Vintage Presance Control Differances

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RockinRocket
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Vintage Presance Control Differances

Post by RockinRocket »

I'm assuming you guys will know the vintage Marshall presence circuits of the 70s of the top of you head here..

5k pot with a .1uf DC on the pot

25k pot with 4k7 parallel - .68uf no DC on the pot

My question here is why does the 25k pot have a 4k7 resistor in parallel here? What purpose does this do?

A 25k pot in parallel with a 4k7 resistor now makes the pot act as a 3k9 pot?
Close enough as a 5k pot id assume Marshall was thinking, if my understanding is correct.
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martin manning
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Re: Vintage Presance Control Differances

Post by martin manning »

The PI must have a DC path to ground, and if you use a pot for that, it's noisy. The modern presence has the 4k7 for the DC path and a separate path for the AC shunt where the cap blocks DC from the pot. The two circuits don't sound the same, and many people will put up with the scratchy pot to have the vintage presence.
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Colossal
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Re: Vintage Presance Control Differances

Post by Colossal »

martin manning wrote: Sat Mar 04, 2017 11:35 pm The PI must have a DC path to ground, and if you use a pot for that, it's noisy. The modern presence has the 4k7 for the DC path and a separate path for the AC shunt where the cap blocks DC from the pot. The two circuits don't sound the same, and many people will put up with the scratchy pot to have the vintage presence.
I prefer the vintage 5kB/0.1uF arrangement. There is more effective range to the knob versus the later 4k7||25kB->0.1uF.
RockinRocket
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Re: Vintage Presance Control Differances

Post by RockinRocket »

Thanks Martin that is very helpful.

Now that I did deeper I may not have done enough homework before posting.

The 2nd version of Marshalls presence circuit may still have used a 5k pot- not 25k.
The .68uf cap is also after the pot.

The pot wouldn't be scratchy this way? The 4k7 to ground and the .68uf after the pot removes DC scratchiness on the pot?
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martin manning
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Re: Vintage Presance Control Differances

Post by martin manning »

I think you are referring to the 1987 MkII? Same idea there- a 4k7 for the DC path and a pot + cap in series for the parallel AC only path. Just different values.
RockinRocket
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Re: Vintage Presance Control Differances

Post by RockinRocket »

Yes I am referring to the 1987 MK2 circuit.

It seems Marshall used lots of variations of the presence control. I associate a cap in series with a 25k pot and 4k7 as Marhsalls first fix for DC on the pot.
After closer examining photos of 70s 1987 and 2204 circuits that didn't happen till later and pot was probably still a 5k.

Now why wouldn't the AC just ground through the 4k7 resistor and bypass the pot and cap as this route is shorter?
What affect does the 4k7 and pot in parallel have on one another compared to the circuit with DC on the pot?

Ill have to play with these circuits when I build another one but I really want to understand these circuits before I go in to play with them :D
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martin manning
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Re: Vintage Presance Control Differances

Post by martin manning »

RockinRocket wrote: Sun Mar 05, 2017 5:36 amNow why wouldn't the AC just ground through the 4k7 resistor and bypass the pot and cap as this route is shorter?
What affect does the 4k7 and pot in parallel have on one another compared to the circuit with DC on the pot?
Th 4k7 looks like 4k7 to all frequencies in the feedback signal while the impedance of the cap and pot is frequency dependent. With the Presence full-up, the vintage and modern networks look the same (4k7//0.1uF), and they are almost the same when the Presence is all the way down and there is 25k in series with the cap. It's the intermediate settings where the response of vintage circuit is better. I think Marshall changed to and settled on the modern presence (with a 22k pot) in the 1987 era.
Cameron
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Re: Vintage Presance Control Differances

Post by Cameron »

RockinRocket wrote: Sat Mar 04, 2017 10:38 pm I'm assuming you guys will know the vintage Marshall presence circuits of the 70s of the top of you head here..

5k pot with a .1uf DC on the pot

25k pot with 4k7 parallel - .68uf no DC on the pot

My question here is why does the 25k pot have a 4k7 resistor in parallel here? What purpose does this do?

A 25k pot in parallel with a 4k7 resistor now makes the pot act as a 3k9 pot?
Close enough as a 5k pot id assume Marshall was thinking, if my understanding is correct.
1488807925307.jpg
I see this mistake a lot ....it's not the same circuit ....same end result technically......but not the same wiring....and also have different character.... The pot is not in parallel with the 4.7k resistor by itself ...it's isolated by the cap to ground. So it doesn't change the value of the 4.7k..either way it's basically 5k ... To make this simple...I'll just show a schematic ....I think you will see what I'm saying......you could also reverse the position of the cap and pot in the new version..but it's the same thing...
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RockinRocket
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Re: Vintage Presance Control Differances

Post by RockinRocket »

Hi thanks for adding more clarification and taking the time for the drawing.

I will have to hear the two circuits side by side to determine what actually make the circuit sound different.
The Internet is full of wives tails on how the old one sounds better. If it just removes DC from the pot and works basically the same then I want to be able to hear it for my own decision. Is the DC on the pot actually noticeable when the pot isnt adjusted? Or is it how the range of the pot works that make folks prefer the first one?
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roberto
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Re: Vintage Presance Control Differances

Post by roberto »

A good compromise is 10k and 100nF, of course with 4k7 in parallel.
25k has really a small range of adjustement, and the scratchy pot can be annoying for some users.
I've personally no issues with that, but everyone has his own preferencies and personal misophonia :mrgreen:
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rooster
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Re: Vintage Presance Control Differances

Post by rooster »

I definitely prefer the 25K pot control and, yes, I think it was an improvement to the circuit. Just be sure to use a lineal pot.

If you can hear that scratch on the 5K pot when you move it, then IMO, it also follows that there is noise in the circuit when it is static.
Most people stall out when fixing a mistake that they've made. Why?
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roberto
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Re: Vintage Presance Control Differances

Post by roberto »

No noise if it stays static, because the wiper doesn't go to a different dc value.
gtomax
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Re: Vintage Presance Control Differances

Post by gtomax »

Remember that the JTM-45 was derived from the bassman/twin 5F8-A/5F6-A design. In the 5E8/5E6 design there's a 5k lin presence pot and no resistor. Fender was loosey goosey in the late 50's and over capacity on production as well as CHEAP. They must have for a while run out of 5k pots because they'd just grab what was next to it - in this case a 25k pot for the mid control one hole over - and put it in presence. With the 4.7k resistor it drops the pot value as you note down into the 5k range. You can find lots of examples of 58/9 twins and bassman amps that have 25k/4.7R combo where-as earlier 5E- designs were correct 5k/no resistor combos. While these seem like they correct the issue the response is actually different between the two implementations.

So when Marshall copied the fender schematic/example etc. they may have later seen the resistor on that pot and tried it out would be my guess vs. the schematic/layout/earlier amps that were 5k/no resistor.

Who knows, but that's my guess.
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