Post-Distortion EQ Recommendations

[chipaudette]

Hey All,

I'm looking for more recommendations for my modded amp. Specifically, I'm looking for anyone's feedback as to what kind of post-distortion EQ they might have tried on their amp.

My amp is a Fender Deluxe Reverb Reissue (DRRI) that I've heavily modded. Basically, I've taken the 12AX7 from "Normal" channel and reused it to make a Mesa Boogie Mark IIc-IV lead channel by (as done in the Boogies) cascading it after clean channel. I rewired the 12AX7 to put in all the correct (supposedly) tone shaping caps and resistors and whatnot. I even reuse the "volume" pot from the "Normal" channel to be the 2nd gain control for the lead channel just like in the Boogie. It all works pretty well. I'm fairly happy with it.

A critical part of the Boogie sound, though, is the 5-band graphic EQ that occurs after the overdrive but before the power amp. It's what allows you to sculpt the distorted sound from a midrange-heavy singing lead tone into a scooped-mids metal tone. The post-distortion EQ is the key to achieving anything besides a Santana sound.

In my amp, I have no 5-band EQ. I do, however, have an extra tone stack (Treble and Bass) left over from the "Normal" channel. So, I chose to loop the now-distorted lead signal back through the tone stack. When I fired it up, I found it to be fairly effective. Good. Of course, it really kills the overall signal level, so I had to boost it back up using half of the 12AX7 used by Vibrato circuit. No problem. I even re-wired the vibrato's "Speed" control pot as a volume pot for the lead channel. Sweet. It all works pretty well.

OK, now we come to my question...I like using the Fender tone stack as a post-distortion EQ....but, I'm sure that I could get an even better sound by tweaking it. The classic Fender tone stack was designed for clean tones...there's got to be room for improvement with distorted tones. As a first attempt, I used Duncan's Tone Stack simulator and found a set of values that seemed good...but once installed, it sounded really crappy. Since my amp is a PCB build, I can't swap components too many times without killing the PCB. So, maybe you all have some experience to help guide this journey with fewer iterations.

My question: Do any of you have a good recommendations for post-distortion EQ? Even if you haven't done an amp like mine, consider that a distortion pedal plugged into the front of your amp ends up having the amp's tone controls as a post-distortion EQ. Have any of you ever tweaked your tone stack to sound better with distortion pedals? What values worked best? What kind of EQ approach works best to sculpt overdrive?

Thanks,

Chip

[Chuck H]

As you no doubt know, a question like this is bound to be answered with opinions...Here's mine:

Combo amps sound like, well, combo amps. They have a nice top and bottom end roll off, a little lacking in bass and there's usually a low mid hump that makes them sound wolfy in the midrange. That said, I usually try to pull back the low mids and use an efficient speaker. The combination just seems to make combos sound much bigger and still loud enough to work with for anything over 15 watts.

Different amps and speakers will have specific characteristics that you may want to accentuate or play down. So anything I relate is just generic. But I'll share anyway.

For post distortion EQ with a standard passive tonestack (ala Fender, Marshall) try .047 bass, .033 mid, 330pf or 390pf treble and a 100k slope resistor. If the amp has a "presence" control try a 47k or 68k slope resistor. The treble pot would be a 500k linear taper, the bass pot would be a 500k audio taper and the mid pot would be 25k, audio taper if you can get it, otherwise linear will be fine.

For a circuit like yours where your seeking versatility I would use a push/pull pot for the treble control and have it switch in different tone stack values for different purposes. Like changing from 330pf treble cap and .033 mid cap to 180pf treble cap and a .047 mid cap, for a more traditional "Fender" sound. Why 180pf when the stock Fender cap is a 250pf? Because the 500k treble pot shifts the mid frequency notch and treble shelf much the same way the slope resistor value does.

This tonestack arrangement seems to help eliminate low end that the combo amp can't reproduce anyway and actually strengthen the low end thats left over. The low mids are trimmed a little to help with articulation but the mid control still has lots of swing for versitility. The treble shelf with 330pf adds just enough bite to clearify the lower notes without seeming imbalanced. But you should try a 390pf for the treble cap also to see which sounds better.

Just my humble 2C

Chuck

[chipaudette]

Hi Chuck H,

Is this what you were suggesting? It does shift the mid scoop lower (cutting the lower mids as you described)...and at higher Treble settings, it's really pushed lower. Is this what you intended?



Note that my amp only has Treble and Bass pots...no Mid. The "Mid" is a fixed resistor at 6.8k. So, that's what I've got shown on these EQ curves. I could switch that fixed resistor to another value...what dial setting on your Mid pot do you normally use?

Thanks!

Chip

[Chuck H]

Yes, The higher level of upper mids is intentional. I suppose it doesn't really cut low mids as much as a BF Fender. But a BF Fender is a really scooped sound. Probably why they've always been the benchmark by which other combo's are judged. But by increasing upper mids you can greatly improve the amps ability to cut through the mix and sound articulate without getting spikey or harsh. Alot of guys will try to boost all the mids, or even the lower mids just changing to a .022 mid cap and/or using a 25k mid pot. But those things by themselves seem to make BF-ish combos sound honky and undefined. Not bad for leads though, blues etc. Since your using a preamp for most of your gain driving the upper mids and keeping some cut in the low mids helps keep the amp articulate. Especially when you overdrive the PI and power tubes.

It's too bad you don't have a mid pot. That adds alot of versitility to this stack because it has almost as much mid swing as the stock BF does. Most stacks that don't use BF type values have very little mid control. Sometimes only a few db. Or the mid pot acts more like a second volume control and just seems to raise and lower everything a few db.

You could use a push/pull pot on the treble control and use it to switch between two "mid pot" settings using fixed resistors. Just temporarily solder in a 25k pot where the mid resistor is now and play with it to find an ideal low setting (mid cut) and an ideal mid boost setting and use fixed resistors of those values on the switch.

Chuck

[TheTinMan]

Chuck - thanks so much for sharing your thoughts on component values for a BF Fender tone stack

Chip - have you thought about a "James" type tone stack? IIRC it's a lot lower loss than the FMV you're using now. Here's an interesting summary of several varieties of tone stacks: http://amps.zugster.net/articles/tone-stacks#Overview How about looking at tone stacks in distortion pedals?

HTH

Chip (not audette)