Mark wrote: ↑Fri Apr 14, 2017 11:26 pm
Basically what is happening is, negative feedback from the secondary of the output transformer is applied to the P.I. this reduces the gain of the P.I. stage, by earthing out a particular frequency this frequency is boosted. The cap chooses the frequencies with the resistor provides a path to earth of lower resistance than the 4.7K resistor. For this reason the pot has to be turned up quite a bit before it is lower than 4.7K.
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I thought the presence control with 4k7 to ground would have only a AC coupled cap/pot presence network. As Pompeii mentioned its a RC filter? So I'm trying to understand why the range of pot doesn't start being noticeable until it reaches 4k7??
When the presence control is turned to 10 or max what is the resistance the pot has 5k or 0ohms??
When the pot is on ten, the resistance is "0".
When the pot is on 25K the easiest path to earth is the 4.7K resistor.
Mark wrote: ↑Fri Apr 14, 2017 11:26 pm
Basically what is happening is, negative feedback from the secondary of the output transformer is applied to the P.I. this reduces the gain of the P.I. stage, by earthing out a particular frequency this frequency is boosted. The cap chooses the frequencies with the resistor provides a path to earth of lower resistance than the 4.7K resistor. For this reason the pot has to be turned up quite a bit before it is lower than 4.7K.
Sent from my iPhone using Tapatalk
I thought the presence control with 4k7 to ground would have only a AC coupled cap/pot presence network. As Pompeii mentioned its a RC filter? So I'm trying to understand why the range of pot doesn't start being noticeable until it reaches 4k7??
When the presence control is turned to 10 or max what is the resistance the pot has 5k or 0ohms??
When the pot is on ten, the resistance is "0".
When the pot is on 25K the easiest path to earth is the 4.7K resistor.
Sent from my iPhone using Tapatalk
Boom, you just explained it in a way that made perfect sense! Thanks. Path of least resistance.
Mark wrote: ↑Sat Apr 15, 2017 1:09 am
When the pot is on ten, the resistance is "0".
When the pot is on 25K the easiest path to earth is the 4.7K resistor.
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Ok so the AC signals avoid the 25k pot until its set a least resistive setting.
Makes you wonder why Marshall or who ever used a 25k pot then? If it was useless until the last portion of the pot.
Perhaps since they already bought 25k pots for the Middle control it was just easier to buy more 25ks.
Hi Robin, I'm not sure why 25K was chosen for the presence control. As you say it could have been because they were using the same value as a mid pot or perhaps because the 6G Fender amps used 25K. I'm not sure why Fender used them though I had a Boogie 1 that had a 10K presence pot to get a better range.
Once again we are back to pot tapers and effective adjustment throughout the travel of the pot.
Dave, the "ground the 1 log of the pot to its own bolt " is not grounded because the 4k7 is the new ground.
Simply attach the .1uf to both wiper and far left terminal in series and leave the other end right where it is on the buss bar.
I prefer the early scratchy pot version with the 5k pot as the PI tail and the 0.1uF from the pot wiper to ground (5F6-A circuit). The modern circuit is essentially identical when the presence is rotated full CW, but the effective range of rotation is larger and the character of the effect at partly-on settings is more pleasing to my ear with the earlier version. The modern circuit doesn't take the cap completely out of the circuit at full CCW either.
