ming da mc 845 aa modification

[bottaccio]

An externally hosted image should be here but it was not working when we last tested it.

Good morning. someone has changed an amplifier with 845 by changing the driver stage? in my ming from there are 2 6SN7 vore and replace them with 34 or other power tubes have a pattern circuit THANKS

 

[bottaccio]

you have the circuit mc845-c12? thank you

 

[Wavebourn]

I once repaired their 845 amps, they used 300B in drivers.

Speaking of modifications, I would couple the cathode follower directly to the output tube's grid, moving coupling cap before the follower. I believe the amp has beefy enough negative voltage source for that.

 

[bottaccio]

you know how it calibrates the bias ? thanks

 

[FullRangeMan]

you know how it calibrates the bias ? thanks

Under the chassis near 300B tube there is a golden pot,
it adjust the 300B. The 300B tube bias the 845.

 

[FullRangeMan]

Good morning. someone has changed an amplifier with 845 by changing the driver stage? in my ming from there are 2 6SN7 vore and replace them with 34 or other power tubes have a pattern circuit THANKS

The Lampizator site is great for anything tube amps.
If you Google Lampizator 805 you will find this useful Ming-Da report with extensive MC3008 805 topcap mods inclusive at input stage.
ming-da
Though I dont see why one would replace the 6SN7 tube, the 6SN7 family are inexpensive and one of the sweetest tube sound avaliable.
300B are expensive and manufacture quality varies.

 

[bottaccio]

I read that to give their best the 845 must be driven in powe

 

[ColinA]

There are many amplifiers out there named ming da.
They may come from different factories or even small back room makers.
There are many different designs and circuits with different valves.

Warning!
They (the Chinese) are well known for using components especially rectifiers that only just meet specification. Any change in supply voltage or load can push them to fail.
Check the quality of all components and replace any that do not look safe to use.

 

[Wavebourn]

845 Ming Da that I repaired had exceptional output transformers, by the way. I would not say so about the rest, though... Film caps were glued by a glue that melts their insulation, limited asymmetric slew rate due to tiny currents, etc...

 

[bottaccio]

exactly what are the capacitors that should be replaced ? tks

 

[Wavebourn]

exactly what are the capacitors that should be replaced ? tks

There were coupling capacitors, black axial with golden letters, but if they are fine don't replace.

 

[bottaccio]

thanks I would like to calibrate the bias but does not know the bias voltage of 845 someone knows

 

[Osirison]

thanks I would like to calibrate the bias but does not know the bias voltage of 845 someone knows

Measure between the marked bias point and ground, be careful at some parts inside there is over a 1000 volts!
Adjust the 33k pot.

Hope this helps.

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