Presence Pot 5K vs 25K

[DaveWell]

Thanks Martin...

My conclusion is to keep the 5K pot

[roberto]

Ok so the AC signals avoid the 25k pot until its set a least resistive setting.

the AC signal doesn't avoid anything, and you can see it if you remove the pot: the sound changes.
And the pot doesn't need to go down to 5k to see something happens. IMO 5k is too low for hi-gain amps, where you can dial in good sounds by hitting the PI with more mids and highs while thaming them on the power amp. With a 5k pot you loose part of this opportunity. That's why I use 10k.

Makes you wonder why Marshall or who ever used a 25k pot then? If it was useless until the last portion of the pot.
Perhaps since they already bought 25k pots for the Middle control it was just easier to buy more 25ks.

Look at the 2203 (and following): you only need 1 MOhm log, 25 kOhm lin and 250 kOhm lin pots for the entyre amp: cheaper components, simpler storehouse, easier to be installed. Imagine that you sell 1 million amps and you pay three dollars more for components for each amp. How much have you lost at the end?

[martin manning]

Marketing and accounting will often drive engineering to do things they'd rather not, but in this case it's a reasonable choice.

[RockinRocket]

Here's 3 common presence circuits we see in Marshalls from the 60s to current production with everything being equal (5k resistor in place of 4k7) but set up different.
The first is the classic 60s. Second is what Marshall used in some reissues of the 60s amps and last being what they use the 2013s DSL series.

Other than no DC on the pot do all 3 respond the same with the same frequency regardless of where the cap is placed??

[roberto]

It's a cap in series with a (variable) resistor vs a (variable) resistor in series with a cap.
Their total impedance to ground will be the same.

[RockinRocket]

It's a cap in series with a (variable) resistor vs a (variable) resistor in series with a cap.
Their total impedance to ground will be the same.

Probably a silly question but the frequency sweep will respond the same between all 3?
Makes one wonder why they chose one over the other for example 2 and 3.

[martin manning]

2 and 3 will have exactly the same response. The choice might have been made for convenience in the physical layout.

The frequency response of the modern circuit is different from the vintage as I said above. Also, you are showing a 5k pot instead of the usual 25k, which further limits the ability to turn the presence all the way off.

[RockinRocket]

The modern circuit is essentially identical when the presence is rotated full CW, but the effective range of rotation is larger.. The modern circuit doesn't take the cap completely out of the circuit at full CCW either.

2 and 3 will have exactly the same response. The frequency response of the modern circuit is different from the vintage as I said above. Also, you are showing a 5k pot instead of the usual 25k, which further limits the ability to turn the presence all the way off.

I did purposely make all 3 circuits use the same value 5k pot (or 5k resistor) for comparison sake.
What exactly makes the frequency response different from example 1 to 2 or 3?



So to recap,
25k pot allows to use less Presence top end than a 5k.

Any drastic frequency change happen between 1 and 2?
I ask as some Marshalls took DC of the pot on latter reissues.
Really trying to pinpoint what exactly changes the tone or if it matters really.

Maybe ill wire up a dpdt switch and try and hear for any differences.

[martin manning]

5k allows less bottom end than 25k. Full CW all three are the same. At intermediate settings the vintage circuit rolls up the highs before the mids, whereas the modern rolls up highs and mids together. Definitely try it both ways so you can hear the difference.

[RockinRocket]

5k allows less bottom end than 25k. Full CW all three are the same. At intermediate settings the vintage circuit rolls up the highs before the mids, whereas the modern rolls up highs and mids together. Definitely try it both ways so you can hear the difference.

I'm sorry Martin I'm confused, are you comparing the 5k/DC to 25k/4k7 in that statement above??
In question was a 5k/DC on pot to 5k/4k7 as shown in the photo I made.
Thanks

[martin manning]

Same comment either way. Using 5k in place of the 25k in the modern circuit is like having only the 8-10 portion of the rotation of the 25k, but it is spread out from 0-10.

[DaveWell]

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

[RockinRocket]

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''

[pdf64]

Less effective in that the range of control is reduced; even at min, there will be some presence boost (compared to the scratchy pot arrangement).

[Gaz]

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.