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James Tonestack Question

6/12/2004 8:35 PM
bob p James Tonestack Question
i've got a tech question that i'm hoping somebody here can answer. i've been looking at the James Tonestack and experimenting with Duncan's Tonestack Calculator.  
here's a schem courtesy of Tone Lizard's website:  
the effect of increasing the value of the "isolation resistor" (the resistor between the BASS and TREBLE wipers) from a typical 100k value seems to lessen the interaction between the bass and treble controls. the effect seems to be that a larger resistance value results in more isolation (less interaction) between the tone controls, while a smaller value seems to result in more interaction between the tone controls.  
there are also some secondary effects -- by increasing the resistance between the bass and treble wipers, the amplitude of the control's effect is muted in addition to providing less interaction between the controls.  
i've noticed a variant of this tonestack in some boutique amps -- the Route 66, for example, replaces this resistor with a cap.  
i was wondering what the net effect would be of substituting a cap for the "isolation resistor." unfortunately, Duncan's Tonestack Calculator won't allow modeling the tonestack with a cap in that location. So i was hoping that someone here could explain it from a theoretical aspect.  
6/12/2004 10:17 PM
Cosmic Zero
The "James" EQ cuit looks almost exactly like the baxandahl circuit used in many hi-fi amps of the 40s-50s. I believe the standard value for the isolation resistor is 120K. I've also seen it as high as 180K.  
Also, the Baxandahl EQ was standard in most Ampeg amps. It's prime benefit is isolation between controls, which would lead me to believe that you don't want to go lower than 120K on the resistor in question or else risk loosing the best feature of the circuit.  
Also the center tapped resistor pair and cap pair must be set to be a ratio of 10 to 1 to work properly. I've used this type of EQ in the past with good results. It does not seem to have an extremely wide tonal range, but on the other hand is one of the easiest EQ's to use to dial in a good sound.  
See the Ampeg portaflex schematic for more info.  
I'd hesitate replacing the isolation resistor with a cap since it would interact with the other caps in the circuit throwing the EQ out of wack.  
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6/13/2004 3:16 AM
"I'd hesitate replacing the isolation resistor with a cap since it would interact with the other caps in the circuit throwing the EQ out of wack."
i thought so too. so now i'm wondering... does Dr.Z knows something we dont...?
9/23/2004 8:59 PM

This is a long thread but I think I know what's going on. There is a way to make a midrange control that you can add on to the standard James bass and treble network. It looks like the bass control network except there is just one cap going end to end on the pot. Then another cap (10X the value of the first cap) connects the wiper of the pot to the output. I've tried it and it doesn't work as well as I'd like. You have to tradeoff gain for bandwidth so you can't get much +/- adjustment without affecting (interacting with) the bass and treble.  
Try to find a copy of the National Semiconductor "Audio Handbook" dated 1977. This is a very rare book that fetches big bucks on ebay. Or checkout the Marshall 2001 schematic dated 1981 in The Tube Amp Book by Pittman. These are Baxandall feedback designs but the same principals apply.
6/13/2004 3:49 AM
Bruce /Mission Amps

I've messed with a lot of Bax/James type tone stacks.... not just in theory or on some computer driven modeling BS platform either.  
Real time, real parts, real ears and plenty of wood shedding.  
Yes, they can be extremely usefull tone controls, for a simple bass and treble pot setup. Especially when you are trying to keep a tonal balance through a preamp stage.  
Although changing that grounded resistor does alter the tone I don't think it does hardly a thing for the midrange and making it adjustable is almost worthless except as another type of gain control.  
I'm telling you, you're standing on the shoulders of giants when it comes to the classic guitar tone controls in tube amps! The wheel has been invented.  
If you want an adjustable midrange tone stack, just build a tweed Fender/Marshal/VOX stack and be done with it.  
6/13/2004 4:21 AM
i Bob  
Actually there are 2 isolation resistors, the one before the bass pot and one after.  
Together they define the amount of bass contol compared to the trebles.  
If you replace the isolation resistor with a cap, you'll lose your trebles also.  
They'll run down to the ground via mid resistor.  
If the cap is small, you don't have any bass going through the stack.  
Actually it looks to me that a cap there would ruin the whole stack.  
Are you sure about the route 66. I doubt very much.  
6/16/2004 4:54 PM
"Are you sure about the route 66."
His description is accurate. The values are altered a bit. I've built it in an amp and it's very nice.
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