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6V6 line preamp
6V6 line preamp
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Old 18th November 2019, 12:00 AM   #3581
democles is offline democles  Netherlands
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I did some N.feedback and it sounds nice.
The bass gets tighter and the voices really sound. But the tube sound remains.
Total amplification is now 1,76x.
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File Type: jpg DSC07553.jpg (980.1 KB, 263 views)
File Type: jpg terugkoppeling 2.jpg (702.3 KB, 236 views)

Last edited by democles; 18th November 2019 at 12:05 AM.
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Old 18th November 2019, 10:48 AM   #3582
Salas is offline Salas  Greece
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6V6 line preamp
The 100k pot's source impedance and the higher gain +Miller when open loop surely makes a 11kHz range square slower vs using feedback. On 20k - 50k pots its better. Not a concern in your case, you have feedback now. Is there any ringing on the leading edge if you zoom in on the yellow trace?

Do you really need the fix bias scheme? Did you end up choosing a really different operating point than the circa 22mA original for auto bias with the 680Ω Rcathode?
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Old 18th November 2019, 12:45 PM   #3583
democles is offline democles  Netherlands
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Thank you Salas for the feedback.
I am a member of a Dutch forum.

We have developed a bias machine for PP amplifiers with a number of people.
I wondered if it would also work for triode circuits.


This circuit remains stable.

And yes I can now easily set a different work point if I wanted to.

But at 22/23 mA it sounds good.

On the scope photo, the yellow is the signal input.

So just for the pleasure of building, I post it here.
The next step would be to put your HV shunt in between and listen what it brings.
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Old 18th November 2019, 02:29 PM   #3584
Salas is offline Salas  Greece
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6V6 line preamp
Thanks for your info. Since the blue trace is the output and not a comparison between FB/NoFB channels as I thought, it indicates some roll off. Even below 20kHz maybe. But you can find exactly by increasing the gen's sinus frequency until the 6V6's output signal falls 0.707 from 1kHz (-3dB).

To increase bandwidth do try with a lower value log pot also. At max volume all pots will present the same bandwidth though because they effectively go out of circuit then.
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Old 18th November 2019, 02:44 PM   #3585
democles is offline democles  Netherlands
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I turned the negative feedback out, to much roll off. We are working on a different setup.
I will try a different pot.

Thanks Salas.
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Old 22nd November 2019, 12:26 AM   #3586
democles is offline democles  Netherlands
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I have changed the feedback

100K pot with 100K resistor.

Roll off is less.

-3dB is at 160KHz

The 50K log volume pot has been ordered.
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File Type: jpg geel uit.jpg (941.3 KB, 110 views)
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Old 22nd November 2019, 12:45 AM   #3587
Salas is offline Salas  Greece
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6V6 line preamp
Much better now
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