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Calling All Tube Experts
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Author:  Juancho [ 10 Nov 2017, 12:36 ]
Post subject:  Calling All Tube Experts

Attachment:
MC3008-A5B15D.jpg
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

Author:  Roscoe Primrose [ 10 Nov 2017, 15:39 ]
Post subject:  Re: Calling All Tube Experts

Looking at the schematic, you should be able adjust the 805 bias current by adjusting the 33k potentiometer to the left of the 470K resistor connected to the 300B grid.

Roscoe

Author:  gofar99 [ 10 Nov 2017, 17:23 ]
Post subject:  Re: Calling All Tube Experts

Hi, Rather than mess with that....I would increase the 47R resistors in the cathode of the output to something like 62R or even 75R. This would case a shift in the bias on the output tube and reduce the idle current. The exact values will have to be determined experimentally. As you noted a fairly odd circuit.

Good listening
Bruce

Author:  blackdog [ 10 Nov 2017, 19:17 ]
Post subject:  Re: Calling All Tube Experts

Adjusting the 33k is how they want you to adjust the bias. I've run into this before with a different make of amp. The caution is that you don't put too low a bias on the 300B and over burden that tube. If that happens, increase the cathode resistor on the 300B.

Author:  Juancho [ 11 Nov 2017, 09:39 ]
Post subject:  Re: Calling All Tube Experts

To clarify the 33k pot only allows adjustment down to 126ma, which is still way too high. Whilst the bias circuit looks simple, any changes can alter the SRPP characteristics or 300b operation. So my quest is to find the best solution to lower the 805 positive bias with least change to the SRPP or 300b operation

Author:  Juancho [ 11 Nov 2017, 11:36 ]
Post subject:  Re: Calling All Tube Experts

gofar99 wrote:
Hi, Rather than mess with that....I would increase the 47R resistors in the cathode of the output to something like 62R or even 75R. This would case a shift in the bias on the output tube and reduce the idle current. The exact values will have to be determined experimentally. As you noted a fairly odd circuit.

Good listening
Bruce

This looks the simplest most pragmatic solution to me
Thanks

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