Help me with my tone stack.

[Exothermia]

I need a little help tweaking the tone stack on my amp. It's a silverface Fender PA135 head that I converted to guitar use after some extensive modifications, including a new tone stack. Here's what I have now, after the first gain stage:

http://img518.imageshack.us/img518/3059/aamptone2sc1.gif

I want to keep the three control scheme, in fact i'd like to keep the circuit intact if possible, but pot/cap/resistor values are up for debate. If you'll note I'm basically using a classic Marshall set in terms of values. My guitar is equipped with humbuckers. I'm currently satisfied with the bass response, the treble response is pretty good although it would nice if it could go a little treblier. I play it with the treble on 10 and I feel like it could use just a tad more. The mid control is what is pissing me off. I play with the mid on 0 and it still has too much mids. Some of that is my speakers, WGS V30 clones which are pretty midrangey themselves, but even so, the control is all the way down and I've still got too much mids.

My head is at the band practice place right now but I'm going to order whatever components I need to tweak it, then when they arrive pick it up from there and fix it so it can be ready to rock and roll for the next practice.

So, anybody got any ideas on value changes to get my tone more to my liking?

[Trout]

I need a little help tweaking the tone stack on my amp. It's a silverface Fender PA135 head that I converted to guitar use after some extensive modifications, including a new tone stack. Here's what I have now, after the first gain stage:

http://img518.imageshack.us/img518/3059/aamptone2sc1.gif

I want to keep the three control scheme, in fact i'd like to keep the circuit intact if possible, but pot/cap/resistor values are up for debate. If you'll note I'm basically using a classic Marshall set in terms of values. My guitar is equipped with humbuckers. I'm currently satisfied with the bass response, the treble response is pretty good although it would nice if it could go a little treblier. I play it with the treble on 10 and I feel like it could use just a tad more. The mid control is what is pissing me off. I play with the mid on 0 and it still has too much mids. Some of that is my speakers, WGS V30 clones which are pretty midrangey themselves, but even so, the control is all the way down and I've still got too much mids.

My head is at the band practice place right now but I'm going to order whatever components I need to tweak it, then when they arrive pick it up from there and fix it so it can be ready to rock and roll for the next practice.

So, anybody got any ideas on value changes to get my tone more to my liking?


You could try bumping up the 250pf to 380pf or 500pf. That will sharpen it up a bunch. I never go under 68K on that slope resistor. I always felt it shifted to much midrange to the bass pot.

Trout

[Exothermia]

Hmm. One of you says decrease the value, the other says increase the value? If I understand correctly, a higher value should filter more out, so I would be tempted to believe a lower value would increase the treble, is that correct?

Also, you believe I should increase the value of the resistor? I wasn't sure if that would affect the EQ much or not but I guess it should. I will try to tweak that too. Any other ideas? Keep in mind I would like to both increase treble slightly and decrease mids.

[phsyconoodler]

Change the bass pot to 250k and the mids will go away some.That makes it basically the same as a super reverb.
The 250pf cap should be ok as is.

[gldtp99]

The Tone Stack Calculator is a free download from Duncan---- it's an easy to use tool to see what happens when values are manipulated in various common guitar amp tone stacks--- it has saved me a lot of time.

http://www.duncanamps.com/tsc/index.html

[epluribus]

Personally, though I'm a big believer in crunching the numbers and knowing the theory, for this I'd just breadboard the thing and start swappin' parts till you get it right. Of course it'll help if you have a rough idea of how the swaps will behave and interact. I have a tone stack sitting on my workbench right now, so I can just lift a couple of connections in an amp and jumper the entire breadboarded stack into the circuit. Yeah it's RF-y and noisy, but nowhere near enough to keep you from hearing the EQ correctly. Then once you get the specs down, you only have to de-solder once.

--Ray

[Rosewood]

I'd move that slope resistor up in value and maybe look at the rest of the amp to increase the treble. I've never looked at the pa135 schematic so not sure about the output stage.

[Blue Strat]

Stock Fender values would certainly bring out more brightness. Maybe start there and then tweak.

[Schwalbe]

I'd move that slope resistor up in value and maybe look at the rest of the amp to increase the treble. I've never looked at the pa135 schematic so not sure about the output stage.

Yep, try a 100K like in a BF Fender instead of that Marshall/Bassman 56K, or anything in between. You could even go a little higher.
You could also add a bright cap.

[Exothermia]

Okay, thanks for the info guys. I'm going to start by raising the value of the resistor and lowering the value of the 250pf cap to start with and see where that puts me. I kinda wish I had just wired these up on a breadboard so I could have tweaked values before installing it but I didn't. If those changes don't help me then I'll pull the wiring and probably do just that so I can get it figured out.

[epluribus]

While we're on the subject, anybody got a link to pages with a good summary of tone stack theory? I've learned a bit so far, but I'm still having fun digging into gain and amplification issues, and haven't really gotten deep into shaping freq response yet. (Other than that if I can build my circuit without a tone stack at all I feel like I musta done something right. I hate attenuation. But I heard somewhere that EQ may be handy once in a while. ;))

Anyhoo, my grasp of tone stack design and circuit interaction (like insertion loss) could use some...er...better grasping. :)

--Ray

Yes my stereos smile. Mea culpa.

[epluribus]

I kinda wish I had...

(fill in the blank)

...so I could have...

(fill in the blank) ...before...

(fill in the blank)

...but I didn't.



Boy, don't we all...


--Ray

[Exothermia]

Lol. Yeah, there was a lot of that going around on this project. Luckily it's pretty much all done except for this tone stack.

[Wakarusa]

Duncan's tone stack calculator is nice, but doesn't give you a lot of freedom (the layouts are fixed and there are only a few). A nice alternative is a SPICE simulator (like the free one from the folks at Linear) since it lets you muck about with the layout and do odd things. The ugly alternative is to dust off your linear algebra and solve the circuit by hand (bleh...).

The point is to not limit yourself to the basic Fender/Marshall layout or values. Not only can you try some other basic designs (poke around in old audio design books for all sorts of odd takes on the problem), but I'd argue that you need to include both the driving and driven circuits in your design. The "tone stack" is just the part with movable bits, but the amp's tone is shaped by stuff like coupling caps, interesting applications of feedback, etc.

[JJman]

http://home.comcast.net/~jjsant3250/2007.10.28%2021.54.44.jpg

The graph with the mid-scoop to the left is the common BF/SF. The one with the scoop to the right is yours. Both are shown using the pot settings on the bottom left. Mid on 0 and Treble maxed.

[jjasilli]

I'd move that slope resistor up in value and maybe look at the rest of the amp to increase the treble. I've never looked at the pa135 schematic so not sure about the output stage.

Yes, not enough info, especially as to how the tone stack is driven. The Marshal value 56K slope resistor assumes that the tone stack is cathode driven. If your tone stack is driven by the plate of the previous gain stage then a 100k slope resistor is the "norm". Reducing the value of the slope resistor will darken the amp & vice-versa.

(The output impedance of the previous gain stage couples with the value of the slope resistor. So, the tonal response of the 56k cathode driven slope resistor apprx. equals that of the 100k plate driven slope resistor. Note further that the Marshall circuit with the smaller slope resistor + the larger mid pot to ground yields less signal loss in the tone stack, especially in the mids. Hence Marshals seem to have more gain and are more mid-rangey than typical Fenders)

[Old Tele man]

...excellent point about the differences in source impedance (cathode vs. plate) and its loading affects upon the tonestack characteristics.

[Mike T]

I'd bump that slope resister up to 100k. Try that first. If you raise the 250pf you'll just add more mids. You could also change the mid pot to 10k. And then maybe the bass pot to 250k. Basic Fender values except for the .22uFs.

[epluribus]

Hey guys, this is a really useful subject...the interaction of various stages in a circuit. Those of us who are just getting a grip on circuit design get to some point where we're getting a handle on fine-tuning a gain stage or an input grid, but the really knotty part is moving from there to understanding how the individual stages interact in a dynamic environment.

I suspect a huge part of this is understanding impedance and loading, including all this "reflected" stuff. I've just found a couple of links on the subject, but no Rosetta Stone as yet...found any good online discussions of these things?

'Preciate what ya'll've done on this thread, great stuff.

--Ray

[Old Tele man]

..."loading" is sorta like a drunk...when HE leans on you, you SAG down a little (or a lot) and the more he weighs, the more YOU SAG.

...you're NEVER able to stand up straight as long as he's leaning on you...samething happens with "loading," the impedance (AC-resistance) can NEVER be 100% (like 12AX7 rp = 62K%) if there's ANY kind of loading, and it gets lower (just like you sagging) the LOWER the loading is, like resistances in PARALLEL, ie:

Z.effective = Za || Zb = (Za*Zb)/(Za + Zb) where Za = tube and Zb = "loading"

...two "tricks" to remember:

(A) the HIGHER the impedance is for Za, the MORE easily it is affected by Zb; and, conversely, the LOWER the impedance is for Za, the LESS easily it is affected by Zb.

(B) the HIGHER the impedance is for Zb, the LESS affect it'll have upon Za; and, conversely, the LOWER the impedance is for Zb, the MORE affect (loading) it will have upon Za.