Presence Circuit and Shunt Resistor

[glpg80]

This also shows visually what I assumed is true - case 3 would perform poorly in AC response.

I do like case 1 but DC on a presence pot is a deal breaker for me - I want to adjust knobs without having to worry about that. So I run with case 2.

I am an engineer by day and the theory of what is in practice today is somewhat divorced because not much is taught in college that draws the line all the way back to tube theory because it’s seen as irrelevant history instead of skills in a modern world. Instead there’s a lot of self teach involving the limitations of tubes and how to quantify modern theory into tube performance and characterization to design stages. Then there’s the sonic aspect of how something sounds related to theory.

The circuit I’ve created in my 74 from scratch is nothing like anything - it isn’t a copy of anything and is a morph of theory, trial, and error as to what sounds right and what doesn’t. If anything having the background helps a bit with debugging but honestly I believe tube amps are a level playing field for all.

 

[FourT6and2]

This also shows visually what I assumed is true - case 3 would perform poorly in AC response.

I do like case 1 but DC on a presence pot is a deal breaker for me - I want to adjust knobs without having to worry about that. So I run with case 2.

I am an engineer by day and the theory of what is in practice today is somewhat divorced because not much is taught in college that draws the line all the way back to tube theory because it’s seen as irrelevant history instead of skills in a modern world. Instead there’s a lot of self teach involving the limitations of tubes and how to quantify modern theory into tube performance and characterization to design stages. Then there’s the sonic aspect of how something sounds related to theory.

The circuit I’ve created in my 74 from scratch is nothing like anything - it isn’t a copy of anything and is a morph of theory, trial, and error as to what sounds right and what doesn’t. If anything having the background helps a bit with debugging but honestly I believe tube amps are a level playing field for all.

What's the issue with having DC on the presence pot? Sure, it makes an audible sound when you adjust it. But you aren't doing that during a song so it shouldn't matter. It might be a safety thing but I wonder how many times that's been an issue since Marshall started making amps in 1962...

 

[FourT6and2]

Also found this nice explanation on Robinette's site:

"The 4.7k resistor found on many 5F6A Bassman amp presence pots cuts the total presence resistance to 4k (25k pot in parallel with 4.7k resistor). The added resistor also quickens the action of the presence control by changing the pot's taper from linear to slightly logarithmic. The reduced presence resistance also adds to the phase inverter's gain by reducing "tail resistance". More importantly, the 27k NFB resistor and 4K presence resistance form a voltage divider for the NFB signal so cutting the presence resistance by 20% also cuts the 5F6A's negative feedback by 20% which would thicken the clean tone and slow the already lazy transition from clean to dirty tone. This circuit change makes the difference in negative feedback levels between the 5F6A and the Marshall JTM45 even greater. The JTM45 has 2.8 times more negative feedback than the original 5F6A but has 3.4 times more NFB than a 5F6A with the 4.7k presence resistor across the 25k Presence pot."

 

[RedPlated]

If you want to see a totally unconventional presence circuit check out a Naylor. It’s the key to their sound. Very little bass in the preamp (.0022 coupling caps on both first two stages, .47uF and .1uF cathode bypass caps). 22uF cap on presence. All the low end comes in there.

 

[glpg80]

I fucked around with all kinds of presence caps and I prefer the stock value the most for a Marshall. The presence cap can definitely have a large impact on overall tone.

Only until after you fuck with a lot of the various stages in a Marshall do you realize just how well they tuned the original circuit sonically, hence how correct those who prefer a pedal out front can be for an intended easy goal.

 

[SpiderWars]

If you want to see a totally unconventional presence circuit check out a Naylor. It’s the key to their sound. Very little bass in the preamp (.0022 coupling caps on both first two stages, .47uF and .1uF cathode bypass caps). 22uF cap on presence. All the low end comes in there.

That looks like the NFB is thru a 0.0015uF cap and then there is that 0.005 cap straight to ground. Very different. Is it almost like a fixed Depth and super high Presence built-in?

 

[mcstinger]

Just for fun, parametrized presence capacitor. Presence and Depth pots are set up for somehow linear frequency response in midrange for 100n cap.

 

[RedPlated]

That looks like the NFB is thru a 0.0015uF cap and then there is that 0.005 cap straight to ground. Very different. Is it almost like a fixed Depth and super high Presence built-in?

It is a depth circuit and presence all in one essentially. But, it doesn’t actually bring in the highs like a Marshall until you get at least 3/4 of the way up on the pot. It’s really interesting the way it works.

I think the way the Naylor is designed was genius. They really thought outside the box. Zero harsh highs. And very tight bass as most of it comes from the NFB section.

 

[Racerxrated]

It is a depth circuit and presence all in one essentially. But, it doesn’t actually bring in the highs like a Marshall until you get at least 3/4 of the way up on the pot. It’s really interesting the way it works.

I think the way the Naylor is designed was genius. They really thought outside the box. Zero harsh highs. And very tight bass as most of it comes from the NFB section.

I have to Dime mine (and the 1995 SD I had) to even approach the highs on any Marshall. Presence Dimed, and treb/mids on 4-6 depending on speakers

 

[RedPlated]

I have to Dime mine (and the 1995 SD I had) to even approach the highs on any Marshall. Presence Dimed, and treb/mids on 4-6 depending on speakers

The first time I had a Naylor I thought something was wrong because I had the presence and treble so high.

I have a Marshall 1959 with a Naked mod. All Naylor preamp with stock Marshall power section, with exception of the presence and NFB which are also Naylor spec. The presence is currently on about 9. Really need to push it. Especially at lower volumes. The amp does open up quite a bit with some volume though.

 

[Racerxrated]

The first time I had a Naylor I thought something was wrong because I had the presence and treble so high.

I have a Marshall 1959 with a Naked mod. All
Naylor preamp with stock Marshall power section, with exception of the presence and NFB which are also Naylor spec. The presence is currently on about 9. Really need to push it. Especially at lower volumes. The amp does open up quite a bit with some volume though.

My SD60 (Dave King version 2022) reminds me of a darker, tighter boosted SLO with tons of lows. Or at least like a Marshall with a depth mod

 

[Code001]

I'm simple. This is how Bruce taught me how to do it, and it's basically what I do. Works well and no scratchy pots.

 

[RedPlated]

I really prefer a 5k pot vs the 25k with 4.7k resistor to ground. The taper is so much better. I've never understood the scratchy presence pot thing to be honest. If it doesn't make a scratchy sound while playing I don't see the concern.

I see he's cutting some highs in the NFB section as well with the 100pf over the 47k NFB resistor.

 

[Code001]

I'm the opposite. I never understood why someone would identify a noise, then leave it as-is without fixing it if it's a simple fix. You found a noise that occurs when you interact with the amp. You have identified the noise occurs because there is DC present. I'm assuming we all know the basics of how resistors and capacitors work, so it should be a relatively straightforward fix in this instance, but you'd choose to not fix it? I dunno, the concept seems wild to me.

 

[jasonP]

I'm the opposite. I never understood why someone would identify a noise, then leave it as-is without fixing it if it's a simple fix. You found a noise that occurs when you interact with the amp. You have identified the noise occurs because there is DC present. I'm assuming we all know the basics of how resistors and capacitors work, so it should be a relatively straightforward fix in this instance, but you'd choose to not fix it? I dunno, the concept seems wild to me.

http://forum.metropoulos.net/viewtopic.php?t=28214

Please see the post larry makes about using this cap to block dc on the pot in the feedback loop, it's pretty interesting.

RedPlated, I agree 100%.

 

[RedPlated]

I'm the opposite. I never understood why someone would identify a noise, then leave it as-is without fixing it if it's a simple fix. You found a noise that occurs when you interact with the amp. You have identified the noise occurs because there is DC present. I'm assuming we all know the basics of how resistors and capacitors work, so it should be a relatively straightforward fix in this instance, but you'd choose to not fix it? I dunno, the concept seems wild to me.

It's the fact that you're solving a ,"problem" by creating a worse problem.

If you prefer a weird, non-linear taper where the presence is boosted mostly on the last 30% or so of the pot's rotation, in order to remove DC noise not present when you're playing it, that's totally fine. I'd much rather have a consistently linear effect from the presence control. And I think this is the common consensus between most builders/ modders.

 

[Code001]

And I think this is the common consensus between most builders/ modders.

I am not sure what this means, exactly?

 

[glpg80]

I don’t want DC on my presence pot so I use a DC blocking cap. I’m able to get a tone a like with it which is all that matters. It would be different if the cap was the reason my tone sucked or limited my ability to dial something in that I like, which isn’t the case for me.

 

[RedPlated]

I am not sure what this means, exactly?

My point being that you think it's wild that the DC issue on the pot isn't, "corrected" by someone who knows how resistors and capacitors work.

Look at a lot of currently built amps. Wizard and Friedman, for example. They do not run a pot with a resistor to ground on the presence. You'll find that the 25k/4.7 combo isn't favored by a lot of people who build and mod amps these days. The taper just isn't desirable. I always remove the 25k/4.7k setup and replace with a 5k pot on mods that I do for customers.

 

[Code001]

Look at a lot of currently built amps. Wizard and Friedman, for example. They do not run a pot with a resistor to ground on the presence. You'll find that the 25k/4.7 combo isn't favored by a lot of people who build and mod amps these days. The taper just isn't desirable. I always remove the 25k/4.7k setup and replace with a 5k pot on mods that I do for customers.

I still don't get it. So because there are builders out there who do it that way, I am supposed to re-evaluate my opinion on DC on pots? Wild