Seeking a different tone.

[Boy Howdy]

I recently acquired an old (early Seventies) Acoustic 150 head. I'd been curious about them for decades because one of my jazz heros used one, so when I found one for cheap I grabbed it.


I've been playing it for a month or so now. And my observations: It of course has that hard, clinical, transistory treble, which I don't love (though I must say I do enjoy the extended high treble frequencies that you don't get with tube amps). But I've found when comparing it to my tube amps that it has a much better TONE for instrumental playing. I'm talking exclusively about clean playing here, no distortion at all. It lacks the mid dip of the usual Fender/Marshall tone stack. Acoustic got something way right here. The upper mid/low treble region is just perfect. The notes practically jump off the guitar.

I'm starting to wonder how that might be duplicated in a tube amp - to lose that ugly hard, clinical treble, but keep those beautiful upper mid freqs. (There does seem to be less low mids muddying things up.)

So, a couple of questions. I know nothing about solid state tech. Can anything be learned from looking at the schematic that could be applied to a tube tone stack? And what are some possible approaches to getting that tone with a conventional or alternative tone stack, such as say the James (which I've never built and have only limited experience with)?

fetch (1041×762) (music-electronics-forum.com)

[The Dude]

A good tool for tone stack calculating is the Duncan tone stack calculator. It's a free download here:

https://www.duncanamps.com/tsc/

While I do understand your post, it's hard to offer better tips without a schematic or specific frequency information. I will also say that a different speaker can make a huge difference in amp tone, volume, and frequency response. Try different speakers to see which best suits your needs.

[gui_tarzan]

Different brands and types of tubes make a difference in tone as well. Tone is also subjective, what one person likes another may not.

One of my favorite clean tones comes from a vintage Strat going through a Roland JC-120 amp with just a touch of chorus. I also have to say my Fender 112SE with an EV speaker has a beautiful clean tone with my MIJ Tele w/ Texas Specials. Sparkly and pure. If I could get that sparkly clean tone out of a tube amp with a silky-smooth breakup when driven, I'd be in heaven on Earth. That's my goal with the home build I've been working on. I like the TSC for looking at how different values change the tone response, but you still need to hear it to see what actually sounds best.

[Boy Howdy]

Thanks, guys. But what I'm trying to get at is that the usual Fender/Marshall tone stack has an inherent mid-frequency dip. And that dip can't be done away with a change of speakers or tubes. It's always there. It could perhaps be addressed, at least to some degree with an outboard equalizer, but what I'm trying to learn is if it's possible to get rid of the mid dip by either a change to a different type of tone controls such as the baxandall, or - to take this in a different direction - to modify the usual tone stack to shift the frequency (not amplitude) of the midrange control downward. This would, conceivably, add some upper mids to the treble control, while allowing me to cut the lower, muddy, mids.

[Helmholtz]

Playing with the TSC shows that the typical TMB stack has a mid dip and while it can be adjusted for flat response, a mid boost is not possible.
Options to lower the mid dip frequency by different component values seem limited. You might try to increase the value of the treble cap.

The James stack OTOH allows for mid boost as well as mid dip.

Typical for these passive stacks is that the effects of component changes and pot settings are to some degree interdependent.

[loudthud]

The Fender type tone stack creates a crevasse for the midrange much like a twin T network. This sounds quite a bit different compared to the baxandall with treble and bass turned up. The schematic you linked in post Home shows a non standard tone control, not that different from some graphic equalizers. An inductor controls the bass and a cap controls the treble. To reverse engineer it, you would need to use a simulator and would have to know the particulars of the inductor used.

A circuit like the Mesa Boogie graphic equalizer or what Fender used in some of their 180 Watt amps (Super Twin ?) would be a good starting point.

Something else to try is a true parametric midrange control with variable frequency and bandwidth controls.

Edit: One technique that can be useful is the synthetic inductor approach. Since most of these circuits use an inductor to ground, you use an opamp and simple circuit to replace the inductor, and you can control most parameters.

[Boy Howdy]

Some good ideas, guys. Some of it a bit over my head, but that's just part of it, of course. l wonder if ya'll have looked at any of Gar Gillies Baxandall variations. He has a bunch of them (his book is great!) some of them have added midrange controls. I've only owned one baxandall amp, a Seventies Ampeg GU-12, which while it had a problem that I could never get fixed, when it was working properly it sounded fantastic. But that was a quarter-century or more ago. And of course I wasn't aware of the bax. so couldn't exactly evaluate it critically.
Ampeg-GU-12-Vintage-20-watt-1x12-combo-front-768x797.jpeg (768×797) (atlantadiscountmusic.com)

Any thoughts on the fact that a Gallien Krueger stereo chorus amp I used to own had two midrange controls?
vorne.jpg (1600×450) (amazona.de)

[Jazz P Bass]

"Any thoughts on the fact that a Gallien Krueger stereo chorus amp I used to own had two midrange controls?"
The GK uses a full range parametric eq circuit.
It would be interesting to sweep the range & scope it to see if/ how the bands overlap.

[Boy Howdy]

Hmm, interesting. I'll have to see if I can find a GK schematic. Wondering how one would incorporate a parametric EQ into a passive tone stack reminds me that I used to own an Ampeg V-2 (Actually, I've had a couple at various points in time.) and they have a parametric midrange. I didn't love that amp particularly, but it did give good tone, and I liked having that extra bit of control over the EQ.

I'm liking Helmholtz's idea more and more about just simply using a larger treble cap, that would bring in more of those upper mids while perhaps even overwhelming, or eliminating altogether, the mid dip. I typically like a fairly large cap as it is - I don't think I've tried going bigger than a 500pico though. . . .No wait, I do have a highly modified Garnet Rebel PA 90R (~35 watts), that I used a 680pico on one channel. I was going for a more Hiwatt kind of sound for that channel. It turned out really nice. I'll have to take another look at the schematic I drew for that. And, of course, plug into it and give it a good listen.

This for me is more a theoretical discussion. I don't have any plans at the moment to do anything in particular. But you never know where these discussions might lead. It's good to exercise the brain, right?

[Boy Howdy]

Gallien-Krueger 250ML schematic:
https://hosting.photobucket.com/albums/v210/bionuker/insanity/schem2.jpg

Ampeg V-4 schematic:
https://www.ampegv4.com/images/schematics/V2.jpg

[eschertron]

... perhaps even overwhelming, or eliminating altogether, the mid dip.

Adding a resistor to the tail of the mid control (between the pot and ground) can have an immense effect on the tone by reducing the dip. I have an amp where I replaced the stock Fender mid pot with a 50k value. Allows me to dial the sound up considerably. An added fixed resistor may be easier to implement, and could be switchable. I did use the Duncan TSC during development.

[Boy Howdy]

Well, this inspires a revelation, of a sort. Some of my amps already have 50K mid pots. And I usually run the mids around 4 on most amps (sacrilege, I know). So, why am I looking for more mids then? Obviously - here's the revelation part - I either need two mids, or I need the midrange at a different frequency from the usual. I'm liking these high mid freqs that the Acoustic 150 is giving, but I don't like the broad (bandwidth) midrange that most amps give. I think what I'd like to do is boost the upper mids, but cut the low mids. And something as simple as a bigger treble cap might do it! I'm just all excited with this idea.

But that points to a gap in my knowledge: What does a bigger mid pot do vs. a smaller one? Since passive tone controls only 'cut', does that mean a 50K cuts more than a 10K, while both give the same amount of mids when turned all the way up?

[Dave H]

What does a bigger mid pot do vs. a smaller one? Since passive tone controls only 'cut', does that mean a 50K cuts more than a 10K, while both give the same amount of mids when turned all the way up?

The bigger mid pot has more mids than the smaller one when set to '10' (50k) They both cut the same when set to '0' (zero resistance)

[Boy Howdy]

I thought so. Always good to question the conventional wisdom, though. We guitar player are notoriously full of BS, after all. It's always the stupidest little things that we end up carrying around unresolved for far too long. This is one I can cross off my list. Thanks, Dave.

[Chuck H]

The bigger mid pot has more mids than the smaller one when set to '10' (50k) They both cut the same when set to '0' (zero resistance)

Yes? Or, sort of? But there's another thing happening too that's worth noting. In the plain ol passive TMB stack used in most amplifiers the mid pot is interactive with ALL frequencies. This affect becomes worse as the mid pot value is increased. This essentially makes the mid control more of a volume control at values over 25k when typical component values are used for the rest of the tone stack. Turning the mids above 5 with a 50k mid pot demonstrates less change in frequency band relationship than overall signal amplitude.