Presence Pot 5K vs 25K

[Cameron]

Just dropping in to say the 5K scratchy pot is much better in every way except the scratchy pot part Next best is the nonscratchy arrangement with a 10KA pot ala Sunn Model T.

Audio would be a very bad taper ...the Sunn has a reverse audio 25k with a 10k instead of the 4.7k to ground... but it's basically the same as the Marshall with a little more feedback. I like to use reverse audio pots for the presence wether useing the 5k or 25k ...it gives a nice taper for the presence control. Although some people preceve the amp as bright sounding ..because it's not what they are used to.

[Cameron]

That makes sense. The 4k7 in parallel with the pot makes the presence about half as effective.
So if I'm understanding this correct instead of a 6db boost* through a pot it will only be half a 3 db boost* for the same frequency.

* or did I mean reduction? I just woke up

Hi,
I have found this document,
It's a Gerald Weber Book
'' Tube Amp Talk''

I've drawn this out in another thread ....remember the 4.7k resistor is not in parallel with the pot .....the 4.7k resistor goes to ground but the pot does not....

[Cameron]

1488807925307.jpg

This is showing the 2 different styles ..the older and the newer style. It was a factory mistake to use the 5k pot in the new style....

[RockinRocket]

My reply was in reference to Webers link in Daves post.

It said while the cap blocked Dc from the pot the 4k7 and presence always sees AC.

"Ac has two paths to ground through the 4k7 and cap/pot ... makes the (presence) about have as effective"

1488807925307.jpg
I've drawn this out in another thread ....remember the 4.7k resistor is not in parallel with the pot .....the 4.7k resistor goes to ground but the pot does not....

This is showing the 2 different styles ..the older and the newer style. It was a factory mistake to use the 5k pot in the new style....

[Cameron]

My reply was in reference to Webers link in Daves post.

It said while the cap blocked Dc from the pot the 4k7 and presence always sees AC.

"Ac has two paths to ground through the 4k7 and cap/pot ... makes the (presence) about have as effective"

1488807925307.jpg
I've drawn this out in another thread ....remember the 4.7k resistor is not in parallel with the pot .....the 4.7k resistor goes to ground but the pot does not....

This is showing the 2 different styles ..the older and the newer style. It was a factory mistake to use the 5k pot in the new style....

I don't agree with that ...but try it for yourself and see. You could say it's a tiny bit that's on when the presence is turned down...but not half. Weber is known to say wrong stuff....so I wouldn't go by that ....Try both and see what you end up with ...everybody has given you more then enough information about it here....

[RockinRocket]

Cameron, I think it makes sense.
Ac has two paths to ground a 5k pot (or 25k) and the 4k7.
So the 4k7 is going to affect how much of the presence cap can boost as their is less to work with.
I don't know the exact amount but if you can get a 6 db boost of top end with DC on the pot. Weber says "half" would only be about 3db.

I'm actually more curious at this point at how it works rather than what sounds better.

[martin manning]

Weber's explanation is flawed. When the pot resistance in the modern circuit is zero (presence full CW), then you have virtually the same circuit as the vintage: a ~5k resistance in parallel with the presence cap.

[Cameron]

Cameron, I think it makes sense.
Ac has two paths to ground a 5k pot (or 25k) and the 4k7.
So the 4k7 is going to affect how much of the presence cap can boost as their is less to work with.
I don't know the exact amount but if you can get a 6 db boost of top end with DC on the pot. Weber says "half" would only be about 3db.

I'm actually more curious at this point at how it works rather than what sounds better.

The 4.7k resistor is doing the same job as the 5k pot.....as far as how the PI sees it. The 25k pot is just varying the amount the cap is in parallel to the 4.7k ...it's basically the same as the 5k pot wiper varying the amount the cap is in parallel with the pot itself......you are really over thinking this.....the only difference in performance is ...with the new style you may hear a tiny bit of presence still on ....and most people can't hear it ...the other is taper of the pot .....but it really is basically the same amount of control. The new style does not give you half of what the old did....that is just wrong....

[roberto]

For the sake of clarity and global understanding, in attachment you can find log plots of the different configurations:
- old scratchy 5k + 100n;
- new 4k7 // 25k+100n;
- my 4k7 // 10k+100n;
- unused 4k7 // 5k+100n.

[RockinRocket]

Roberto!
That's is supper helpful! Did you plot that your self? Much appreciated!

It looks like plot 4- 5k /4k7 100n starts where plot 1- 5k/100n is set to 5 (or half on the pot) like Weber stated as half as effective.

For the sake of clarity and global understanding, in attachment you can find log plots of the different configurations:
- old scratchy 5k + 100n;
- new 4k7 // 25k+100n;
- my 4k7 // 10k+100n;
- unused 4k7 // 5k+100n.

[roberto]

I'm happy to see you find it useful.

Yes I plotted myself, please note that all plots has been done with linear tapes.
I can do it with rev log if needed.

[RockinRocket]

I would really like to see-

5k .68uf (Major Major)
5k/4k7 .68uf
25k/4k7 .68uf

Although lookin at your posts shows a great idea how the set ups differ. If you are bored and want to

I'm happy to see you find it useful.

Yes I plotted myself, please note that all plots has been done with linear tapes.
I can do it with rev log if needed.

[Cameron]

Roberto!
That's is supper helpful! Did you plot that your self? Much appreciated!

It looks like plot 4- 5k /4k7 100n starts where plot 1- 5k/100n is set to 5 (or half on the pot) like Weber stated as half as effective.

For the sake of clarity and global understanding, in attachment you can find log plots of the different configurations:
- old scratchy 5k + 100n;
- new 4k7 // 25k+100n;
- my 4k7 // 10k+100n;
- unused 4k7 // 5k+100n.

Maybe I misunderstood ...but I thought you were talking about the 25k with the 4.7k resistor as being half..... The 4.7k // 5k version was a mistake that Marshall eventually noticed. This configuration would make the pot 2.5k...and yes it would clearly limit the pot to half ...but this does not apply for the 4.7k//25k version...as the pot is not grounded....even if it was ..it's still not half .... Just trying to keep this clear....

[roberto]

I don't want to create a debate, but I wouldn't say that it's half effective. It's a confusing definition.

If we consider that the old scratchy has a presence control from 0 to 100% (in other words, if we consider that as a reference for our comparison), new arrangements have a range from 10 to 100%, 20 to 100%, 50 to 100% (just to say some indicative percentages) while rotating the pot from fully CCW to fully CW, depending on the value of the pot (25k, 10k or 5k).

So the full CW effect is the same (so the definition "half effective" can be confusing), but changes the effect when at full CCW: null in the scratchy one, more and more present on modern configurations as far as you decrease the value of the pot.

In my amps I use the old scratchy lovable one, or the 10k lin one.

[roberto]

The other point is that the scratchy solution is linear in its range, while new ones aren't with linear pots.
That's why some builders switched to rev log pots, to linearize the effect while keeping away the scratchy noise.