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Looking for Mingda MC805-AA Bias Procedure and Mods

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Posted on June 2, 2018 at 11:59:45
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 I am about to pick one of these up in good working shape, and would be interested any mods that have been tested for this amp.

I would also like to know if there is a good standard bias procedure. I know there is any internal pot, but specific test points and values would be helpful.

This is the stereo integrated version that uses the 6L6 driver with 12AX7 input and 12AU7 follower.

I have seen a Lampizator page on the MC805-A monoblocks, but they are not quite the same as they use a 300B driver.

I would like to find something that has actually been successfully tried on the MC805-AA.

 

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RE: Looking for Mingda MC805-AA Bias Procedure and Mods, posted on June 4, 2018 at 21:57:12
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 Hopefully you get a manual with these. It looks like they have a 1 ohm resistor under the 805 and you would measure the voltage across it to check/set the bias. The specific voltage that you're looking for will be in the manual. If the seller doesn't have the manual, don't buy the amps.

 

RE: Looking for Mingda MC805-AA Bias Procedure and Mods, posted on June 5, 2018 at 06:14:22
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 Too late - I have the amp in my possession.

Price was too good to pass on - not that much more than the vintage RCA 805s and near new pair of GE7581As that came with it would have cost.

I learned from the previous owner that he obtained the amp from Response Audio, and I can see that there are some mods such as the addition of V-Caps between the 12AX7 and 12AU7. :)

There appear to be discharge resistors on all the power caps (another good sign).

It also appears that the biasing was modified in a similar fashion to a Lampizator article (mods for the 300B driven monoblock version), but I need to study that a little more.

I am hoping that Response Audio has some memory of this unit.

In any event, it does work, and sounds nice, but the filament voltage on the 12AX7, 12AU7 and 6L6 is a little high (about 6.8VDC) when running at 120V input (I won't touch the 805s). This is not surprising since most Chinese amps are designed to run at 110V/220V, so I am in the process of getting a small variac to drop the input voltage. At 110V I fully expect the filament voltage to be spot on.

The previous owner did mention something about being told to preferably run at 110V, so everything is consistent so far.

 

RE: Looking for Mingda MC805-AA Bias Procedure and Mods, posted on June 5, 2018 at 08:08:57
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 It's much less expensive and more convenient to just wire in a small 120V:10V power transformer in as a bucking transformer.

 

Bucking Transformer, posted on June 6, 2018 at 05:56:20
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 Here's some info...

 

RE: Looking for Mingda MC805-AA Bias Procedure and Mods, posted on June 7, 2018 at 07:55:59
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 According to a Ming Da seller in the UK, there is a 1 ohm resistor in the cathode circuit of the 805, and there is supposed to be 90-95 mA flowing through that resistor.

He did provide me a schematic which is supposed to be mostly accuruate.

Anyway, I assume that if I measure voltage across that 1 ohm resistor, then I should get a reading of 90-95 mVDC.

Unfortunately, the measuring points and adjustment pot are inside the chassis. Not sure why Ming Da could not put a couple of bias measuring points and adjustment pots on the chassis - there is plenty of room.

Also unfortunately, the bias needs to be adjusted any time the 6L6s are changed. That is one of the tubes I was hoping to roll on a regular basis.

While I could probably do there external bias check mod myself, I will see if I can find a local pro to help me out. It might actually be nice to set up a couple of small meters to make bias check simple.

 

RE: Looking for Mingda MC805-AA Bias Procedure and Mods, posted on June 7, 2018 at 07:57:32
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 Yes, the 6L6 is directly coupled to the 805, so changing one affects the other. Adding an ammeter to each amp would make a lot of sense.

If you post your approximate location, then I'm sure someone on here can help you out.

 

RE: Looking for Mingda MC805-AA Bias Procedure and Mods, posted on June 7, 2018 at 08:11:58
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 I am in Chadds Ford (PA) in the event anyone wants to volunteer reasonably-priced services and is not scared of 805s.

 

Bias Procedure (In Case Someone Needs It), posted on June 11, 2018 at 16:47:17
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 From a dealer in China that formerly sold this amp:

"First, Please use the red probe of a multimeter to contact the 1Ω resistor near 805 tube socket, use the black probe to contact the chassis, then use the screw driver to bias 33K trimmer potentiometer which is round and gold and with blue color on the top, until the multimeter shows 100mA, 0.1V."

Easy enough.

I just wish that Ming Da had enabled this to be done externally.

 

Final Word - This Amp is a Keeper, posted on June 16, 2018 at 08:14:44
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 Just a quick update.

Still haven't made any bias adjustments or mods (one of these days), but nothing is red plating or being overdriven, and (to my ears) the sound is great when paired up with the right speakers (currently a pair of Tekton Pendragons).

I would definitely go with full range speakers as the bass on this amp is full and clean.

The amp is definitely a keeper.

For people that like 1000V in their lives, I would highly recommend trying one of these out (if you can find one in good working shape).

 

This looks like it could be the schematic, posted on June 19, 2018 at 15:52:41
Tweaker456
 Image result for Mingda MC805--AA schematic

 

RE: This looks like it could be the schematic, posted on June 19, 2018 at 16:43:51
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 Thanks.

That is the monoblock version.

The stereo version is essentially two of those.

I have a hand-drawn version of that schematic I got from a Ming Da dealer, but your printed version is easier to read.

 

RE: Final Word - This Amp is a Keeper, posted on April 16, 2021 at 13:23:28
jandroutsos@hotmail.com
Audiophile

Posts: 1
Location: Ontario
Joined: July 18, 2020
 Hello to all, I picked up one these amps. Very nice indeed. I replaced the 6P3S-E (good sounding but tested low, Russian 6L6 equivalent) with a vintage, smoked glass, made in USA, RCA 6L6G. I also replaced the 12AU7 with vintage RCA clear top 12AU7. Regarding the bias, I communicated with the manufacturer in China via email. They recommended 100mA of bias for the 805 tubes. They instructed that this is to be measured (as equivalent mV on your multimeter, of course), with the RCA input cables removed, placing one probe (+ive red) between the two 47Ω resistors and the 1Ω resistor and placing the other probe (common black) on the chassis common ground terminal or the speaker common black terminal. I used probes with a spring loaded hook, which look like small oscilloscope probes, which "stay" on, so I don't have an accident, and took my time adjusting for the 100mA. I had the amplifier on its side, with the transformer casing resting on a book.
I really enjoy the sound this amplifier makes and consider it a great deal in audio. It is far more full than 300B SE designs and more refined than any push pull I have owned to date, and I have owned quite a few.
I hope I have provided some useful good info here, if you still have this amp, that is.
Cheers

 

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