Need tone shaping insight...

[GibsonGM]

So hey, here I am, a few thousand posts into this hobby, and I've never stopped to think about some stuff until now!

I'm putting together a tube preamp which will be of the stompbox variety (fed to an amp input and not driving a power amp).  It's basically a GTFO-like 4 stage deal.  Sounds really nice, but I did notice some 'fizziness' in the highs, and 'gravel' in the mids.

I've been messing around with it and put an EQ pedal after.  I found that notching out 400 to 800Hz down 10dB sounds really good, and that rolling everything above 3kHz down by 10 dB also gets rid of 'fizz'.   With those changes, the thing is perfect.   To keep these results I intend to roll my own tone controls rather than use a 'standard' setup like an FMV....I'd like a more narrow mid control so's to not lose the good stuff on either side of the bands I found that were 'not so great'.

The question arises here: is there an "order" that one would want to place the controls??  I mean...if I cut bass, then went for mids, then treble..would I end up in a weird position where i am trying to either boost/cut something that has already been 'taken care of'?   Generally I just go to a tried and true setup, and in this respect my info is pretty badly lacking.   Not sure if I should cut mids and bass somewhere between gain stages, cut treble at the end, things like that.  I'm aware of blocking distortion...in fact, this thing could use some bass BOOST, most likely.  Decisions...

I have endless signal to use...68V after my last cap!  So loss is not a problem.

Thanks for any insight!

[krister]

I would try voicing the various stages properly first. You may find you don't need as many tone controls as you think. Please post a schematic of what you built and we can work from there.

[GibsonGM]

Hey Krister - 3rd post down, by Jok3r...I did the Landry version.  It has 2 1M gain pots (for now, til I find out what sounds best).
[url]http://www.diystompboxes.com/smfforum/index.php?topic=93936.400[url]

No mods.

I like the sound overall - pretty kick@ss. But noted some 'fizziness', shrillness, and playing with the gains didn't do too much to it.   And a little 'peakiness' in that 400-800Hz range I mentioned.  This was meant to be a preamp >> power amp, my guess it, and I am building it to stand alone as a stomp.  B+ is about 300V  into V1.   

Now, add the EQ pedal - no boost, just to cut those freq's - and this thing is outrageous!    I know what to do re. general things, like larger coupling caps to increase bass, using cathode bypass for shelving, just not sure what to about those couple of 'issues'.   

Rolling off at 3kHz (drastic sounding, this must have a strong treble emphasis) is simple....1st order LPF.  It's the mid thang that has me stumped.  I need a -10dB cut but not with a very wide bandwidth..Fco would be 600Hz, but the edges at 400 and 800 don't seem to fit well with a bridged T or something.   Set the mids where I want, I'd be ok with just a bass + treble on the output for use with different amps.

[aron]

Can you use the Duncan tone stack simulator to get your EQ? That's what I did when I was looking for a particular EQ setting.

[GibsonGM]

Possibly.  Just curious about WHERE I should place some things, if that makes sense. Like - bass control after the first stage, cut treble at the end.

I'd like to understand more about what happens when you say, cut bass by 10db, but you have a mid cut later that overlaps.   I would assume those frequencies are just "not there" to cut on the L side of the mid scoop...what are 'the rules', that kind of thing.

But there are definitely some very good 'already done' stacks that might work as well!   I'm looking at the James right now.

[thelonious]

I came up with a really effective tone control the other day. I couldn't figure out why my amp sounded so dark... until I remembered I was wearing a hat that covered my ears.

A lot of people expect and even like the interactivity of a tone stack block; it's certainly part of the traditional tone stack feel. Also, it's predictable because you get all your loading/loss in one place and can feed the whole thing with a low-impedance cathode follower and stick a recovery stage after it. But in your situation I agree that separating them is probably a good idea. The end is a good place for the treble cut, since each overdriven stage can create odd order harmonics/fizziness. It can also be helpful to insert basic lowpass filters as you go along in order to control fizz and prevent oscillation. I agree that after the first stage would be a good place for the bass control, for the reasons you stated (getting rid of blocking distortion/fartiness). I guess the only thing I'd be worried about in a high gain pedal is if front-loading the tone controls would add any noise concerns---hiss from the resistors and from carbon pots, oscillation concerns from more having more components and wires up front, etc. But there's not really any way to know until you try, right? Maybe it won't be an issue.

One idea as an alternative to a fixed bass control is selectable coupling caps. You could make the first or second coupling cap selectable by putting a variety of cap values on a rotary switch. Check out http://tagboardeffects.blogspot.com/2014/04/black-arts-toneworks-black-forest.html

[J0K3RX]

A wise man pointed something out to me once... Preamps filter out the low end and the negative feedback loop around the power amp restores the low frequencies.
Have a look at these...

http://hypernova.net/Aiken_Files/Aiken%20Tech/www.aikenamps.com/NegFB1.gif

https://dl.dropboxusercontent.com/u/11178619/Ceriatone/Chupacabra2520.jpg

[midwayfair]

I think this might work ...

Output of tube---->--560pF-->-----12K---Grid of next tube
                          |              |           |
                          |             220K      |
                          |              |           |
                           ---120K---|         1n
                                         |            |
                                      4.7nF      ~22K (I think?)
                                         |            |
                                      Ground    Ground

The 560pF/120K/4.7nF/220K put a -12dB notch around 600Hz, right in the middle of your 400-800Hz.

Then the 12K, plus the 38K output impedance of the preceding tube and 1nF form a low pass at ~3KHz. The 22K is my best guess at -10dB, but I'm not actually sure if it should be that. I'm being lazy I guess.

Obviously this is a bit lossy, but it's not horrible. It's less loss than a Fender stack, for instance.

You could make the 220K a 250K pot and ditch the low-pass filter entirely, but I got the sense you were looking for something fixed.

[Quackzed]

Rolling off at 3kHz (drastic sounding, this must have a strong treble emphasis) is simple....1st order LPF.  It's the mid thang that has me stumped.  I need a -10dB cut but not with a very wide bandwidth..Fco would be 600Hz, but the edges at 400 and 800 don't seem to fit well with a bridged T or something.   Set the mids where I want, I'd be ok with just a bass + treble on the output for use with different amps.

you could also take a look at gyrators, theres even a calculator at muzique... either opamp or transistor, not sure if you could translate for tubes...
http://www.muzique.com/lab/gyrator.htm
you can basically tune them to be wide or narrow, they're used in alot of 'metal' type pedals like the boss metal zone, that need a narrow but large scoop or bump to give either a narrowish bass freq boost without muddiness or narrow treble boost without fizz etc...they dont need to be narrow, but they can do it... you could also take a look at the metal zone for some ideas on controlling the width (Q) or bass/cut etc...

[GibsonGM]

Nice! Thanks for the suggestions, guys.  Definitely in line with my thinking on it.   The thing sounds like I would expect - this is just fine tuning.  If I were a 'rack guy', I'd just run an EQ and have every possibility open, ha ha.

Joker - yeah, NFB does seem to bring back the lows - Fender being a notable example of some pretty classic uses of the technique.

Midway & Quackzed: yes, I'll probably end up with something fixed.  I'll look at the example, Midway, thanks!  I'm hoping to not have to add in opamps etc. - I looked at gyrators (which is what's happening inside the EQ, after all...), and they're a very neat & effective way to do this, altho then you're not being "true to tube" (whoopie doo, but...). 

Just that this thing sounds insane with the changes I made with the EQ pedal, and if they're fixed, the preamp I'm running into can deal with a little bit of bass or mid boost.  An FMV stack doesn't have the narrowness to do what I want, though.  Since I'll limit the output to around 5V, there is a TON of signal there to play with! 

[deafbutpicky]

Another approach at this:
lower frequencies have more power than high ones,so they don't need as much gain for sustain.
But ears are much more sensitive to higher frequencies.
So, for a balanced gain structure I'd cut bass early on in the circuit, mids later on and have a tone control for the
highs at the end.
Cutting highs slightly stage wise is a good idea to keep noise down and the first stage should have the most gain for the
same reason.
These guides don't even touch the idea of a good eq stage for tone shaping but should give you a good starting point.
Also sometimes it might be helpful to think about a bass boost as a cut of everything above the desired frequency if enough
gain is applied to the signal before hand (this is what negative feed back is sometimes used for at the end).

[J0K3RX]

Have you tried messing with the values of C3,C4 and C5? Also, perhaps a tweaked version of the original Soldano GTO tone controls?

[GibsonGM]

Yeah, I'm 'at that level' of thinking, Deaf, and you're 100% right.  People crank up, but keep their low end the same on stage, and it sounds crappy!  I was on the band about it just last night!    I know what NEEDS to be done, but not quite how to implement it, he he....

Joker:  i could see changing them out for 10n or something, but remember, bass isn't really 'the problem' here - I want to make a small notch at a 600Hz center.    I'm probably asking for too much...the bandwidth I'm looking at isn't very wide.   I'm being picky, and the tones I'm after ARE made in the studio - using EQ's and such.....you're probably right in that i need to pop in *some* tone stack and see what happens!  Might just take care of the issue on its own thru losses!   

I'll go thru the Soldano schemas I can find and see what looks like it might work.   Simple BMP tilt control doesn't sound good to me as it takes ALL the mids, some of which I like...

[Transmogrifox]

I looked at gyrators (which is what's happening inside the EQ, after all...), and they're a very neat & effective way to do this, altho then you're not being "true to tube" (whoopie doo, but...). 

No reason you can't make a gyrator with a triode.  Very nearly the same thing as the BJT version. 

It sounds to me like you go the perfect sound you liked with an EQ at the output.  Since you're working on stompbox signal levels, I personally would have just built an op amp mimic of the 2 relevant bands in the EQ and hard-wired them where it sounded best.  Then you can make use of a traditional tone control and pretend it never happened .

If you go messing with EQ between stages you can probably get where you want with this, but everything you do pre-clipping will have a nonlinear effect -- so for example a mid-cut pre distortion may remove the opportunity for some sweet harmonics generated from the original signal in that range while the nasty stuff you don't like is still cross-multiplied back into that range by neighboring bands.  A lot of experimentation needed to find out exactly where it's coming from.

I think the safest bet would be treble cuts between stages since harmonics from the first stage of clipping may be creating the fizz in later stages.

Just my "off-the-cuff" thoughts that come to mind

[krister]

I would shape the tone at the end to notch out what you want. Then make up the gain after that and send it to the output. The fizz is probably coming from pushing one the stages too hard.

[Philippe]

a tube pre-amp into an SS power amp never made much sense to me as clipping transistors towards/at the end of the line usually sound shitty...unless that is the tone/sound one is seeking. hybrid amp manufacturers (i.e. tube pre-amp to SS power amp) have been conning unsuspecting/ignorant buyers for over a decade...on the other hand, a solid state TS into a good tube amp is another story.

[GibsonGM]

a tube pre-amp into an SS power amp never made much sense to me as clipping transistors towards/at the end of the line usually sound shitty...unless that is the tone/sound one is seeking. hybrid amp manufacturers (i.e. tube pre-amp to SS power amp) have been conning unsuspecting/ignorant buyers for over a decade...on the other hand, a solid state TS into a good tube amp is another story.

I agree with you entirely, Phillipe.  I am using this to, essentially, change the preamp in my (tube) Hot Rod Deluxe from a blues rig to a more modern sound.   I'll ust run it into the clean channel. <yes, that will also affect things since the existing tone controls are there; they go pretty 'flat' if you tweak them, tho>     I don't like the stock distortion on them for modern stuff (too flabby, IMO).   I would never, ever consider pushing a transistorized amp - it sounds so crappy I don't even want to think about it.    However, if you keep the PA clean and don't clip it, you can get good preamp distortion using tubes and then just keep what you got, only louder, if you want to go that route...

The main purpose for doing this project is....to do it   

Yup, you are right, Krister...in getting the DISTORTION I want, I'm also adding a lot of top end fizz...can't see a way around that, altho I'll pop in an anode bypass and see what happens.   Also, yes, I'll just see what I can do at the output.

[petemoore]

  ROG, AMZ, many other places have lots of examples to draw from.
Everything from basic tone shaping [input cap, caps from SP to ground], to tone controls and two pole filters.
I ''would'' say cut the bass immediately [or early], if I was inclined to because ''everyone else does''. Generally, the narrower the passband the easier it is for any amplifiers, but easy might not be the ticket, though this can  be made to not matter in stompboxes much more easily than say a bass amp. Lions share of circuits use .1uf [input or staging caps] or smaller unless intended for bass registers.
Most schematics show a bass register created by staging capacitor values in a small window of Uf, but many other 'flavors' are created using the amplifier shortcomings as an advantage.
A look at the many FF derivitaves shows quite wide ranges between the input and output capacitor cutoff slopes, sometimes it's preferred to allow lots of bass in to 'swamp' the otherwise already distorting amp with larger current and voltage swings.
Often there's already a wire coming from the board/input cap, made large to begin with, add series cap can trim input bass rolloff.
A switch's lugs provide a place to mount a board splice-out, making a place at the switch to play around with values [but loosens up/spreads out the noise equation].
CISSS principle: Complicated Initiatives for Sound Systems Scrutinizing, KISS principle [keep it simple stupid], are kind of like a ying/yang discovery for tone tasters.