Tube amp help

[shadowlight]

I have a Ming Da 3008 se amp and need help in biasing the 805 tubes.  The bias is  pots are inside the amp and I am not comfortable mucking around inside while it is powered on.  Any recommendations on a local tech who can help me?

[sleepyguy24]

Hi Shadowlight

Where are you located again? The guy I go to for everything tube related is Scott Gramlich from SG Custom Sound.

http://www.yelp.com/biz/sg-custom-sound-douglaston

A nice straight shooting quiet guy in Douglaston NY. He modded my Jolida 502P amp and has familiarity with US based tube gear like McIntosh too. I believe he can definitely help with your MingDa amp if you can get it too him. I've mailed stuff to him in the past also but you would just have to coordinate with Scott as to go about it.

[shadowlight]

Thanks Sleepy.

I am located in Princeton, NJ and the amp is pretty heavy so would prefer not to ship it.

[richidoo]

http://www.mei-xing.com/english/english/cp/1_3008-AB.html

http://lampizator.eu/AMPLIFIERS/CHINA/Mingdamc300-C/ming-da.html

This review of old mono version of MC3008-A originally had no adjustable bias for 805s. Might be interesting to you or your tech as it shows a schematic, if your manual does not include a schematic, yours might be a derivative circuit of this old one. The old one had fixed bias, and some of the MingDa amps are autobias, so are you sure that yours has adjustable bias pots inside?  Does the manual say anything about bias? I know it's all Chinese, but maybe there is a picture with arrows or something that would indicate that it is in fact adjustable bias.

A guy on audiogon forum found it difficult to get service from local US MingDa dealers who did not sell him the amp. A good tech will not need the schematic, it is a relatively simple design.

The amp weighs ~100 pounds and it needs to be run upside down to adjust the bias. So probably wise to have a tech show you how to do it the first time.  While he's in there ask him to mark the meter probe connections positive and negative probe points, and also mark which way to turn the bias pot to get to zero. With that info you can do it yourself safely - IF YOU DARE!!     Be careful of those wires coming out of the top of the 805s, they are 900V.

I bet it sounds great!

I remember on AC there were some threads about finding audio techs. Maybe you can dig up the old thread.
Rich

[Emil]

C'mon, Shadowlight. Grow a pair!

Just dont bias standing in water.

[mfsoa]

Shadow - I'd think that Maxamillion here might be able to help - He's in Bridgewater.

-Mike

[shadowlight]

I had reached out to MingDa (got a response back from them within 12 hrs  ) and here is what they had to say about biasing the amp.

"
Here is the procedure for biasing MC3008-ABSE after you install a 805 tube.

First, please use the red probe of a multimeter to contact the 1 ohm resistor which is near the tube socket of 805 tube.

Second, please use the black probe of the multimeter to contact the chassis.

Third, there is a 33K adjustable resistor which is round and gold near the tube socket of 300B.
Please use a screw drive to bias the 33K resistor until the multimeter shows 100mA, 0.1V.

That's all."

There is no way I am poking around inside the amp when it is upside down, opened and powered on.  I had Lukasz modify the amp so we can use the simple 805 tube without the connection on the top but we were not able to test it due to one of the new psvane tubes being bad (had to wait almost 3+ weeks for replacement to get to me).  I have a copy of the updated schematics that Lukasz marked up in case we needed to revert back.

[BobM]

[shadowlight]

Bob -

Maxamillion is going to help me get it up and running.

Thx Mike.

[shadowlight]

It seems that the amp has some kind of short and needs to be looked at by a tech.  The bias was jumping all over the place and we also saw AC.

[rollo]

Topround has a Tech that can help you out. PM him.


charles