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It is currently 22 May 2019, 09:44

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PostPosted: 24 Jan 2019, 08:05 
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Joined: 07 Mar 2017, 16:35
Posts: 9
Location: France
Hi Everyone

I've just completed this tube amp (http://diyaudioprojects.com/Tubes/6T9-P ... -Tube-Amp/)


The sound pass through but the amp starts squealing after few seconds, even with volume pot at "zero".

I read on internet that it was probably a positive feedback so I switched the wires linked between OT primary leads and 6T9 plates, still squealing.
I also tried to switch OT secondary leads but still the same.

If I disconnect the negative feedback wires (just one channel or both), the amp stop squealing..

Voltages are good and I checked the wiring twice.

Any thoughts ?

Regards

Pierre


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PostPosted: 24 Jan 2019, 08:25 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
Swap the wires to from OP-tranny to GND/FB. You could possibly have the OP tranny connected in a way that the feedback is feeding POSITIVE and not NEGATIV feedback, making your amp a siren.

Sorry, didn't see you'd already done that ...

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PostPosted: 24 Jan 2019, 10:07 
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Location: France
I forgot to tell that the squeal is happening with and without input signal.


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PostPosted: 07 Feb 2019, 06:33 
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Joined: 07 Mar 2017, 16:35
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Location: France
Hi

I made a mistake using the word "squeal", in fact the amp make a loud sound in the mid frequency range like a alarm.

The attached file is a recording of that sound.

I tried different value for the NFB cap : 80pf, 150pf, 220pf and NFB resistor : 10k, 20k.
There is no change at all.

If I disconnect the NFB wires, the amp has a lot of gain but works fine.


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PostPosted: 07 Feb 2019, 08:55 
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Joined: 19 May 2011, 05:38
Posts: 36
Make sure that the screen grid taps are going to the correct tube!


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PostPosted: 07 Feb 2019, 10:21 
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Joined: 07 Mar 2017, 16:35
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Location: France
Thank's for your response.

When you said "correct tube", you think that maybe I reversed primary leads between the two channels and the NFB of the right channel is feeding the left channel and vice versa ?


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PostPosted: 07 Feb 2019, 16:30 
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Joined: 13 Feb 2013, 11:02
Posts: 46
Location: Canada
No, reverse the primary an screen taps between the two output tubes in the same channel. You have those reversed and instead of negative feedback you have positive feedback creating a high power oscillator.

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DTS Audio Electronics
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PostPosted: 08 Feb 2019, 03:39 
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Joined: 07 Mar 2017, 16:35
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Location: France
Hi blackdog

First thing I did was to swap primary leads (blue with brown and blue/yellow with brown/yellow).

The siren still there.

I'm not sure it's a positive feedback or perhaps I misunderstood that part.

People talk about "ringing/overshoot " because of the negative feedback. Perhaps this is the reason ?


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PostPosted: 08 Feb 2019, 11:47 
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Joined: 07 Mar 2017, 16:35
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Location: France
Problem solved.. NFB links were reversed.

NFB of the left channel were wired to the right channel and vice versa...oops :D

The amp works great now.

Thank's everyone


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