I came across this recently: "Early Champs and Princeton often use a single knob tone circuit. The mid scoop was coming from the speaker. Jensen speakers had their typical response which lent to the early fender sound. When they changed over to speakers which were flatter in the mids so the tone controls were redesigned to scoop out more middle. So if you have an Eminence speaker with a flatter mid, then a later design tone stack may be more appropriate.". Does that mean you have to have a "mid" control to get the "scoop"? And are bass & treble controls merely bass-cut and treble-cut circuits, so if you max both of them you have the same signal that entered the tone stack? I would like to have tone controls that "boosted" the bass & treble (not cut). Do I have to go "active" with OP amps and such? I've seen amps with a "mid" control, and also a "center freq" knob. This might do it.... But how?
All passive tone controls are cut only. If you want boost, you need an active circuit. But why worry, if you want boost, just boost everything and cut away that which you don't want. And that is the conventional tone stack.
You might find a Fender amp with only bass and treble controls, but the mids is still there in the form of a resistor, or place a resistor could be, at least.
Once you see center freq knobs, you have left the world of simple tone stack. Go over to Duncanamps.com and download the tone stack calculator - TSC - and play with it.
You might find a Fender amp with only bass and treble controls, but the mids is still there in the form of a resistor, or place a resistor could be, at least.
Once you see center freq knobs, you have left the world of simple tone stack. Go over to Duncanamps.com and download the tone stack calculator - TSC - and play with it.
Enzo - thanks! You've confirmed a lot of what I think I know. I recently downloaded Duncan's PSU designer, and will definitely play with the TSC. Question about a Mission Amps mod to their "Tweedy Deluxe": what does changing the cathode cap on the 2nd gain stage (in the PI tube) from 25uF to 0.68uF do to the tone? Thanks again!
If you look at something like the Super Reverb , as Enzo alluded to, there is a 6.8K resistor at the bottom of the tone stack for the Normal channel. Compare that with the Vibrato channel, which has a 10K pot. So the Normal channel sets the mid somewhere in the middle of the range. Adjust that resistor in a dual control tone stack to set your desired mid cut.
JJ - I agree. I looked at some response curves, seems "scooped" may mean more of a flattened-out mid freq hump. Irregardless, if that's part of what a good tone sound like, I plan on putting a 3-pot stack on my deluxe. Now to decide the cap and pot values (have lots to choose from..). Will help temper the gain I get from 3 stages of preamp!
You've got me intrigued.... one pot that's an adjustable bandpass filter. Hmmm, I'll have to go thru all the info I have on passive filters. A problem I see is, unlike high or low-pass filters that you can run "out of the way" high or low, a bandpass will always be there somewhere in the range of frequencies. Unless you can adjust its "emphasis" as well. Any way you can post a simple schematic? Thanks! Jeff
You said you have "Champs/Princeton" amp but never said what model #, there are a half a dozen of each. For instance; the "Princeton 5E2" has the type of control I'm talking about. The problem with this though is at center position it has a -3dB cut on both high and low frequencies where mid should be. This is due to the 2 cap values
.05uF (low) .005uF (high). A simple trick is to change one of them to a value that will Xover the other.
.05uF (low) .005uF (high). A simple trick is to change one of them to a value that will Xover the other.
Hi LA,
Maybe I should have been more specific when I made that statement you had quoted.
In early Fender designs they were using Jensen speaker, mated to amps with only small amounts of NFB (that is whole other subject). Running an amp this way with low damping factor, the speaker response is quite emphasized. More NFB (higher damping factor) would tends to flatten out the overall frequency response.
Have a look at vintage alnico or early ceramic Jensens. There is a lower frequency bump where the resonant peak is, and a rising bump at about 2KHz. In between those points the response has got some ''scoop'' or droop to it.
It is just my conjecture, but once the power increased in the Brownface and Blackface designs, they are push-pull outputs with more NFB and using more powerful speakers which had a flatter mid (500 to 2K) response. If you run Duncan tone stack, and enter stock values for the Treb Mid Bass. Try it for Brownface with a fixed resistor mid (treb and bass control pots only). You see that inherent scoop? That I think would have been done on purpose, to preserve the Fender sound at higher watts with flatter overall response.
Put it in historical context, in the late sixties and seventies, if you were running a 20watt amp say a late brown or an early black, jamming with your mates, you put all the controls to ''eleven'' right? OK maybe not all the way if you were playing jazz or country..but with subtractive tone stack cutting quite a lot of dB from your sound, we do what we can to keep up with the band. That scooped sound on an overdriven amp is very much part of the early American Rock sound. Marshalls TMB: copy of Fender but look at the mid tone pot value, generally it is a bit more mid presence and different position in the preamp.
Maybe I should have been more specific when I made that statement you had quoted.
In early Fender designs they were using Jensen speaker, mated to amps with only small amounts of NFB (that is whole other subject). Running an amp this way with low damping factor, the speaker response is quite emphasized. More NFB (higher damping factor) would tends to flatten out the overall frequency response.
Have a look at vintage alnico or early ceramic Jensens. There is a lower frequency bump where the resonant peak is, and a rising bump at about 2KHz. In between those points the response has got some ''scoop'' or droop to it.
It is just my conjecture, but once the power increased in the Brownface and Blackface designs, they are push-pull outputs with more NFB and using more powerful speakers which had a flatter mid (500 to 2K) response. If you run Duncan tone stack, and enter stock values for the Treb Mid Bass. Try it for Brownface with a fixed resistor mid (treb and bass control pots only). You see that inherent scoop? That I think would have been done on purpose, to preserve the Fender sound at higher watts with flatter overall response.
Put it in historical context, in the late sixties and seventies, if you were running a 20watt amp say a late brown or an early black, jamming with your mates, you put all the controls to ''eleven'' right? OK maybe not all the way if you were playing jazz or country..but with subtractive tone stack cutting quite a lot of dB from your sound, we do what we can to keep up with the band. That scooped sound on an overdriven amp is very much part of the early American Rock sound. Marshalls TMB: copy of Fender but look at the mid tone pot value, generally it is a bit more mid presence and different position in the preamp.
Shanx: I got my OP quote from Practical Tone Controls. .... That was you? Then I thank you for taking interest in my quest for understanding.... I've been re-reading it, as well as playing with Duncan's software. Have compiled a number of different Fender stacks (see pics), to compare and figure out. Question: a simple RC filter has a fixed slope (say 3dB/octave), the pot only changes the cutoff frequency? So you cannot change the "amount" of the cut, only its center frequency? I'm leaning towards playing with a Treble/Bass/Mid 3 control stack, picking the Mid cut freq I like, making it fixed, leaving a 2 control Bass/Treble stack (like has been done before). I just want to play with the mid and the others to pick the caps that work for my cabinet & speakers. Thanks
Hi LA,
That is really funny cause I had never run across that Practical Tone Controls site, but basically stated the same thing on my post the other day from ''modding a frontman 15 thread....
If you think about the simpler single pot tone stacks you show the pot resistance is fixed and the wiper is generally allowing more highs to pass through when the wiper turned toward the low value cap, or shunting lower frequencies to ground when the pot wiper is turned towards the bigger cap. They don't generally change the centre frequency or cuttoff frequency, just the amount of bleed through or shunt to ground signal. It is possible to do a circuit with a sweepable center frequency and able to vary the amount of cut. Or hi low cut-off , just get to be more involved..
That is really funny cause I had never run across that Practical Tone Controls site, but basically stated the same thing on my post the other day from ''modding a frontman 15 thread....
If you think about the simpler single pot tone stacks you show the pot resistance is fixed and the wiper is generally allowing more highs to pass through when the wiper turned toward the low value cap, or shunting lower frequencies to ground when the pot wiper is turned towards the bigger cap. They don't generally change the centre frequency or cuttoff frequency, just the amount of bleed through or shunt to ground signal. It is possible to do a circuit with a sweepable center frequency and able to vary the amount of cut. Or hi low cut-off , just get to be more involved..
Shanks: LOL I realized I had taken your post and pasted it at the top of that Practical Tone Controls file.... wanting to include it in my documents....
Shows I do appreciate all the Forum input I get! I do want to be able to play with mid cut and where it occurs, to simulate the older speakers (and then part of that classic Fender sound?). Always cut the mids for my music listening, so why not for guitar? Problem is there are SO many different circuits, I wish I had a white-noise generator and spectrum analyzer... I can sweep my SigGen and look at amplitude changes on my scope, but is more tedious. My current stack 250pF treble RC works well, and the 25K/.047uF Mid RC does it's attenuation thing, but my 1M/0.1uF Bass RC isn't doing what I want. Have a table on several amp component selections for these RC filter pairs, and need to play with them some more.
Shows I do appreciate all the Forum input I get! I do want to be able to play with mid cut and where it occurs, to simulate the older speakers (and then part of that classic Fender sound?). Always cut the mids for my music listening, so why not for guitar? Problem is there are SO many different circuits, I wish I had a white-noise generator and spectrum analyzer... I can sweep my SigGen and look at amplitude changes on my scope, but is more tedious. My current stack 250pF treble RC works well, and the 25K/.047uF Mid RC does it's attenuation thing, but my 1M/0.1uF Bass RC isn't doing what I want. Have a table on several amp component selections for these RC filter pairs, and need to play with them some more.Hi I have just spent 3months perfecting and incorporating the Fender Based Tone stack into my newly designed Pre Amp(PreQ) with great success.
https://www.circuitlab.com/circuit/e2nq9c/ca-20020-aa/
It has a very rich natural sound with no excess brightness while still maintaining good HF detail. Its not your papa's Bass/Mid/Treble type of bandpass controls as there is interaction between Pots in my design.
Music Hall Tube DAC > PreQ (Mine) > Emoptiva BiAmp > Paradigm Studio 100's
I will definitely look at your design. I really opened up a can of worms when I started playing with the 3 pot stack... making tables of sine wave amplitude vs frequency with different cap values, then finding out the pots were interactive (I remember reading that somewhere!), then noticing they overlapped...then a bad solder joint....Arrrgh! Thought this would be easy... Starting to look at that one-pot design LOL.... Gonna look at Duncan's software - no soldering!
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