Presence control on 5F4

[Ed_Chambley]

Is there any way to make the presence control be more responsive?  The fender layout shows the .0uF cap connected to the center and left of the 5k pot, the weber layout shows only left left lug connected to cap then to ground.  Would this make any difference?

[Boots Deville]

I don't know about making it more responsive, but the two methods of wiring the pot that you mention are both functionally equivalent ways of wiring a pot as a variable resistor.

[OldHouseScott]

Change the pot to a 5K reverse audio taper.

[Ed_Chambley]

Change the pot to a 5K reverse audio taper.

I would, but I do not know where to source one.  I can reverse the linear, but I do not understand what this would accomplish.

[jojokeo]

The feedback resistor going to the #1/right lug looking at the back of the pot creates a voltage divider w/ the cap being on the center/wiper and grounded on the far left/#3 lug of the pot which also has this lug grounded. You would do best by changing the feedback resistor feeding into the pot by lowering it a little bit. Another thing is increasing the cap value also but this doesn't make quite as much difference as the feedback resistor.

[Ed_Chambley]

The feedback resistor going to the #1/right lug looking at the back of the pot creates a voltage divider w/ the cap being on the center/wiper and grounded on the far left/#3 lug of the pot which also has this lug grounded. You would do best by changing the feedback resistor feeding into the pot by lowering it a little bit. Another thing is increasing the cap value also but this doesn't make quite as much difference as the feedback resistor.

There are 2 resistors with a common connection.  A 56k as well as 1.5k in series. Do you have a recommendation of which one and what value would you try first?  I am not sure which one would be considered the feedback resistor.

[stingray_65]

Hi Ed,

It may help a bit if you understand just how a presence control works.

Fist off, a presence control is a Negative FeedBack with a tone control.

The negative feedback (NFB) takes a sample signal from the speaker out (this signal is inverted compared to the incoming signal to the phase inverter) and inserts it to the cathode.

because the signal is inverted, it cancels out the incoming signal. since the signals arent exactly the same and are not phased (timed) exactly the signal isn't cancelled 100%, it just dampened or muted.

So when we add a tone circuit (the 5Kpot and .1 cap) we cut certain frequencies out of the sampled signal.

those missing frequencies (mid-range in this case) do not dampen the incoming signal and so those frequencies seem more pronounced.

SO:

Changing the capacitor value affects the tone, lowering the values passes higher frequencies to ground and the result will be a brighter sound.

changing the NFB resistor (56K) changes how much.


I'm not one of the Forum's Technical Gurus so I may have missed a few details, but I did experiment around with a NFB circuit on a small amp (5F2A) I built so I could understand this concept a lot better.

OH!

You can put a switch between the speaker connection and the 56K. this will remove NFB and defeat the presence. Effect is louder and less headroom.

You can sub a 20K resistor and a 25K A taper pot for the 56K. wire the center lug the speaker and one of the other lugs to the 20K, leave one of the outside lugs open.

OH!

I found these two links to be VERY useful,

http://www.diystompboxes.com/pedals/tubedummy.html

http://www.aikenamps.com/AmpTerms.html

I still refer to these places to make sure I'm on the same page when I get replies to questions I have.

Ray

[stingray_65]

ED,

You posted there is a 56K and 1.5K resistors in series.

Is the junction between those two resistor going to your PI tube? pin 3 or 8?

No connection means the presence control would not work.

[Ed_Chambley]

ED,

You posted there is a 56K and 1.5K resistors in series.

Is the junction between those two resistor going to your PI tube? pin 3 or 8?

No connection means the presence control would not work.

I have a connection from the presence lug 3 to the intersecting of the 1.5k and the 56k then continues on to pin 3 of the phase inverter.  It does "work" if you want to call it that, but it has VERY little effect.

[Ed_Chambley]

Hi Ed,

It may help a bit if you understand just how a presence control works.

Fist off, a presence control is a Negative FeedBack with a tone control.

The negative feedback (NFB) takes a sample signal from the speaker out (this signal is inverted compared to the incoming signal to the phase inverter) and inserts it to the cathode.

because the signal is inverted, it cancels out the incoming signal. since the signals arent exactly the same and are not phased (timed) exactly the signal isn't cancelled 100%, it just dampened or muted.

So when we add a tone circuit (the 5Kpot and .1 cap) we cut certain frequencies out of the sampled signal.

those missing frequencies (mid-range in this case) do not dampen the incoming signal and so those frequencies seem more pronounced.

SO:

Changing the capacitor value affects the tone, lowering the values passes higher frequencies to ground and the result will be a brighter sound.

changing the NFB resistor (56K) changes how much.


I'm not one of the Forum's Technical Gurus so I may have missed a few details, but I did experiment around with a NFB circuit on a small amp (5F2A) I built so I could understand this concept a lot better.

OH!

You can put a switch between the speaker connection and the 56K. this will remove NFB and defeat the presence. Effect is louder and less headroom.

You can sub a 20K resistor and a 25K A taper pot for the 56K. wire the center lug the speaker and one of the other lugs to the 20K, leave one of the outside lugs open.

OH!

I found these two links to be VERY useful,

http://www.diystompboxes.com/pedals/tubedummy.html

http://www.aikenamps.com/AmpTerms.html

I still refer to these places to make sure I'm on the same page when I get replies to questions I have.

Ray

Thanks Ray, I did have an idea of how presence works.  Your additional info is helpful.  The links you provided are bookmarked and I refer to them all the time.  I guess I can wire in a 50 k resistor and turn it and see if changing the resistor is going to give me something I may prefer.

Thanks you for your reply.