Can't Hear Presence Control. '73 Circuit Help (Really Probably Solved This TIme!)

[mickeydg5]

Yeah I hear ya. I made an order last night for some parts. 5k pot, 25k pot, new 4k7, a .1uF cap, just everything in case I need it. Unless there is some other mystery section of the circuit that could affect presence, that seems like all I can do. And I've been really hoping I didn't have to mess up these solder joints, but oh well. I'll try all those things out.

Do you know what my resistance should be across the .68uf cappacitor in circuit? Because I get:

4.34k presence at 10
10.2k presence at 0

But my other Synergry cap I ordered, out of circuit resistance reading, is overload. Is that standard?

I understand most do not understand electronics and how they work together.
You see, your meter works off of DC current to measure. The capacitor in the presence circuit is in-line and capacitors block DC. We had to short the capacitor to remove it's reactance for the meter to read the resistance properly.
So, the capacitor has reactance and interferes if not taken out of the circuit.
To get more in depth, your meter operates on a specific frequency according to its specifications when measuring which also affects the readings given in a certain circuit. That is getting too deep for most to understand.

 

[Mjh36]

I understand most do not understand electronics and how they work together.
You see, your meter works off of DC current to measure. The capacitor in the presence circuit is in-line and capacitors block DC. We had to short the capacitor to remove it's reactance for the meter to read the resistance properly.
So, the capacitor has reactance and interferes if not taken out of the circuit.

Ohm's Law huh, what??? Ha, so true, so much to learn electronically it can be overwhelming. I get some aspects but other stuff goes straight over my head.

Potentionally solved......

 

[Mjh36]

I understand most do not understand electronics and how they work together.
You see, your meter works off of DC current to measure. The capacitor in the presence circuit is in-line and capacitors block DC. We had to short the capacitor to remove it's reactance for the meter to read the resistance properly.
So, the capacitor has reactance and interferes if not taken out of the circuit.

Ok so I disconnected the NFB wire to the pot.
Connected it to a 5k6 resistor and my other .68uF cap, in parallel with each other, then to ground.
Basically a "fixed" presence circuit rather than a pot.
And I can hear it clearly go bright and dark, or loud to softer, as I disconnect the 5k6 resistor.

Question though I'm confused:

It sounds brighter with the resistor/capacitor I'm testing connected, so essentially 5k/.68uF on the presence circuit.
But my stock presence pot in the amp reads 5k when it's turned down, and almost zero ohms turned up.

It seems like one of these is working backwards? Should I hear more clarity with a higher or lower resistance value?

 

[mickeydg5]

Ok so I disconnected the NFB wire to the pot.
Connected it to a 5k6 resistor and my other .68uF cap, in parallel with each other, then to ground.
Basically a "fixed" presence circuit rather than a pot.
And I can hear it clearly go bright and dark, or loud to softer, as I disconnect the 5k6 resistor.

Honestly I need to see your connections to understand exactly what is being done.
It makes not much sense unless see the progressions.

 

[Mjh36]

Honestly I need to see your connections to understand exactly what is being done.
It makes not much sense unless see the progressions.

What about this? This kind of makes it a fixed presence circuit right?

And I can clearly hear it go darker when I unhook the 5k6, bright when I connect it again.

 

[mickeydg5]

What about this? This kind of makes it a fixed presence circuit right?

And I can clearly hear it go darker when I unhook the 5k6, bright when I connect it again.
View attachment 131680

Ok, I see what you are doing.
The better idea would be to place the capacitor and resistor in series, not parallel. Then disconnect.

 

[Mjh36]

Ok, I see what you are doing.
The better idea would be to place the capacitor and resistor in series, not parallel. Then disconnect.

Ok I did that too, sounds good, but it howls when I disconnect the resistor. And I don't know how I could switch resistor values quickly to tell a difference in tone.

The way in the picture it doesn't howl when disconnected. Maybe because the capacitor is always connected to ground. But it sounds like if I went from 10 to 0 on the presence if I take the resistor out.

But an actual real audible difference is happening. I'll try it tomorrow when I can play louder. Thanks for your help so far!

This also worked, the 5k6 + .68uF in series like you said, with another 5k6 in parallel. Noticeable tone shift in disconnecting the parallel resistor.

 

[XTRXTR]

Try two resistors in parallel to ground with the 'CAP in series on the NFB side' of the two, say a 10 ohm and a 5.6k ohm, that should properly mimic the presence circuit at 0 and 10. Leave the 5.6k in place with the cap in series and add or remove the 10 ohm across the 5.6k. Adding in the 10 ohm should be like the presence circuit at 10, removing it like presence at 0. Doesn't have to be 10 ohm but that is closer to the value you measured on the pot at 0. Anything small from 1 ohm to 100 ohms would be a similar effect. Mimic the pot at 5 using the other 5.6k you have.

The presence pot is usually wired to go toward least resistance CW 0-10 this allows more AC signal to pass through the cap toward ground and less to the PI. Less to the PI boosts frequencies not sent to ground via the presence pot - more presence of mids and highs are boosted and a bit more gnarl.

Cap value determines the frequencies sent to ground. Larger cap sends more lows to ground and also allows more low to mid as well as the highs boosted via lack of NFB at the PI. 680nF cap could be too big in that spot. Many of the Marshall drawings have a 100nF in that position. A 100nF allows more mid to high boost. This way you can turn down the presence if you want more low and and up for more mid and high boost. A 680nF may be allowing too much low to boost even when the pot is cranked to 10 or 0.

 

[mickeydg5]

Ok I did that too, sounds good, but it howls when I disconnect the resistor. And I don't know how I could switch resistor values quickly to tell a difference in tone.

The way in the picture it doesn't howl when disconnected. Maybe because the capacitor is always connected to ground. But it sounds like if I went from 10 to 0 on the presence if I take the resistor out.

But an actual real audible difference is happening. I'll try it tomorrow when I can play louder. Thanks for your help so far!

This also worked, the 5k6 + .68uF in series like you said, with another 5k6 in parallel. Noticeable tone shift in disconnecting the parallel resistor.
View attachment 131681

You mentioned the howl before when disconnecting the feedback loop.
Do you have all the volumes turned to zero?
Did that happen during any playing before?

 

[mickeydg5]

Try two resistors in parallel to ground with the 'CAP in series on the NFB side' of the two, say a 10 ohm and a 5.6k ohm, that should properly mimic the presence circuit at 0 and 10. Leave the 5.6k in place with the cap in series and add or remove the 10 ohm across the 5.6k. Adding in the 10 ohm should be like the presence circuit at 10, removing it like presence at 0. Doesn't have to be 10 ohm but that is closer to the value you measured on the pot at 0. Anything small from 1 ohm to 100 ohms would be a similar effect. Mimic the pot at 5 using the other 5.6k you have.

The presence pot is usually wired to go toward least resistance CW 0-10 this allows more AC signal to pass through the cap toward ground and less to the PI. Less to the PI boosts frequencies not sent to ground via the presence pot - more presence of mids and highs are boosted and a bit more gnarl.

Cap value determines the frequencies sent to ground. Larger cap sends more lows to ground and also allows more low to mid as well as the highs boosted via lack of NFB at the PI. 680nF cap could be too big in that spot. Many of the Marshall drawings have a 100nF in that position. A 100nF allows more mid to high boost. This way you can turn down the presence if you want more low and and up for more mid and high boost. A 680nF may be allowing too much low to boost even when the pot is cranked to 10 or 0.

All of this has been established already in the thread but you did explain and sum up quite nicely.

 

[Mjh36]

You mentioned the howl before when disconnecting the feedback loop.
Do you have all the volumes turned to zero?
Did that happen during any playing before?

Oh I got some results and graphics I'm trying to compile right now get ready for a big post!

 

[XTRXTR]

All of this has been established already in the thread but you did explain and sum up quite nicely.

Sometimes I can't see the tree for the forest

 

[Mjh36]

Cap value determines the frequencies sent to ground. Larger cap sends more lows to ground and also allows more low to mid as well as the highs boosted via lack of NFB at the PI. 680nF cap could be too big in that spot. Many of the Marshall drawings have a 100nF in that position. A 100nF allows more mid to high boost. This way you can turn down the presence if you want more low and and up for more mid and high boost. A 680nF may be allowing too much low to boost even when the pot is cranked to 10 or 0.

You mentioned the howl before when disconnecting the feedback loop.
Do you have all the volumes turned to zero?
Did that happen during any playing before?

Believe it or not, preliminary conclusions are that the stock presence circuit works, and is just too small of taper and/or frequency to hear gradually. This has been a main theme suggested and I think it might be the case!

Tests:



So I ran some "fixed" presence circuit tests. I put it on a switch to quickly hear the differences between theoretical "0" and "'10". I could hear the frequency shift with all these circuits above. They may seem redundant but I was trying all the variables. The shift wasn't huge but it was there. I almost feel that more resistance felt louder (and perhaps brighter?). That is the 5k6 in series to the .68uF. Switching in the parallel resistors seems to dampen the tone maybe ever so slightly.

Howl:

This gets good and I think solidified my understanding of what's going on. Mentioned earlier in the thread, was to prepare for some howl if you disconnect the NFB circuit. And it does do that. Remember, my master volume mod is set to almost max for the purposes of this presence diagnosis, as it seems to be the best chance of hearing the frequencies. Since lower master volume means less negative feedback, I want to simulate as close to stock as I can.

But @mickeydg5, I just found there's a threshold. I turned down my master volume about halfway, and there's no howl when disconnecting the NFB circuit to the stock presence pot. There's normal guitar signal, just darker. When I connect it again, jump in brightness. If I turn the master much past half, it's start squealing and howling.

With the lower master so I can hear regular guitar signal, it sounds exactly how I would expect having "no presence" to adding an upper midrange boost to the circuit when I connect and disconnect the wire. It's quite dramatic and definitely brighter. However, after the initial boost from connecting it, the adjustment is poor. I can barely hear it, which was the original problem. But I think it is there, now that I've had the ability to put fixed presence on a switch and hear an example of 0 to 10 back and forth.

So, I think the presence circuit is operational. I think by default it gives you that midrange boost that it's supposed to, but the combined 5k pot/wiring setup make for a near inaudible taper.

Chronological Order of Marshall Presence:

I'm not going to go back and look for this. But if I rub my last two bong-soaked brain cells together this is what I remember, it goes:

5k/.1uF -----> 5k/.68uF -----> 25k/.68uF -----> 5k/.1uF

In general, I think this was the evolution of the JMP presence control. Eventually settling on 5k/.1uF which seems the most popular today from all the clones and circuits I see. Along the way there seemed to be the omission of the .68uF on V2. Other forum posts I've read seem to state a .68uF V2 cathode bypass and .68uF presence cap is too much. My particular '73 is quite transitional. Brown turret board, but no .68uF V2, a 5k/.68uF presence, and a big bright cap. I believe they switched to the 25k pot for a better adjustment range. And if mine has the 5k pot, this might be the problem. It could very well be almost an always-on fixed presence unless you get the bigger pot.

 

[Mjh36]

I have parts on order from Valvestorm. I'll be able to try both a 25k/.68uF combo and .1uf on the stock presence here in a few days. I may end up liking that more if it is anything like the 5k/.1uF on the Ceriatone clone. But then there's the dilemma of whether that's worth changing the stock nature of this old amp or if I should let it be. Looks like $80 for a NOS .68uF if you can find one. I could just leave it so it's put to good use, since I'm now pretty certain it's not malfunctioning. I'll have to hear the difference when I get the parts in.

 

[dtier]

In you experiments you need to always have the 4k7 resistor, (or 5k pot,) connected between your purple wire and ground to complete the tail of the inverter stage to ground. The cap can only be in parallel to bypass AC signal around resistance. Otherwise you will source your DC bias current from the output trans and the 100k resistor.

I hope your still enjoying and not getting frustrated! The presence control on high gain amps like your Ceriatone are typically much more pronounced than an overdriven standard SL because most of the distortion is happening in the preamp, while the inverter and power amp and are typically running clean.

 

[FourT6and2]

Have you tried simply replacing the presence cap? I know you might not want to disturb the original solder joints, but really... it's not gonna hurt anything.

Also have you confirmed the pot is indeed 25K?

 

[mickeydg5]

Believe it or not, preliminary conclusions are that the stock presence circuit works, and is just too small of taper and/or frequency to hear gradually. This has been a main theme suggested and I think it might be the case!

Tests:

View attachment 131805 View attachment 131806 View attachment 131807 View attachment 131808 View attachment 131809

So I ran some "fixed" presence circuit tests. I put it on a switch to quickly hear the differences between theoretical "0" and "'10". I could hear the frequency shift with all these circuits above. They may seem redundant but I was trying all the variables. The shift wasn't huge but it was there. I almost feel that more resistance felt louder (and perhaps brighter?). That is the 5k6 in series to the .68uF. Switching in the parallel resistors seems to dampen the tone maybe ever so slightly.

Howl:

This gets good and I think solidified my understanding of what's going on. Mentioned earlier in the thread, was to prepare for some howl if you disconnect the NFB circuit. And it does do that. Remember, my master volume mod is set to almost max for the purposes of this presence diagnosis, as it seems to be the best chance of hearing the frequencies. Since lower master volume means less negative feedback, I want to simulate as close to stock as I can.

But @mickeydg5, I just found there's a threshold. I turned down my master volume about halfway, and there's no howl when disconnecting the NFB circuit to the stock presence pot. There's normal guitar signal, just darker. When I connect it again, jump in brightness. If I turn the master much past half, it's start squealing and howling.

With the lower master so I can hear regular guitar signal, it sounds exactly how I would expect having "no presence" to adding an upper midrange boost to the circuit when I connect and disconnect the wire. It's quite dramatic and definitely brighter. However, after the initial boost from connecting it, the adjustment is poor. I can barely hear it, which was the original problem. But I think it is there, now that I've had the ability to put fixed presence on a switch and hear an example of 0 to 10 back and forth.

So, I think the presence circuit is operational. I think by default it gives you that midrange boost that it's supposed to, but the combined 5k pot/wiring setup make for a near inaudible taper.

Chronological Order of Marshall Presence:

I'm not going to go back and look for this. But if I rub my last two bong-soaked brain cells together this is what I remember, it goes:

5k/.1uF -----> 5k/.68uF -----> 25k/.68uF -----> 5k/.1uF

In general, I think this was the evolution of the JMP presence control. Eventually settling on 5k/.1uF which seems the most popular today from all the clones and circuits I see. Along the way there seemed to be the omission of the .68uF on V2. Other forum posts I've read seem to state a .68uF V2 cathode bypass and .68uF presence cap is too much. My particular '73 is quite transitional. Brown turret board, but no .68uF V2, a 5k/.68uF presence, and a big bright cap. I believe they switched to the 25k pot for a better adjustment range. And if mine has the 5k pot, this might be the problem. It could very well be almost an always-on fixed presence unless you get the bigger pot.

You did not tell me anything that I do not already know but your conclusions are mostly correct.
Yes the circuit is operational but far less than desirable.
No the taper is not in question and most PRESENCE potentiometers have "B" (linear) tapers.
Also a larger potentiometer on drives the shelf of the filter circuit down in frequencies, to those not available to guitar.

Yes, that is why I asked about volume control placement and also to mention with a NMV the Treble/Middle/Bass controls matter as well.

All you need to understand is that almost all PRESENCE circuits are based on the .1uF and 25k potentiometer nowadays. I am sure there is a reason for this.

Howl can also be caused by reverse push-pull connection to the power tubes. The connections can be finicky with relation to the output transformer winding/construction.

 

[Mjh36]

In you experiments you need to always have the 4k7 resistor, (or 5k pot,) connected between your purple wire and ground to complete the tail of the inverter stage to ground. The cap can only be in parallel to bypass AC signal around resistance. Otherwise you will source your DC bias current from the output trans and the 100k resistor.

I hope your still enjoying and not getting frustrated! The presence control on high gain amps like your Ceriatone are typically much more pronounced than an overdriven standard SL because most of the distortion is happening in the preamp, while the inverter and power amp and are typically running clean.

Thank you for those explanations, I hope I didn't do anything to damage the amp while I had it hooked up that way. Would this be a closer way to wire a fixed presence on a switch, from about 0 to 10? This had about the same result as the others sound-wise. I could hear a slight shift switching it but it's subtle.

 

[mickeydg5]

I have parts on order from Valvestorm. I'll be able to try both a 25k/.68uF combo and .1uf on the stock presence here in a few days. I may end up liking that more if it is anything like the 5k/.1uF on the Ceriatone clone. But then there's the dilemma of whether that's worth changing the stock nature of this old amp or if I should let it be. Looks like $80 for a NOS .68uF if you can find one. I could just leave it so it's put to good use, since I'm now pretty certain it's not malfunctioning. I'll have to hear the difference when I get the parts in.

The 5k/.1uF is straight from the Fender designs. Marshall copied that one first then started experimenting.

 

[Mjh36]

Have you tried simply replacing the presence cap? I know you might not want to disturb the original solder joints, but really... it's not gonna hurt anything.

Also have you confirmed the pot is indeed 25K?

I'll test some new values in a couple days when I get the parts. If it sounds a lot better I'll change it but the amp always sounded great. As far as I know we did some in circuit measurements of the pot and I think it's 5k.