One-Knob Tone Stack Idea

[kleptocracy]

I'm designing an amp sort of based on the Sunn Model T. I really want to minimize the number of knobs on the front panel, so I need a 1 knob tone stack (also considered going without one entirely but read somewhere on here about need to cut some mids to get a normal guitar sound). Additionally, I wanted a tone stack that would load down the cathode follower as little as possible so I can use relatively large value coupling caps without fear of blocking distortion (I'm basically building a bass amp for guitar).

I played around with the Big Muff tonestack in the Duncan Amps calculator, and was dismayed that most configurations seemed very current hungry. So starting with that circuit I popped over to TINA 7 and after playing around and massaging the parts around, I finally settled on the following arrangement. At one end of the knob's range, the tone stack barely loads the previous stage at all, and even at the other end the load is still relatively light. The mid-cut also shifts it's Fs in addition to depth over the knob's range, so it's hopefully a bit more versatile than the Muff stack.

Check out the attached images and lemme know what you think. I'm concerned I might not be cutting enough mid even at the one extreme.

I'm also open to suggestions for totally different arrangements if there's something out there that would fit the bill even better than what I've got here.

Thanks!

FYI - Pay close attention to the left axis -- each graph has very different indices! Aslo, the 38K is the load resistor for the CF, and isn't really part of the tone stack, but I wanted to design around making sure it didn't have much of an effect.

[tedmich]

very cool...you should build it and test it out.

maybe a multi ganged pot would help, here is a very nice alps:
http://brigarelectronics.com/mm5/mer...ategory_Code=P

[tubeswell]

Well not really knowing what you want, there are some suggestions on this page.

http://amps.zugster.net/articles/tone-stacks

(Also check out the one knob tone stack suggestion under "Other' at the bottom of that page)

[Steve Conner]

I see your mid cut is measured in millidecibels, that's unlikely to make much of an aural impact

If you find a circuit intended for solid-state use "current hungry" with tubes, then you can use a technique called impedance scaling. Multiply all resistor, pot and inductor values by 10, and divide all capacitor values by 10. The resulting circuit will have the same frequency shaping characteristics, but 10 times the input impedance. (Or whatever number, 10 is just a suggestion.)

I'd start by scaling the Big Muff and Tube Screamer tone controls, and see how those go. You can also check out the bridged-T that Gibson used in some of their oddball amps, to produce a notch in the midrange.

[Merlinb]

I played around with the Big Muff tonestack in the Duncan Amps calculator, and was dismayed that most configurations seemed very current hungry.

Can't you just scale all the resistances up to make it less heavy? Try 470k for all the resistances, 3.3nF for the shunt cap and 1n for the series cap.

The circuit you've come up with looks a bit limited, you basically have a flat response (near as dammit) or a big treble cut with a slight mid scoop. No treble lift option??

[loudthud]

The schematic in the thread below shows a variation of the Big Muff circuit scaled for a tube amp.

http://music-electronics-forum.com/s...ead.php?t=6733

[GibsonLover]

Jack Orman works over the Muff tone control pretty well here too:
http://www.muzique.com/lab/tone3.htm
The article is called AMZ presence Control.
Jack makes a good point about the low inpedance of the drive - it may not be what you think, and the beautiful symmetry you see in a sim may be completely different on the bench.

[kleptocracy]

Thanks. I'll dig through those pages.

Also, an excellent point about the signal impedance changing the frequency response.

In reality, I'm not exactly sure what I want. I play through such an odd-ball rig and play such odd-ball music, I'm not sure what my tone-shaping needs are going to be. So...I'm just going to have to plug some parts in a go for it. If it's no good or too limited...that's what the soldering iron is for.

But, once again, thanks for all the info and suggestions. I'm going to dig through this stuff and see if I can have a modified circuit up here soon.

[kleptocracy]

So once again, thanks for all the input and suggestions. After looking through those pages and playing around with those circuits and my own a fair amount, I came to the conclusion that I still like mine the best...or, at least, I think it gives me the best shot of getting what I want out of the amp (loooooads of bass with lots of warmth, but still sounding like a guitar and a guitar amp...at crazy high volumes)....but with some modifications. Like, another knob, for instance.

So here we go, a very similar circuit...but with two knobs (plus volume). I actually bothered to normalize the y-axis on the graphs this time, so these are a bit easier to read. Basically, the "bass" knob actually sort of defines the range in which the "treb" knob can work. Also, turning the bass knob all the way up gives essentially a flat, very-low-loss response regardless of the treble knob.

In other words, imagine you have a tone stack with a fixed bass and mid response, but with an adjustable treble response. The "bass" knob then gives you the ability to blend in as much of that tone stack as you want, or blend it out entirely. Looking at the circuit and response, that's pretty much how I expect this to work, and I think it's pretty neat...or at least unique. Yeah, there's a volume change for the tone knobs...but it's kind of unavoidable. At least the volume knob should pretty much function independently.

As far as input impedance goes, increasing input impedance should sap the treble response, and so if this happens I just need to raise the value of R2 to compensate...or vice versa if the impedance is lower than expected.

[Old Tele man]

As far as input impedance goes, increasing input impedance should sap the treble response, and so if this happens I just need to raise the value of R2 to compensate...or vice versa if the impedance is lower than expected.

...buffer both the input and the output...or, enclose the whole circuit in a feedback loop between a tube input and output.