tone stack question

[shooter]

I built a multi preamp, all 3 work!  Now I’m trying to tune/voice them.  Preamp 1, all the knobs work but it seems way to bright, almost fingers on chalkboard bright.  To get to what I think is close, I have to set treble to min, bass and mid to max.  The schematic is about a direct copy of a mesa formula pre section.

What do the 8.1M/.002uf (10M on original), do?  I’m guessing a band pass filter?
Will increasing the value of the treble pot and/or 250pF cap get me the right direction?  I’d like to get the treble to roll off sooner instead of boosting bass.  haven’t scoped it yet, should have my “baseline” scoped tomorrow. I’ll be doing my homework on tone stacks next.

Thanks

[sluckey]

To get to what I think is close, I have to set treble to min, bass and mid to max.

That tone stack is so close to the Fender AB763 TMB stack that I think something may not be not wired IAW the schematic.

[jjasilli]

1.  I suspect the main culprit is the 2.7K resistor between the treble & bass pots.   Maybe try shorting it out by jumpering it.  It may be both: a) blocking treble from bleeding to ground; and b) impeding mids & bass from re-entering the mix from below it.

2.  The bypass cap across the vol. pot is 2X the AA763 value.  This might be passing too much mids.


3.  The slope resistor may be too large at 150K.  It will make the tonestack brighter than the standard value of 100K, which might better match the rest of the component values.  Dropping the value below 100K will darken the tone.

4. I don't get the the 8M resistor & cap circuit added the top of the tonestack.  The resistor seems large enough to render that part of the circuit superfluous.

[shooter]

Thanks guys, i'll go back over the build n see if I missed something, also the suggestions on what to try.  Thanks for the help

[HotBluePlates]

Preamp 1, all the knobs work but it seems way to bright, almost fingers on chalkboard bright.  To get to what I think is close, I have to set treble to min, bass and mid to max.

Remove the 39pF cap across your 2.2MΩ. That boosts things above ~1.86kHz (the -3dB point of 39pF and 2.2MΩ) while the 82pF cap is rolling off starting about 5.9kHz (the -3dB point of 82pF and 330kΩ). That's a nice big presence peak you may not want (Mesa had a graphic EQ after all this to tailor things if you wanted, which you don't have).

Or just remove the 39pF and the 82pF caps (cause you still would have the 5.9kHz rolloff which will compound the natural rolloff of the speaker at/above that frequency).

What do the 8.1M/.002uf (10M on original), do?  I’m guessing a band pass filter?

They don't do anything. The 250pF cap bypasses them almost completely.

The 250pF cap doesn't look like 8.1MΩ (to evenly split signal between the resistor/cap and the 250pF) until the signal frequency is down to 78.6Hz. But at 78.6Hz, the 0.022uF cap looks like a bit over 1MΩ (so that total path is over 9MΩ). Any signal that low has already taken the path of least resistance through the 150kΩ resistor to the caps connected to the Bass and Mid pots.

The schematic is about a direct copy of a mesa formula pre section.

That explains it. You copied parts values, but didn't copy the relays for switching. The 0.002uF and 10MΩ do nothing until the relay shorts out the 10MΩ. At that point, the 0.002uF cap bypasses the 250pF and shifts the response of the Treble pot downward. So it acts like a mid boost with the 10MΩ shorted.

... the 2.7K resistor between the treble & bass pots.   ...

I don't think this part does much of anything either.

In a Fender tonestack, the Bass pot varies from 250kΩ (max  bass) to 0Ω (minimum bass). Here, the Bass pot varies from 252.7kΩ (max bass) to 2.7kΩ, which is kinda like the Fender Bass pot just a hair above 0.

[shooter]

Thanks hotbluesplate, I went back to the original after sluckey mentioned possible wrong  setup n figured out the 8.1m/.002.  I did use an old nos cap for the 250pF and with a magnifier found "sweat beads" on it.  I'll swap it n re-check the sound then move to the down-steam filter if it's still bright.

thanks again

[HotBluePlates]

The 250pF cap is not the problem.