What are the differences in tonestack circuit placement?

[Gaz]

As far as plate-driven tonestacks are concerned, do they normally come after the first gain-stage for a reason, rather than after multiple stages? I'd like to learn more about EQ placement in a typical guitar preamp.

So, I do have a specific reason for this inquiry: I currently have a Plexi-type build 'staged' like this:

paralleled triodes-->triode-->plate driven Baxandall stack-->'gain recovery' triode-->phase inverter...

Would there be a tonal change if I were to move the stack after the paralled stages? I realize there would be less gain preceding the EQ, but I'm not sure how it would effect the tone.

Thanks.

[Chuck H]

This is a question with complex answer I don't posess all the knowledge to cover. But I'll go over some basics that I do know.

The typical BF type circuit with the TS right after the first triode is a great 'clean' amp. It allows for amplification of the signal prior to the TS so the circuit can accept some losses and still allow for changes in dynamics. But... With a high gainer that has multiple cascade triodes placing the TS early in the circuit means that once the signal is overdriven any effect the TS may have had on the final EQ gets lost in a square wave. In that instance the TS reacts more like 'distortion character' circuit than an actual EQ circuit. This isn't all bad. Trainwreck amps are designed this way and they enjoy great desirability. I have a recent design that is similar and I really think it's a winner. But it's important to regognize that the final EQ of the amp will be controlled by design aspects other than the TS in this instance.

If it hasn't become obvious already, placing the TS later in the amp allows for more control over the final EQ at the sacrifice of not having the ability to alter distortion character. If you look at some of the cascade type high gainers with a master volume you will see that the usually place the tonestack at the end of the chain right behind the master volume. That way the EQ set by the TS isn't lost via overdrive. It's important to note at this point that these are usually channel switching amps. So a 'clean' channel is available. The distortion tones all tend to have a very similar character in these amps.

Basically, if you plan to overdrive the amp, the earlier you place the TS in the circuit the more effect it will have on the distortion character and the less effect it will have on the final EQ tone. The later you place the TS the less effect it will have on the distortion character and the more it will have on the final EQ. If you don't plan to overdrive the amp much it is not nearly as important.

Chuck

[cbarrow7625]

Nice explanation Chuck!

[chipaudette]

Yes indeed, good explanation.

To continue Chuck's line of discussion...here are some examples:

Tone Stack Early: Blackface/Silverface Fenders, Trainwreck, Mesa Boogie Mark series
Tone Stack Late: Most Marshalls, Soldano, Bogner, Peavy 5150, Mesa Boogie Rectifier series

And, to amplifiy (ha!) what Chuck said, putting the tone stack early can indeed give you tremendous control over the characteristics and feel of the high-gain distortion. But, it can also make the amps fiendishly difficult to dial-in a good tone. This is a common complaint of the Mesa Boogie mark series of amps. People always talk about how hard they are to use. Of course, the dual gain controls and all those pull pots don't make it any easier. I see it as being the same as that typical trade seen in everyday life...if you give me more freedom, then I've got just as many opportunities to screw up as I do to get it right. Some people like having the extra freedom and some people don't see the value. I'm a tweaker...I like the freedom...color me the libertarian amp user.

Back on topic...

Of course, putting the tone stack late in the circuit doesn't let you avoid doing some difficult tweaking. Nope. It's still VERY important that SOMEONE shape the frequency response of the guitar signal prior to the distortion stages in the preamp. If you just send in the full-width guitar signal, the resulting distortion is often tubby and unfocused. Ick.

So, if you put the tone controls late, the amp designer must decide what to do with the early part of the circuit. The amp designer must hard-wire in some tone circuit elements in place of the now-missing user controls. Therefore, it's now YOU -- the amp designer -- who has to make the decision in place of the individual user. YOU have to now decide what tone shaping will be right regardless of who plugs into your amp and regardless of the type of guitar or the type of pickups. Wow, that sounds like a hard task. Luckily, you've got a lot of good sounding Marshalls (and other amps) to borrow ideas from. And, if you're only building the amp for yourself, that makes your job a lot easier, too, because you're only dealing with your tastes and your guitars.

Of course, you can always try to get the best of both worlds...you could have TWO tone stacks. You could have one tone stack early in order to control the character of the distortion and you could add another tone stack late in the circuit to control the overall EQ and tonal balance. This is why Mesa put the Graphic EQ into their Mark series...the tone knobs are early in the circuit and the Graphic EQ is late. The tone knobs control the character of the distortion while the EQ controls the final tonal balance (ie, the thump, scoop, and sizzle).

In my own high-gain amp, I have a treble and bass control right after the first gain stage (a la Fender). Then, I added a treble and bass control (plus a make-up gain stage) at the end of the pre-amp to control the final sound of my pre-amp's distortion. Works great! Plus, I've got more knobs...and, for me, more knobs are better.

Good luck!

Chip

[daz]

A good example would be the way a EQ works at an amp's input or at the effects loop if it has one. Another example you can easily try is a distortion pedal and an EQ pedal and try them both ways....EQ before, then after the distortion pedal. One way you are EQing the guitar and the other way you are EQ'ing the distortion. EQi'ng the distortion allows a far greater range of character change, tho if you have a thin sounding guitar it won't help with that the same way as a pre distortion EQ does. Of course there IS a gain stage B4 the EQ in the fender amp example, but you get the point. I'm not sure how the two would vary for clean tones, but if Chuck says the earlier fender style is better i'll take his word. Besides, no one would dispute fender's reign as king of clean.

[Gaz]

Thanks for the informative responses everyone. In light of all that info, I think I'm going to leave the 'James' stack where it is - post distortion - because I favor more control of the square waves.

The James stack is definitely giving me a lot more than a standard FMV stack, especially with the addtion of a mid-boost switch as Adam Alpern describes on his tonestacks site.

Thanks again.

[pentatone]

I hope noone is offended by my hijacking but maybe this topic can enlighten me.
Last year I soldered me a Gibson GA40 clone without tremolo with one tone knob like shown in the schematic below. I am really happy with the amp but still I am wondering why the tone pot has no significant effect. If I understand the schematic correctly it looks like the tone regulation only works for one half of the PI and only one power tube.

Maybe someone of the experts can bring some light into my observation. Is it systemimmanent that the tone knob doesn't do more?

Thanks and excuse me for hijacking.

GA40 schematic - http://www.harpamps.com/gibson/pg_0181.jpg

[Steve Conner]

Pentatone: This tone circuit is a bit unusual but it should affect both sides of the PI. The reason is that with this PI circuit (called "floating paraphase") the second triode is driven off the plate of the first one.

The output impedance of a 12AX7 stage is less than 100k, so I expect the 1M tone pot will do very little over most of its travel, but it should kick in eventually. If it really does nothing, maybe you've connected it to the wrong side of the 12AX7? It should be the one whose grid is fed by the input signal.

Chuck H: +1, I like this explanation too! I'm a fan of the two tone stack concept, one at the beginning to set distortion character and another at the end to set the actual tone. Knobbage permitting, as it means more knobs, and it's hard to decide what to call them: one of my early Toaster prototypes had them labelled "Fart" and "Sizzle".

Although if the power tubes are getting overdriven to hell, the second tone stack is "distortion character" too.

[pentatone]

Steve -

thank you for your response.

"... so I expect the 1M tone pot will do very little over most of its travel, but it should kick in eventually."
That's exactly what it does. I like the sound of the amp so I see no reason to change the "tone stack" but I am happy now that I can stop chasing for an error.

[Chuck H]

Although if the power tubes are getting overdriven to hell, the second tone stack is "distortion character" too.

+1, ABSOLUTELY.

Ever since realizing the benefits of power tube distortion I have been morphing my build strategy toward controlling the final EQ with the power tube type (@ a given voltage), speaker/s, NFB loop, OT and cabinet. The only trouble with this approach is that it offers little adjustability (mic at a different angle or placement, different speaker cab, cab tilted, elevated or on the ground). Even a presence control can become more like a "character" control when your over driving the piss out of the whole amp. Still, I get more personally satisfactory results this way. I don't do the dual tonestack because I like things simple. Fewer knobs is better for me. As long as I don't feel a big need to have variable control over a given parameter, I hard wire it fixed at a considered level.

And for those proponants of the dual tonestack... I think A. Dumble would agree with you (look at the ODS schematic)

Chuck

[Gaz]

I guess I've inadvertently gotten the best of both worlds by placing my tonestack smack in the middle of the circuit...

As I mentioned, I'm currently using a James stack and have been pleased with the results. From the amalgamated opinion of every possible thread I could find on the Internet about the James stack, it seems as though it's not favorable in high gain amps, or even guitar amps in general. But I'm normally running my master volume on 10 (as if it didn't exist), and find the James' controls do a wonderful job of attenuating frequencies before getting sent to the output section, shaping the character of the power tube distortion.

I also have a presence and hi-cut (ala Vox AC30) to tame the power tube distortion and to keep the amp 'listenable' at such high db's, which has always been my design goal.

While I'm rambling: Do either of the brilliant ampage heavy weights currently posting in this thread know a cleaver way to effectively shift the 'mid point' of a James stack? My mid boost is more of a mud boost

Thanks,


Alex

[Steve Conner]

Chuck H: Yes, I got turned onto the two tone stack idea from looking at Dumble schematics in the first place. Or maybe it was Mesa or whatever, they all have a shared ancestor in Randall Smith's daisy-chaining one channel of a Fender into the other for more gain (which puts two tone stacks into the circuit)

The midband of any two-knob tone stack is just the frequency range that's not affected by either the bass or treble network. So you can't change it independently. To move it up, you'd need to increase the bass and treble break frequencies, and that may change the tone in ways you don't like at other knob settings.

[Gaz]

Thanks. I meant to write, shift the apex of the mid scoop or percieved boost, not it's volume. Mine is currently centered too low in the frquency spectrum, which is great for a scoop, but not the perceived boost, only accentuating muddy frequencies. Perhaps this is why FMV stacks don't really do a boost; it just muddies things up.

I've also included a mid boost switch that adds a cap to create a mid hump when the controls are flat. Additionally, I've added a resistor in parallel to the 'hinge' resistor (the one in the middle of the stack) attempting to shift the hump forward in the frequency spectrum into the high-mids, but to no avail.

Is there a way with the James to shift the hump forward? I'd love to be able to get a little more 'bite' out of the mid switch. Thanks for any ideas.

Perhaps I could lower the treble into high mids? I do have a presence control which takes care of the high-highs...

[Steve Conner]

Thanks. I meant to write, shift the apex of the mid scoop or percieved boost, not it's volume.

That's what I thought you meant, and my explanation addresses that.

Mine is currently centered too low in the frquency spectrum, which is great for a scoop, but not the perceived boost, only accentuating muddy frequencies. Perhaps this is why FMV stacks don't really do a boost; it just muddies things up.

As I thought I tried to explain, the Baxandall/James stack can only boost the same frequencies that it scoops. In fact it doesn't boost or scoop midrange at all: the bass and treble knobs move the high and low ends of the spectrum up and down, while the middle stays where it is. So you can't have the peak of the "mid boost" curve (produced with bass and treble = 0) at a different frequency than the bottom of the "mid scoop" you get with bass and treble set to 10.

Well, maybe you can, but I don't know how. Possibly a shift switch that swaps all the capacitors for smaller ones?

The regular Fender/Marshall stack can't do a mid boost. With the bass and treble at 0 and the mid at 10, the frequency response is flat, and all other settings produce a mid scoop. Only the Bax/James can do a mid hump.

I've also included a mid boost switch that adds a cap to create a mid hump when the controls are flat. Additionally, I've added a resistor in parallel to the 'hinge' resistor (the one in the middle of the stack) attempting to shift the hump forward in the frequency spectrum into the high-mids, but to no avail.

I'm not familiar with any of these mods, can you post a link to schematics? And what way is "forward" while we're at it? I'm assuming it's up.

Is there a way with the James to shift the hump forward? I'd love to be able to get a little more 'bite' out of the mid switch. Thanks for any ideas.

The mid shift switch I mentioned above is the only idea I can think of. Hope this helps, or maybe someone more familiar with the Bax/James can help. I seem to remember a web page somewhere with these James mods and frequency response plots done on the Duncan tone stack calculator.

For mid boost, I prefer a LC resonant circuit like the Ampeg SVT midrange.

[Gaz]

There's a post I made earlier in the thread with a link to the James stack and mid boost switch I used, and I think it's from the website you referred to. Thanks for the help, and yes, maybe someone with more experience using the James stack might chime in... in the meantime, I'll check out that SVT mid control.