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PostPosted: Thu Feb 26, 2015 2:56 pm 
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Mark IV

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Location: Tucson, AZ
3124+ wrote:
When comparing the presence pot values, are you guys using vintage mode, or modern?


The value is independent of the negative feedback (mode), because the pot is mechanical and will always give the same comparison. The only thing I hear different is the way CH2 opens up at the end of it's taper. Otherwise, the rule of thumb previously noted is a good comparison.

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PostPosted: Thu Feb 26, 2015 4:25 pm 
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Mark II

Joined: Mon Feb 05, 2007 11:02 pm
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In CH3 Vintage, the 100K presence pot actually sounds warmer than the 25K. Increasing pot resistance blocks signal through the .1uf presence capacitor to ground. This means less high frequency attenuation in the negative feedback loop and therefore less high frequency boost in the power amp.

In CH3 Modern, the negative feedback loop is disabled and the presence control becomes a treble bleed off the tone stack. Increasing pot resistance blocks high frequency attenuation through the .003uf cap to ground.


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PostPosted: Fri Feb 27, 2015 12:19 am 
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Mark IV

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Comparing the tapers between differing values will never be dead-on-exact as the maximum resistance of the pot affects the sound on it's own. However, rule of thumb, the approximate sound as described in the manual is close. The only major difference in taper I hear on mine is when peaking the presence in Vintage mode. Modern doesn't emphasize the mids the same way Vintage does, because of the difference in operation you have stated.

Maybe I'm misunderstanding your question. If someone swaps presence pots out to make CH3 like CH2, the mode doesn't matter. They'll be really close to each other in any mode.

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PostPosted: Wed Aug 12, 2015 9:39 pm 
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Mark II

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The mode matters because on vintage setting the presence circuit works in the traditional way, by attenuating treble frequencies in the negative feedback loop to consequently boost overall treble in the power amp. When switching to modern mode on ch 3, the negative feedback loop is disconnected and the presence pot moves to control the r/c filter on the treble pot wiper.


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PostPosted: Thu Aug 13, 2015 12:11 pm 
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Mark IV

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I think I understand what you meant, now. I think they're comparing the same modes, not modern vs vintage. That's apples to oranges as you pointed out.

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PostPosted: Thu Aug 13, 2015 12:58 pm 
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Mark II

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afu wrote:
I think I understand what you meant, now. I think they're comparing the same modes, not modern vs vintage. That's apples to oranges as you pointed out.


Even comparing same modes is apples to oranges because... ch2 modern keeps the negative feedback loop connected and the presence control functions in the traditional way as part of the power amp. But ch3 modern disconnects the negative feedback loop in the power amp and the presence control moves to the treble pot wiper.


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PostPosted: Mon Nov 16, 2015 11:28 am 
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Mark IV

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http://wp.me/p4h97k-c8

My article about the Presence pot circuit. Some notes in regards to the modification are included.

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PostPosted: Tue Jul 12, 2016 6:41 pm 
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Mark IV

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https://wordpress.com/post/warpedmusician.wordpress.com/4738

I finally got around to plotting the Vintage presence control's filtering. In the pictures, as the frequency drops away it is able to pass the phase inverter more easily. R2 is variable and represents each 30 degrees on the control.

I haven't done the Modern version yet and I'm beat. I'll get back to this very soon.

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PostPosted: Wed Sep 14, 2016 1:22 pm 
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Bottle Rocket

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A quick question here (please bear with me - I don't want to dig through all the posts ;) ) -

the basic channel 3 presence mod is the same one that the manual describes?
Fluff got his pot. straight from the Mesa guys?

Thanks!


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PostPosted: Thu Sep 15, 2016 2:21 pm 
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Mark IV

Joined: Thu Aug 15, 2013 12:20 am
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EooN wrote:
A quick question here (please bear with me - I don't want to dig through all the posts ;) ) -

the basic channel 3 presence mod is the same one that the manual describes?
Fluff got his pot. straight from the Mesa guys?

Thanks!


Yes.

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