Tone stack vs. preamp filtering - which has a greater effect on the sound of an amp?

[tjs]

I know, I know...it's the circuit as a whole that defines the sound of an amp and not specific individual sections. That said, I was wondering which aspect generally does more to define the sound of an amp: the tone stack or the other components involved in preamp frequency filtering (particularly the cathode bypass cap and coupling caps).

For example, let's start with a typical Blackface Fender circuit. Then suppose we were to perform the following modifications:

1) Change the tone stack to typical Marshall values, keeping the rest of the circuit the same.

2) Change the cathode bypass caps and possibly the coupling caps to typical Marshall values, keeping the rest of the circuit the same.

Gain aside, how big of a tonal difference would each of those changes make?

Another question: suppose I had an amp with a Blackface-style clean channel that cascaded into multiple Marshall-style gain stages when switched to OD mode. What would the overdrive of such an amp sound like? Would the Blackface input stage dominate or would the Marshall gain stages impart more of a bite to the tone? Where would the OD sound lie along the Fender to Marshall spectrum?

[Blue Strat]

Is this a rhetorical question?

Try everything. Everything listed makes a difference. How much will depend on who you ask.

There's a lot that goes into the "Marshall sound" besides the tone stack and coupling caps. If all you look at are these 2 aspects, everything you've listed matters.

[BJF]

Hi,

You might want to go about an amp more like this:
Hm, a little too much bass.........where do we adjust bass?
Ah, too much treble..........how can that be lessened?
Not quite or definately not the desired distortion.......where is distortion created and how can it be adjusted?

There'd be a lot more to do to make your Fender sound like a Marshall, but the biggest step would be the speakers.

Consider the Miles Platting amp- it has a Fender style preamp and a Marshall style power amp and doesn't sound like either.........

Just a few thoughts

Have fun
BJ

Affiliations
www.mpamp.com (http://www.mpamp.com)
www.bjfelectronics.com (http://www.bjfelectronics.com)

[tjs]

Is this a rhetorical question?

Not so much rhetorical as hypothetical. I'm not looking to mod a specific amp at this point, and in any event I certainly wouldn't try to turn an amp into something completely different from what it is. I'm just trying to get a conceptual idea of how the various parts of an amp's circuit interact and what sort of effects changes in different areas might have. There are a number of places at which to tweak bass or treble frequencies; what I'm curious about is how adjusting them at one point in the circuit differs from adjusting them at another.

For example, blackface Fenders are known for farting out in the low end when driven. Suppose I had an amp based on such a circuit (I wouldn't go near an actual BF with a soldering iron, of course), and I wanted to minimize this problem while maintaining as much as possible the characteristic blackface clean tone. How would I go about doing so? Would I decrease the cathode bypass cap at the first stage or increase the slope resistor/decrease the bass cap in the tone stack? Or would I decrease bass at one point and increase it at another?

I realize that one could write an entire book about this sort of thing and that the best way to learn is by doing. I'd just like to get a basic idea of how someone would go about determining where/what to tweak in a circuit to achieve a desired effect. I've read a lot of books and internet resources and while I've learned a lot about how adjusting various components will affect the frequency response, none of them go into the thought process behind deciding where to make these adjustments or how making them at one point will differ from making them at another.

Thanks.

[BJF]

Hi,

Originally posted by tjs
"For example, blackface Fenders are known for farting out in the low end when driven. Suppose I had an amp based on such a circuit (I wouldn't go near an actual BF with a soldering iron, of course), and I wanted to minimize this problem while maintaining as much as possible the characteristic blackface clean tone. How would I go about doing so? Would I decrease the cathode bypass cap at the first stage or increase the slope resistor/decrease the bass cap in the tone stack? Or would I decrease bass at one point and increase it at another?"

Well, then giving the reverb mixer and reverb recovery stages seperate catode resistors ( this would need to be scaled) and then using a 4u7 elko for each would go a long way and if you'd then adjust the other catodes in the preamp in a similar way farting might be almost gone..........while amp would sound just the same at lower levels

Consider that a 22uF Elko from this period can have pretty wide tolerans ( deviation from stamped value).
You'd then find amps with varying capacitors and the ones with lower catode cap values sound tighter.

Oh, is it vintage or does it sound good?:crazyguy

Have fun
BJ
Affiliations
www.bjfelectronics.com
www.mpamp.com

[ChickenLover]

I'm not sure if it's what you're asking but VERY generally speaking, for distorted tones you'd prefer to cut bass early in the preamp to prevent farting or just a muddy mess. But distortion generates harmonics that sometimes can sound buzzy or sound too bright so you'd like to cut treble late in the circuit.

Look at a Marshall preamp (bright channel that is)...smaller couplers and smaller cathode bypass caps which helps the distortion stay tight and not overly muddy. But since a non-MV Marshall gets much/most of the distortion in the PI, often you need to run the bass control at low settings. That often cures the muddy/fuzzy low end but then since you don't have any tone controls after the PI you can't dial out very much high end.

Also, the Marshall tone stack is driven by a cathode follower as opposed to a regular gain stage in a BF Fender. A cathode follower has a lower output impedance so the tone stack tends to load it down less. That cathode follower driven tone stack at the tail end of the preamp is a big contributor to the Marshall sound and is often preferable for distorted tones. Other amps with a cathode follower driven tone stack at the tail end of the preamp include JCM800 MV Marshalls, Soldano SLO, Bogner Ecstasy Red and Blue channels, Mesa Dual Rectifiers...you get the idea.

There was also a recent thread regarding Dumble iterations over the years and it appeared that some of the more experienced Dumble (or clone) users preferred the HRM circuit which adds a tone stack at the tail end of the dirty channel preamp (not cathode follower driven though). I'm pretty sure glaswerks contributed to that thread.

One last thing...even if the distortion is happening well 'downstream' of the first gain stage...that first gain stage usually has a HUGE affect on the quality of the distortion. Usually you need to cut bass but having a bunch of treble there can be OK since you're going to be cutting that later on anyway...that extra treble can help with having a crisp attack. Some high gain amps have a pretty huge 'bright' cap across the gain control...big enough that it would slice your head off if it were in a BF Fender.

[tjs]

One last thing...even if the distortion is happening well 'downstream' of the first gain stage...that first gain stage usually has a HUGE affect on the quality of the distortion. Usually you need to cut bass but having a bunch of treble there can be OK since you're going to be cutting that later on anyway...that extra treble can help with having a crisp attack. Some high gain amps have a pretty huge 'bright' cap across the gain control...big enough that it would slice your head off if it were in a BF Fender.

So ideally, if you wanted to have a single channel amp that got a big clean sound but didn't fart out in overdrive, you would want to cut bass before the OD stages and increase it after them (which would require that the distortion be generated after the first stage and before the PI)?

[tjs]

Also, some amps (usually BF/SF Fenders and amps inspired by them) overdrive in such a way that the midrange stays extremely clear. What causes that? Is it the location frequency-wise of the mid cut in the tone stack?

Thanks.

[ChickenLover]

tjs,
I didn't really say to increase the bass after the OD stages...there's usually plenty of lows in a guitar amp power section. Sorry to cut it short here but IMO...you just can't have your cake and eat it too. If you want a big clean tone...there's one amp or one channel. If you want a 'good' distortion, there's another amp or channel.

But I should point out that my favorite distorted tones are very different from some others. Some people like it wild and nasty...the more 'Neil Young-ish' it is...the better. I'm at the other end of that spectrum. I want it like the best Marshall, rodded-Marshall, or Dumble or some combination of those.

[mark norwine]

What causes that?

See, it's just not that simple.

Here's a bad analogy:

Bite into a pizza..."hey! what causes that spicy taste?" you ask. I answer, "Tangy red sauce & red peppers."

But amps are not that simplistic.

Your question is akin to asking, "Acetone....CH3COCH3 Which Hydrogen atom gives acetone its solvent abilities"? I answer, "it's not any one single atom....it's the sum of the parts. Take away one H atom, and you no longer have acetone at all."

In an amp, there's certainly architectural "building blocks" at work, but there's no "magic bullets" to which one can point and say "that component gives it a marshall sound", etc.

Bad anaology....but maybe it makes sense....

[ChickenLover]

In an amp, there's certainly architectural "building blocks" at work, but there's no "magic bullets" to which one can point and say "that component gives it a marshall sound", etc.
So true...I know I've certainly built prototype preamps using 'building blocks' from this circuit and that circuit that sounded so incredibly awful when I built them. Other times I've just looked at a schematic and thought; "How can this possibly sound good."...yet it does.

[tjs]

Thanks for all the responses, everyone.:)

See, it's just not that simple.

Here's a bad analogy:

Bite into a pizza..."hey! what causes that spicy taste?" you ask. I answer, "Tangy red sauce & red peppers."

But amps are not that simplistic.

Your question is akin to asking, "Acetone....CH3COCH3 Which Hydrogen atom gives acetone its solvent abilities"? I answer, "it's not any one single atom....it's the sum of the parts. Take away one H atom, and you no longer have acetone at all."

In an amp, there's certainly architectural "building blocks" at work, but there's no "magic bullets" to which one can point and say "that component gives it a marshall sound", etc.


I see I'm not the only one with pizza on the brain after reading that thread in the Pub.:D

I guess a better way of phrasing my question would be, "Generally speaking, what contributes to such a tone?"

[jca345]

I created tone controls that allow the preamp to compliment and interact the PI and Power amp in very subtle and "right" ways. What this essentially does is voice the amplifier from predominantly classic American styles to British styles with little effort. You can actually shift between a very pumped up Fender, to a jangly AC-30, to a demonic Plexi type of sound simply by playing with guitar and amp volume, hi/lo, lean/plush, treble, bass, and the trim control. Nothing changes in the cathode bypass, stage coupling, or basic preamp toplogy. It is a simple matter of frequency response, drive levels, and overall touch sensitivity. That is where the magic of most deisgns lives. In otherwords, most guys beat their heads against the wall designing a power amp to compliment their preamps. I went the other route--designed a preamp to compliment the power amp and what I wanted to accomplish with it. Doing so will allow you to get the maximum potential out of a particular set of tubes--in this case 6V6's. Each amplifier is a "study" in a particular tube and design topology to me.

Granted the bypass and coupling values themselves are extremely important--and allow the amp to breathe properly, it's the levels, freq. response, and feel factor that goes from one extreme to another.

:BEER

[John Phillips]

Consider the Miles Platting amp- Wow! You've even heard of them :).

A friend I do a lot of work for has two of them (probably both dead at the moment)... I think one has a matching cabinet.

Interested? ;)

[BJF]

Wow! You've even heard of them :).

A friend I do a lot of work for has two of them (probably both dead at the moment)... I think one has a matching cabinet.

Interested? ;)

Hi,

Oh, I have serviced one. I guess Sweden where one of the few countries those where exported too. Might find a Slade fan to buy those you know considering Slade endorsed them at the time. Wohoo matching cabinett! Nice

Have fun
BJ

[fakeox]

Wow, shades of Ampage!