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Post subject: NFB Switch
Posted: Mon Apr 28, 2014 5:50 pm
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Aspiring Musician
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Just wondering if this would work fine for a switchable feedback mod. The top is the way it is now.

Thanks for any advice.

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Post subject: Re: NFB Switch
Posted: Mon Apr 28, 2014 6:44 pm
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Looks fine to me. 8)

On second thought, although your circuit would work, it might be better to put the switch and another 1.5K resistor, in parallel with the existing resistor.

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Post subject: Re: NFB Switch
Posted: Mon Apr 28, 2014 7:30 pm
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Aspiring Musician
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Being parallel then, it would drop to 750 ohms. Would using a 1.8k be better, or maybe it's not that critical in a NFB loop, being only 70 ohms off? On your suggestion I could use a SPST on/off switch also.

This is on my '65 reissue/'68 custom project. Hoping it will quiet it down for certain situations.

Thanks for the help.


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Post subject: Re: NFB Switch
Posted: Mon Apr 28, 2014 8:02 pm
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upnorth wrote:
Being parallel then, it would drop to 750 ohms. Would using a 1.8k be better, or maybe it's not that critical in a NFB loop, being only 70 ohms off? On your suggestion I could use a SPST on/off switch also.

This is on my '65 reissue/'68 custom project. Hoping it will quiet it down for certain situations.

Thanks for the help.


Not sure what you are trying to accomplish but the loop gain with a 1.5Kohm resistor is about 10% greater than with a 820 ohm resistor. If both amps use the 820 ohm resistor (which is what the 65 DRRI uses), by going to the 1.5K resistor, you will be increasing the overall gain of the power amp stage which I don't think is what you want.

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Post subject: Re: NFB Switch
Posted: Mon Apr 28, 2014 8:16 pm
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Actually what I'm up to is this: I swapped out all the boards in my '65 reissue with boards from a '68 Custom DR. (Boredom and availability) The '68 has the NFB set with a 1.5k, the '65 reissue had it's set with a 820 ohm. Just wanted to switch between the two. (Boredom again maybe?)


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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 12:52 am
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upnorth wrote:
Being parallel then, it would drop to 750 ohms. Would using a 1.8k be better, or maybe it's not that critical in a NFB loop, being only 70 ohms off? On your suggestion I could use a SPST on/off switch also.

This is on my '65 reissue/'68 custom project. Hoping it will quiet it down for certain situations.

Thanks for the help.


Yes, a 1.8K would get you closer to the 820 ohms that you want. Consider the tolerances also, the actual resistances might be different than indicated.

Let us know about the difference if you try this. I think that it's kind of a cool idea.

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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 10:02 am
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Just an update. I installed a switch and put a 1720 ohm resistor in parallel with the '68 factory NFB 1.5k resistor. The only thing close to 1.8k I had was a 1.5k carbon comp that had drifted to 1720. I labeled the switch '65 and '68.

It does make a difference. Less gain/just a touch of volume drop, just what one would expect I guess. It also does quiet the amp down some in the '65 NFB position.

I think I'll put some type of in line quick connectors so I can try some different resistors. I'll post some pic's when I have it apart again.


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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 10:40 am
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Upnorth, JOC, you flip the switch with the amp off? I read about something like this on another site. The poster said the switch made quite a noise, if flipped during use.

Just curious. Thanks!


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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 11:20 am
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BMW2002Ti wrote:
Upnorth, JOC, you flip the switch with the amp off? I read about something like this on another site. The poster said the switch made quite a noise, if flipped during use.

Just curious. Thanks!


Dead quiet. I was actually doing an A/B often while playing and no noise when switching whatsoever. Wonder what amp, kind of switch, etc., he was using.

As I was playing I noticed how much I dislike these Golden Age Parson's Street alnico humbuckers. Harsh highs in any amp I use. I really need to get some 57's or something.


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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 11:39 am
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BMW2002Ti wrote:
Upnorth, JOC, you flip the switch with the amp off? I read about something like this on another site. The poster said the switch made quite a noise, if flipped during use.

Just curious. Thanks!


That was why I suggested the alternate circuit, no chance for an open circuit that might cause noise.

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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 3:39 pm
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shimmilou wrote:
That was why I suggested the alternate circuit, no chance for an open circuit that might cause noise.


Thanks for the help Shimmilou. The open circuit while switching would have been a pain.

Here's some pic's.

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Post subject: Re: NFB Switch
Posted: Tue Apr 29, 2014 6:29 pm
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Nice work! Quite the custom amp. 8)

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