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PostPosted: 30 Sep 2015, 15:29 
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Joined: 04 Feb 2013, 06:52
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Location: North Wales
I've seen some schematics where the feedback from the secondary side of the output transformer goes to the cathode of the input valve through a variable potentiometer. I presume it's a linear potentiometer, is this a worthwhile method of providing feedback? I imagine in practice you adjust it to suit your ears and your speakers. Is a switch also worth considering to give zero feedback, when it's in the open position?


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PostPosted: 30 Sep 2015, 15:38 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
A simple 3-way switch would be fine for adjusting feedback, I think. One postition for no fb and two postions for more and less fb.
One problem when adjusting fb is that you also adjust gain so a variable would require adjustment of volume too. And a potentiometer that simultaneously adjusted fb and gain would be very quirky.

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PostPosted: 02 Oct 2015, 10:03 
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Joined: 04 Feb 2013, 06:52
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Location: North Wales
I don't anticipate using one potentiometer to control both feedback and gain, but rather to determine by experiment, what amount of feedback gives the best resulting sound, then leaving well alone. Could then measure the resistance the potentiometer is adjusted to, and replace with resistors of the nearest standard value.


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PostPosted: 02 Oct 2015, 12:25 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Golden Tube Audio has a 300B with adj. NFB. One pot per channel.

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PostPosted: 02 Oct 2015, 17:27 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, For initial testing purposes a variable resistor will do fine. Then as you indicated, measure the value and use a fixed resistor. Be aware though that like was mentioned the gain will be reduced as you increase the NFB. Additionally if there are non-linear components in the loop (ie capacitors) that are there to shape the sound or overal amplifier response the trunover points will be altered.

Good listening
Bruce

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