively out of circuit?

So did you do the mod already and did you also remove that 3.3uF capacitor?
I guess you could just leave it in as it wouldn't be connected to anything anyway.