Another active Tone Stack idea

[Casey4s]

My last active tone stack got out of control and the preamp was going to have 9 preamp tubes. So I started over with this idea but I had a hard time finding an effective way to introduce a midrange control.

This is what I have at this point I am open to suggestions. This going into a Tweed style (5F6A) amp when I get it working.

[pdf64]

I would have described those tone controls as passive, they follow the 'James' arrangement as per the tone stack calculator. The second control is another bass, rather than mids. Active tone controls usually use feedback, whereas the above operates open loop.

[Casey4s]

Hmmm apparently I misunderstood the terminology. Because this actually produces boost and cut in these frequencys I was under the impression that is what defined active from passive, but apparently not.

I am having one hell of a time getting a midrange inserted into this from the original RCA model. I pretty much thought it might just be an extended bass control.

Does anyone have any thoughts on how to insert a MID into this circuit? Obviously my first shot didn't work out well. LOL

[loudthud]

It's kinda tricky to make a midrange control with just resistors and capacitors. They end up covering too wide a frequency band or don't have very much boost or cut. There is a form of the Bax that looks like the bass control except there is one cap across the ends of the pot and another in series with the wiper to the summing junction. This Marshall schematic is an example where they used switches to change caps.

http://www.el34world.com/charts/Sche.../2001_375w.pdf

[Casey4s]

It's kinda tricky to make a midrange control with just resistors and capacitors. They end up covering too wide a frequency band or don't have very much boost or cut. There is a form of the Bax that looks like the bass control except there is one cap across the ends of the pot and another in series with the wiper to the summing junction. This Marshall schematic is an example where they used switches to change caps.

http://www.el34world.com/charts/Sche.../2001_375w.pdf

Thanks for the post,

My original idea was a powered Bax plus a version of the Ampeg V4 type midrange control, but I got way too many triodes involved to make it realistic. So I started over with an RCA B&T powered design. I am going to have to build a test bed to model this I guess.

[J M Fahey]

If you want to make a tubed midrange control, you'll have to make it the way the old pros did, with inductors.
Study the Pultec and the Altec equalizers.
And please do not refer to Tweeds, as this concept is totally opposite to what was used in them.
Good luck.

[imaradiostar]

The Ampeg is a good example of an active control. I was going to mention the SVT's active mid control.

You basically can add feedback around a gain stage and a passive control to make it an active control.

Many active controls use a pot to pan between boost and cut- take a look at old Neve controls for an example.

jamie