Difference in schematic of classic Fender circuit

[sonolink]

Hi guys,
I'm building this circuit in a pedal, so I'd like to be sure that the pots aren't backwards. The circuit is the classic AB763. I came across 2 copies and each draw the pots in opposite directions (see the little arrow that is supposed to mark CW direction, AFAIK). Any ideas on what is the correct orientation of pins 1 and 3 in the circuit please?








Thanks a lot for your help!
Sono

 

[KIDVOX]

i hope tis helps a little. its just the standard BMT tone stack used by fender, vox , marshall. heres a little photo from a book i have. there may be moreinfo on the web.

the arrows on the schematcs show the direction of the wipper to cut said band. the photo is hw the pots look from the rear

 

[KIDVOX]

remember to take all earth wires to one single point. (star earth ) this will redcuce the chance of an earth loop ( hummmm

 

[sonolink]

Thanks for your reply Kidvox. I know it's the standard Fender tone stack. The problem is that in one schem pins 1 and 3 of all the pots are wired the opposite way than the other schem. If I'm not wrong, wiring them one way or the other will result the pots working CW or CCW. I'm trying to find out which way is the correct to avoid having the pots working backwards when I order the PCB, and that's why I'm asking for help.

 

[KIDVOX]

remember to take all earth wires to one single point. (star earth ) this will redcuce the chance of an earth loop ( hummm) marshall use a 820ohm resistor in parrallel with a 25 mfd cap at R14 position a bit more chrunch

 

[metalockpick]

The top one is correct if the markings refer to counterclockwise. That high cut switch boosts the upper mid-range, it doesn't actually cut the high end.

 

[KIDVOX]

i always ave trouble posting on this forum my phone keeps editig my text, dropping od letters or words, soetimes whole sentances dissapear while im typing.

 

[sonolink]

The top one is correct if the markings refer to counterclockwise.

Ok, so if I want the controls to work properly (i.e. turning the pot CCW equals to less Gain or less Volume, or less whatever) then I should follow the second schematic, correct?

That high cut switch boosts the upper mid-range, it doesn't actually cut the high end.

How do you mean?


Thanks a lot for your help!
Cheers
Sono

 

[metalockpick]

The switch and added capacitor set the high pass filter point of the treble control. Making the capacitor larger lowers that frequency. Fender over the years has used values from 150 PF to 500 PF in this circuit. Many people would say the sound is " brighter", and that's because there's more energy going through the treble control, due to more upper mid-range frequency energy.

Pots in schematics typically are marked to indicate which end of the pot is connected at full clockwise (or counterclockwise) to the center wiper. Think of it this way: If the treble capacitor is direct connected to the wiper, from the wiper being all the way up, that would be full clockwise. Wire it that way. Once you've got that, follow suit with the other two.

 

[MagnetoSound]

The top one. Arrow points in clockwise direction.

Pin 1 is bottom, pin 2 = wiper, pin 3 = fully clockwise end.

 

[sonolink]

The switch and added capacitor set the high pass filter point of the treble control. Making the capacitor larger lowers that frequency. Fender over the years has used values from 150 PF to 500 PF in this circuit. Many people would say the sound is " brighter", and that's because there's more energy going through the treble control, due to more upper mid-range frequency energy.

So if I understand this correctly, since the switch makes the cap bigger or smaller (if it's closed it adds another cap in parallel increasing the cap value, and if it's open it reduces that cap value) and since a bigger value lowers the frequency of the HP filter of the Treble pot, if, let's say the pot is pretty high, when the switch is closed you should notice a "High Cut", but if the pot is kinda low you wouldn't really notice a big change. So the "High Cut" switch is dependant on the Treble pot control. Is that right?

All things considered, if I have a good enough Treble control, do I really need that switch? Maybe that's why the second schematic doesn't have the switch?

 

[sonolink]

The top one is correct if the markings refer to counterclockwise.

The top one. Arrow points in clockwise direction.

Pin 1 is bottom, pin 2 = wiper, pin 3 = fully clockwise end.

Contradictory views? Who is right?

 

[MagnetoSound]

Take your pick

Look at the GAIN control. Would you say turning this control clockwise would be turning it up or turning it down?

 

[sonolink]

Take your pick

Look at the GAIN control. Would you say turning this control clockwise would be turning it up or turning it down?

Up, because the wiper will be closer to the input pin (pin 3) so there will be less resistance on that side......??

 

[sonolink]

Up, because the wiper will be closer to the input pin (pin 3) so there will be less resistance on that side......??

Thanks Magneto

 

[metalockpick]

Damn... I've gotta not do this on my phone when I'm half asleep

 

[sonolink]

Damn... I've gotta not do this on my phone when I'm half asleep

No probs metalockpick
But please do answer my previous question. I'm always eager to learn

So if I understand this correctly, since the switch makes the cap bigger or smaller (if it's closed it adds another cap in parallel increasing the cap value, and if it's open it reduces that cap value) and since a bigger value lowers the frequency of the HP filter of the Treble pot, if, let's say the pot is pretty high, when the switch is closed you should notice a "High Cut", but if the pot is kinda low you wouldn't really notice a big change. So the "High Cut" switch is dependant on the Treble pot control. Is that right?

All things considered, if I have a good enough Treble control, do I really need that switch? Maybe that's why the second schematic doesn't have the switch?

Thanks for your time and help
Cheers
Sono

 

[merlin]

It depends how you mount your pots and knobs. With the silkscreen written around the 'top' then you want clockwise to give you 'more' . With the silkscreen written around the 'bottom' then you want clockwise to give you 'less'.

Wiper slides 'upwards' on circuit diagram for 'more'.

 

[Philbo King]

Merlin has a great point. Expanding on it a bit, it would be simplest to use discrete wires to connect the pots to the PCB. That would add ruggedness to the unit as pots soldered directly to a PCB are a frequent failure point (solder joints crack, etc.). It would also allow to to easily change connections on any pot, making the OP problem moot.

 

[Matador]

It depends how you mount your pots and knobs. With the silkscreen written around the 'top' then you want clockwise to give you 'more' . With the silkscreen written around the 'bottom' then you want clockwise to give you 'less'.

Wiper slides 'upwards' on circuit diagram for 'more'.

I was just going to post this.

For chassis that are 'upside' down (like in a combo), meaning the pot legs face down when installed, then you get one rotation direction. For 'right-side up' config (like a Marshall head) then it will all be the opposite. I'm guessing the two schematic variations might be due to this difference.