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Ming Da MC3008
Ming Da MC3008
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Old 7th November 2017, 01:06 PM   #1
juancho is offline juancho  United Kingdom
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Default Ming Da MC3008

Does anyone know the correct bias setting for these amps? It's currently set at 135ma and sounds great but it's always nice to know it's correct!
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Old 7th November 2017, 04:16 PM   #2
FullRangeMan is offline FullRangeMan  Brazil
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What 3008? This amp has many versions.
If its the 805 amp old version seems too much imo.
Less Bias would not hurt, less heat, less power and a longer life to the 300B/805.
Anyway contact Mrs.Suena at mingda at mei-xing.com
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Old 7th November 2017, 04:20 PM   #3
juancho is offline juancho  United Kingdom
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Thanks for the reply. It's the version with 805s and 300b drivers without the top cap 805s.
The range on the pot is approx 126-145 so 130 is on the low side of the adjustment.i will write to Ming Da as you suggest
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Old 7th November 2017, 07:31 PM   #4
juancho is offline juancho  United Kingdom
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Default Positive Bias for 805 valves

I've never done this before. Does higher +v = lower dissipation or higher?
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Old 7th November 2017, 07:39 PM   #5
FullRangeMan is offline FullRangeMan  Brazil
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Positive grid=much power and shorter tube life.
Seems original US 805 tubes was made to use positive Bias.
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Old 7th November 2017, 09:05 PM   #6
juancho is offline juancho  United Kingdom
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Ming Da sell it with positive bias already set up. UK ones run at ,95ma whereas this grey import is running at 135 with minimum adjustment possible down to 126ma. So I want know whether I should increase positive bias to decrease dissipation or vice versa
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Old 7th November 2017, 09:38 PM   #7
FullRangeMan is offline FullRangeMan  Brazil
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Less Bias = less power

Neg Bias = less power
the more negative Bias is, the less power there will be.
As Bias increases towards positive, power increases too.
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Old 10th November 2017, 04:30 PM   #8
juancho is offline juancho  United Kingdom
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Default Calling All Tube Experts

Here's hoping someone can help! I'm sorting out a Ming Da 3008A amp. These are monoblocks which use a 300b driving an 805 output tube. I've attached the circuit diagram. The amp uses a novel bias circuit as shown using a Zener reference and supplying positive bias to the output tube.

As supplied the amp runs its output tubes too hard at 135ma when 95ma is the point to aim for for decent tube life. My question is: if I want to increase the positive bias I assume I lower the Zener reference to give less negative bias to the 300b and also to raise the positive bias for the 805?

Is that correct?
Thanks
David
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Old 10th November 2017, 04:48 PM   #9
DavinCarter is offline DavinCarter
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You are actually looking to decrease the positive bias voltage. Have
you adjusted the 33K bias POT and just need to go lower? Yes,use a lower voltage zener if needed and it cannot adjust lower.
If you do not need so much power, lowering the bias is fine for sure.
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Old 10th November 2017, 04:54 PM   #10
kevinkr is offline kevinkr  United States
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Ming Da MC3008
Yes you need to decrease the positive bias on the output tube in order to reduce the plate current. (Confirming Davin) A small reduction of a few volts may be all that is required.
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