BSIAB2 with Marshall tone stack

[waltk]

I've built a couple BSIAB2s, and love the sound of them (Thank You Ed), but I sometimes find myself wishing for a little more control over the tone.  I was thinking that if I replace the single-knob tone stack with a Marshall three-knob tone stack, that might get me what I'm looking for.  Has anybody tried this (could only find references to other tone mods in other posts) - or have any guesses as to whether it will work?  Here's a schematic with the basic idea.  The tone stack is straight out of Duncan's Tone Stack Calulator - but I could have messed something up in the implementation with my limited understanding.  So, any advice would be appreciated.  There's a bigger image of the schematic in the gallery here:http://www.aronnelson.com/gallery/main.php/v/waltk/Schematics/BSIAB2+Marshal+tone+stack+1+Schematic.png.html

[johngreene]

A tube amp tone circuit like this is a rather high impedance circuit. So there are two problems to start:

1. the 100K volume pot is going to load it down.
2. the output impedance is going to be really high.

So, you can approach this a couple of different ways. Two of which are:

1. lower the impedance of the tone stack. (lower resistor values, increase cap values)
2. buffer the tone stack to provide high impedance load for tone stack, low impedance drive for output.

This type of tone stack is very lossy as well, so you will probably need more gain after it to make up for the loss. Since the amplitudes here are volts, you are many dB closer to the noise floor than you are with a tube amp that is working in the 100's of volts range. It may be quite noisy when you are done.

--john

[MikeH]

Check out the values in the full stack version of the thunderchief at ROG:

http://www.runoffgroove.com/tc-tone.png

[MikeH]

Oh well wait- it's basically exactly the same except for that 517k resistor.  What you really might want to try, it putting it between gain stages rather than at the end.

[MohiZ]

The 517k resistor doesn't need to be there, so you can just take it out entirely (so that the wire from the treble knob goes only straight to the volume pot). In the Duncan TSC it's there to act as a load resistor, symbolizing anything you plug into the output jack.

[liquids]

What exactly is it that you don't like about the BSIAB sound that you think the Marshall tone stack will add?  That would help.

Looking at the GGG schematic....I find that putting C13 on a on-off-on toggle switch from stock .002uF/no cap/.001uF adds a lot!

Likewise with shrinking C10 to .0047uF or smaller for a more useful tone control on the high end side, and modding C8 to 1uF or larger so the tone pot doesn't roll off any more bass.

Likewise, turn r8 into a 100k pot - instant 'mids' / contour / AMZ presence control; very functional and cool.  Makes it go from an already versatile circuit to a swiss army knife of rock tones.

To me, all those options together make it better than your generic Marshall tone stack, and while I've never tried a thunderchief, it would have the advantage of fewer gain stages and fewer trim pots.

[waltk]

Wow! Thanks for all the great suggestions!  I've been building all manner of stompbox circuits, but am still a raw beginner when it comes to understanding the designs.

JohnGreene - I was so intent on getting control over the frequency curve that I completely ignored what was happening to the impedance.  Thanks for the advice.  As Mike pointed out, these are the same values used in the full tonestack version of the ROG Thunderchief (except for the extra 517K resistor that doesn't need to be there).  Do you have any suggestions about what might be good alternate values for the caps/resistors?  I wasn't as worried about the dB loss because the BSIAB seemed to have plenty of extra gain. (maybe I should worry about that though...)

MikeH - I've been meaning to build a Thor (and have an etched and drilled board).  Maybe I should build that and the full tonestack version of the Thunderchief - just to see how they compare.  Suppose I still want to try this tonestack in the BSIAB; where exactly would you put it? just before the gate of the second micro-amp?

The 517k resistor doesn't need to be there...

MohiZ - Ohhhhh.  Thanks.

What exactly is it that you don't like about the BSIAB sound that you think the Marshall tone stack will add?

liquids - I like everything about the BSIAB sound.  But it sounds different playing through different amps.  I just wanted the flexibility of tweaking the tone to match the equipment I'm playing through.  I like your suggestions, and I have an extra BSIAB PCB, so I'm going to build one with all the mods as you suggested.  Maybe that really is all the flexibility I need.

Thanks again!
-Walt

[johngreene]

JohnGreene - I was so intent on getting control over the frequency curve that I completely ignored what was happening to the impedance.  Thanks for the advice.  As Mike pointed out, these are the same values used in the full tonestack version of the ROG Thunderchief (except for the extra 517K resistor that doesn't need to be there).  Do you have any suggestions about what might be good alternate values for the caps/resistors?  I wasn't as worried about the dB loss because the BSIAB seemed to have plenty of extra gain. (maybe I should worry about that though...)

The 100k pot is basically the same thing as the 574K resistor in your TSC. If you changed the 574K to a 100K in the simulator you will see what it does to your frequency response. A good starting point would be to just scale it backwards. You are changing the 574K to 100K so scale all your resistors by 100K/574K and scale up all the caps by 574K/100K and you should essentially have the same response. Or you could just change the volume pot to a 500K and be done with it. 

Although you really should follow it with a buffer because that high of an output impedance isn't going to drive much.

--john