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Old 18th December 2013, 01:34 AM   #1
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Default Fender Tone Controls

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?
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Old 18th December 2013, 01:48 AM   #2
Enzo is offline Enzo  United States
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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.
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Old 18th December 2013, 03:08 AM   #3
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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!
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Old 18th December 2013, 04:45 AM   #4
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Changing a bypass cap on a gain stage for a smaller value raises the cutoff frequency, the effect is less bass.
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Old 18th December 2013, 06:44 AM   #5
Enzo is offline Enzo  United States
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But you must also calculate the bass rolloff frequency. If the stock cap rolloff is 5Hz, and the new one is 25Hz, for example, it may not affect a guitar very much.
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Old 18th December 2013, 11:17 AM   #6
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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.
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Old 20th December 2013, 12:34 AM   #7
jjman is offline jjman  United States
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"....The mid scoop was coming from the speaker..........."

I have never seen a frequency response curve of a SPEAKER that was scooped. I think people say "scooped" regarding speakers when they are less mid-humped than average.
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Old 20th December 2013, 01:02 AM   #8
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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!
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Old 20th December 2013, 02:00 AM   #9
SRtom is offline SRtom  United States
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I've worked with Fenders and most other amps. With a single knob it is a low pass (cap to ground). But with some simple wiring and a few additional parts, you can turn it into a 'low pass, bandpass and high pass circuit with a single knob-all passive.
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Old 20th December 2013, 03:32 AM   #10
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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
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