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

[FourT6and2]

Thanks for helping me over the desoldering hump. I was just still in a honeymoon phase with this thing, and I'm the king of screw-ups so I didn't want to test my luck. Now that they are desoldered I'll have no problem trying different arrangements out. Parts come in tomorrow, so I can try the .1uF and/or 25k. But I would rather leave the pot in there, less hassle.

I would first try to simply use the existing 0.68uF cap and just rewire it in parallel. But yeah, I agree with you about leaving the original pot in there. The cap is easy enough to swap in/out.

Here are some links, all 1 page short reads, that really got me going down this path, worth a quick glance. Someone else in there mentions this current presence circuit that I have.

Vintage Presance Control Differances - The Amp Garage

Marshall guys- .68uf presence cap? | The Gear Page

1973 Marshall Super Lead 100 Watter Handwired | Rig-Talk

Yup. Those threads confirm what I've been saying. Marshall tried a few different things over the years. Not all of them were good ideas lol... And some of them were actual mistakes (like an employee grabbing a 5K pot when they should have grabbed a 22K or 25K).

 

[Pete Farrington]

I think it’s reasonable to assume the 5k pot wired as a variable resistor in series with the presence cap was a design choice, as it’s shown on a few schematics.
I prefer it to the 22k in series, I find the effective taper is better, as it allows a finer degree of control.

 

[XTRXTR]

I think it’s reasonable to assume the 5k pot wired as a variable resistor in series with the presence cap was a design choice, as it’s shown on a few schematics.
I prefer it to the 22k in series, I find the effective taper is better, as it allows a finer degree of control.

Now y'all got me curious. I'll have to test out the differences. Without a shunt we are changing the DC bias on the PI aren't we? If that is so then the presence pot is not only changing the NFB but also the gain of the PI dynamically with the NFB. That has to have a different tonal soundscape compared to what I have on my JCM 800 based 2204s (22k in series with a 100nF and parallel to the 4.7k)

I've been using one of them as a test amp anyway, might as well put this into the mix of things to test the voice of my amp. Typically I keep it at 0 so I may find a better use.

 

[Pete Farrington]

See https://el34world.com/charts/Schema...rshall_2204_jmp_master_volume_lead_50watt.gif

This ampgarage thread has some great simulations from user roberto https://ampgarage.com/forum/viewtopic.php?f=6&t=35795&p=446027

 

[mickeydg5]

I'm not sure you understand what I'm saying. OP's amp looks like someone could have combined two different presence circuits by accident.

There are three different versions I've seen Marshall use:

1. The original method: 5K pot, cap wired in parallel, no shunt resistor — scratchy pot.
2. Newer method: 25K pot, cap wired in series, 4K7 resistor — no pot scratchiness.
3. More rare version (probably a mistake): 5K pot, cap wired in parallel, WITH 4K7 resistor.

However, OP's amp looks to be an erroneous combination of 5K pot, cap in SERIES, 4K7 resistor

So... either the cap is wired wrong and needs to be changed over to parallel config., or the pot is the wrong value and needs to be switched out for a 25K. Someone wired the amp wrong.

What OP's amplifier has is exactly as shown in the schematic that was posted.

 

[FourT6and2]

See https://el34world.com/charts/Schema...rshall_2204_jmp_master_volume_lead_50watt.gif

This ampgarage thread has some great simulations from user roberto https://ampgarage.com/forum/viewtopic.php?f=6&t=35795&p=446027

Look who started that thread It's because I noticed the different arrangements and how some work better than the others. I also started a similar thread here and on RT.

 

[FourT6and2]

What OP's amplifier has is exactly as shown in the schematic that was posted.

That doesn't mean it's right. Or ideal. OP is experiencing a problem with his amp: Presence control doesn't seem to do much. Just because Marshall did it, doesn't mean it was a good idea. Plenty of amp companies have done weird things over the years.

 

[FourT6and2]

I think it’s reasonable to assume the 5k pot wired as a variable resistor in series with the presence cap was a design choice, as it’s shown on a few schematics.
I prefer it to the 22k in series, I find the effective taper is better, as it allows a finer degree of control.

The range of that pot as it is, is pretty small. That's why OP says his presence control does nothing. Maybe it was a design choice. Maybe it wasn't. Either way, gotta try all 4 of the arrangements to see what works for you and what doesn't.

Personally, my favorite of the bunch is 5K pot, cap in parallel, no shunt resistor.

 

[mickeydg5]

Well count me in as confused. I both get a lot of what is being said, as well as some stuff going completely over my head. I'm tryin' though. One thing I have to get straight is this frequency shift or Q or RC shelf and how that works first.

For example, I've always understood in tinkering pedals that you can shift the EQ by changing a combo of a resistor and capacitor in parallel. This is how I've always understood it:

View attachment 131923

So next, I created a set of "fixed resistor" presence circuits that I think are wired like mine.
First is my stock presence how I think it is currently.
The next three are examples if fixed resistors were used for 0%, 50%, and 100% potentiometer values.
Forget about the 4k7 shunt for now. Basically, I'm wondering, is this a correct visualization of what the potentiometer is doing?

Is it about series resistance before the capacitor that's changing the frequency? Basically going from a range of 100k --> 105k if you add in the NFB and pot values?

View attachment 131928 View attachment 131924 View attachment 131925 View attachment 131926

The 4.7k is the load in this scenario.
The PRESENCE circuit is the 5k pot and .68uF. That is a variable series resonant/low pass RC filter in shunt with the load. The 5k pot in this circuit is the major cause of frequency blocking.
1) with presence at zero the filter resonates at 47Hz passing only low frequency due to the 5k resistance.
2) with presence at halfway the filter resonates at 94Hz passing only low frequency due to the 2.5k resistance.
3) with presence almost max the filter resonates at 11,708Hz passing lower frequency due to 20R (ohms) resistance.

But note that at just 100R (ohms) the frequency cutoff shelf is 2,341Hz and at 500R (ohms) it is 468Hz.

In comparison, using a .1uF capacitor with the pot maxed getting 20R (ohms) the frequency cutoff would be 79,618Hz.

I think the .68uF makes for a much darker circuit in this case especially with the feedback resistor at 100k blocking a lot of the feedback signal while the connection to the 4-ohm tap of the output transformer provides the least amount of feedback.

 

[mickeydg5]

The range of that pot as it is, is pretty small. That's why OP says his presence control does nothing. Maybe it was a design choice. Maybe it wasn't. Either way, gotta try all 4 of the arrangements to see what works for you and what doesn't.

Personally, my favorite of the bunch is 5K pot, cap in parallel, no shunt resistor.

That scenario is the original design as used by Fender.
The problems with this design are the potentiometer has DC running through it which may cause scratchiness and noise and the failure of the potentiometer goes up. So the more modern designs address these issues.

 

[FleshOnGear]

That scenario is the original design as used by Fender.
The problems with this design are the potentiometer has DC running through it which may cause scratchiness and noise and the failure of the potentiometer goes up. So the more modern designs address these issues.

Yeah, on my ‘72 the presence pot had been replaced by a sealed PEC pot. I assume because the original had developed problems. My 2555SL has the 22k pot in series with a 0.1uF cap, and a 4k7 shunt. To me both circuits work and sound fine, but I don’t adjust the presence often - I tend to keep it on 10.

 

[Pete Farrington]

1) with presence at zero the filter resonates at 47Hz passing only low frequency due to the 5k resistance….

Please have a look at the simulations in the ampgarage thread. I get the impression you’re looking at this all upside down, there’s no resonating going on at low presence settings.

 

[FourT6and2]

That scenario is the original design as used by Fender.
The problems with this design are the potentiometer has DC running through it which may cause scratchiness and noise and the failure of the potentiometer goes up. So the more modern designs address these issues.

I don't see it as a real problem. I don't know many people who twist the presence control in the middle of a song and decent potentiometers don't fail that often these days.

 

[mickeydg5]

Please have a look at the simulations in the ampgarage thread. I get the impression you’re looking at this all upside down, there’s no resonating going on at low presence settings.

I do not think so.
Please link your favorite ampgarage simulation pages.

 

[XTRXTR]

Well, hearing is believing. I'll test all logical versions. Shunt, no shunt, cap series, cap parallel, 5k pot, 25k pot, 100nF, 680nF.

 

[FourT6and2]

I do not think so.
Please link your favorite ampgarage simulation pages.

They're in the previous linked thread.

 

[Frank Araneo]

I do not think so.
Please link your favorite ampgarage simulation pages.

I think this is the link:

Presence Circuit and Shunt Resistor - The Amp Garage

Loving this thread! Thanks to all who contribute, and good luck to the OP.

 

[Pete Farrington]

Forget about the 4k7 shunt for now. Basically, I'm wondering, is this a correct visualization of what the potentiometer is doing?

Is it about series resistance before the capacitor that's changing the frequency? Basically going from a range of 100k --> 105k if you add in the NFB and pot values?

Sorry that your query was kinda overlooked.
The feedback network is a potential divider. The input to the potential divider is a signal from the OT secondary, eg at the 4 ohm tap.
The feedback network consists of a series resistor, 100k in this case, and a shunt resistor, 4k7.
In parallel with the shunt is the presence cap and the variable resistance of the presence pot.
The output of the feedback network goes, via a 0.1uF coupling cap, to the grid of the triode of long tail pair that’s operating in common grid mode.
The signal at that grid is the (main) negative feedback signal.
If the negative feedback signal is 0, eg presence on full and the test frequency is high, the gain of the power amp will be its open loop gain.
As the presence control setting is reduced, as the frequency is lowered, the power amp gain will reduced to its closed loop level, ie we can ignore any further shunt effect across the 4k7 feedback shunt resistor due to the presence cap / control.

With respect to the closed loop gain, with presence on max, the 3dB lower corner frequency of the presence boost will be roughly about the frequency at which the presence cap reactive impedance = 4k7.

So about 340Hz for a 100nF cap, 50Hz for 680nF.

See the simulations for further detail.

 

[mickeydg5]

Please have a look at the simulations in the ampgarage thread. I get the impression you’re looking at this all upside down, there’s no resonating going on at low presence settings.

I am glad you feel a need to call me wrong because of some thread on another platform but I am still failing to understand your logic as to how I am backwards with regard to this thread on another platform which in my opinion does not speak of much matter.

Please point out what construes my upside down thinking and then expound upon it.

 

[Mjh36]

Sorry for the delay on getting back to some results. Got the parts in and well... I don't know what to say but I'm hearing a whole lot of "not much" with any combination of presence circuits.
I rigged it up to try a each pot with both caps on a switch. Both old school with DC on the pot and the other way with the shunt resistor.

No shunt, old school way
5k / .1uF
5k / .68uF
25k / .1uF
25k / .68uF

With shunt and pot going in series to capacitor
5k / .1uF
5k / .68uF
25k / .1uF
25k/ .68uF
Some main points I took:

I hear almost no difference in switching the caps. Something is happening but not much. Blindfolded I couldn't tell what pot/cap arrangement I'm using.
Never heard any scratchy DC when turning the pots when wired to do so.
Hardly any change in tone from 0 to 10.
In fact, the background hiss is slightly louder at 0. Adding to that, I would say there's almost a touch more "hairiness" when the presence is at 0.
When using the 25k / no shunt, there's loud squeal and oscillation when turning it down towards 0 on the knob.

I would hit chords, then go and turn the pot and hit the capacitor switch. Tried all combinations. Low master volume, High master volume. With and without an attenuator. I don't know what to say.
This was my setup, I triple checked all wiring. Just really nothing, besides the initial "boost" of connecting the NFB in general. No dark to bright -- 0 to 10 -- at all.

Could there be anything else at all circuit-wise that's impeding the presence circuit from working correctly? I circled an area in the schematic, the phase inverter stuff that's connected.