Quick sanity check re presence control

[Fred G.]

A quick and dumb question:

I'm embarrassed to ask this, but I need a sanity check.

In the schematic on the Ampage of the 50 watt Marshall, having the "presence" control turned all the way to "0" would be the rough equivalent of removing it completely from the circuit, correct? In other words, if one physically removed the presence pot and cap, it would be the "loose" equivalent of turning the control to zero, right?

Thanks and sorry for the dumb-assed obvious question!

Fred G.

[Enzo]

What schematic, where? What model number is in the lower right of the drawing?

I am not aware of a control that goes to open at the end of its travel. At least not on purpose. More likely it becomes a short - a piece of wire. But I wait to see what you refer to before proclaiming anything.

[Fred G.]

Enzo,

The schematic is right here - on the Ampage under the references link.

No, the Marshall "presence" control certainly doesn't go to "open" at the end of it's travel. I'm pretty certain that when turned all the way to "0" that the presence control has no effect - i.e., the feedback and shunt resistor values are in effect by themselves (did I word that correctly?).

[carl gigun]

I think you mean this one? http://ampage.org/schematics/mpwr50.gif

That area of the old Ampage site hasn't been active for many years.

The cap is trying to pass the higher frequency parts of the feedback signal to ground. The pot is controlling how easy it is for the cap to do it.

The presence pot is wired as a variable resistor. Turned down to zero it's a 25k resistor and the cap is pretty well blocked from ground. As you turn the presence up, that path to ground gets better and better. At ten it's basically a short from the cap to ground.

It's important to note that the feedback signal is negative or destructive feedback. Removing highs from the feedback signal means the feedback will cancel LESS highs from the regular signal. So the more highs the cap sends to ground, the more highs you'll hear coming from the speaker.

Maybe you're asking this question because you'd like to remove the presence control. You can do that but the amp would be pretty dark. You can boost the treble earlier in the circuit to compensate, but it doesn't have exactly the same effect.

[Fred G.]

Thanks, Carl - this is exactly what I wanted to know.

Actually, on my 50W JCM800, I usually leave the presence on 0 - 2. I'm wanting to approximate this on a Deluxe Reverb clone which has a post-pi MV and is a tad bit too bright - I want to Marshallize it somewhat, but I don't have the space for a presence control.

[Matt T.]

Fred,
Remember that the post-PI MV is inside the negative feedback loop. When you turn a post-PI MV down really low...the Presence control doesn't even work because there's so little signal being fedback and the 'incoming' signal to the PI (from the preamp) is still the same, super hot signal.

One thing you might try is a 'Cut' control. It's super easy since you already have a post-PI MV. Look at Vox and Matchless schematics and locate the 'Cut' control (it's after the PI). Then think about how you could just 'tap off' of the MV wiring to do your Cut control. Ooops, I just remembered you don't have room for a pot...oh well...nevermind.

In any case...lots of ways to tame brightness. First thing to consider IMO is the speaker though. Also, is the bright cap permanently soldered to the volume pot like a Deluxe Reverb? IIRC, usually the Vibrato channel has it and the Normal channel doesn't.

[carl gigun]

Adding a presence control to your circuit can only make it brighter unfortunately. The cut control is a good suggestion. They're nice because they come so late in the chain, I find they can knock off a lot of fizz or harshness that comes from earlier clipping. If you don't have space for another control on the panel you could find out what values you like with a temporary pot+cap then install a fixed resistor+cap.

If you play with the presence pretty low on your marshall you probably like a clearer, or smoother sound. You should be able to get the same amount of brightness but a different feel by turning down the treble control and turning up the presence. I don't have a marshall, but on other amps I have this gives a fuzzier, less controlled sound that suits some kinds of music.