How do Amp Tone Networks Work?

[MistaGuitah]

From my understanding, not a lot of amp EQs truly operate in a way that it simply adds or takes away certain frequencies. It seems like amp tone networks have a sort of parametric effect on other tone controls, and some amps operate according to what a certain frequency is set to. From what I've been told, amps generally operate where the treble or mid knob setting determines who effective the rest of the tone controls are, and it a certain order. If that is the case then the respective control essentially controls the overall network so if it's the treble control, then a low treble setting might result in weaker mid and even weaker bass controls.

I'd like to know how this actually works. Does it have something to do with how the input signal is routed and then hits one tone control before another or something? I find this interesting as, for instance, Mesa Boogies have a peculiar way of EQ'ing that's quite different from dialing in a Fender amp. How does the tone network operate on say a 1959 plexi? Seeing that they have 4 inputs that emphasize higher or lower frequencies and input sensitivity, it's definitely more complex than an amp with 1 volume and 1 tone control. A lot of guys, regardless of which channels they jumper, seem to prefer the treble down but the presence up in a plexi. I assume this is because they need to be played loud to give up the goods so logically, the louder it's set the more the presence would come into play.

Anyway, I don't know if I have a decent understanding of these concepts but would like to know more. Can someone explain how this basically works?

 

[rstites]

If you want to play around with this, download the Tone Stack Calculator/Emulator from Duncan's page: https://www.duncanamps.com/tsc/

You can see the natural response of traditional amplifier tone stacks and then move the controls around and see how the frequency response shifts. Ask if you have questions. I can elaborate on anything you want out of it.

 

[BSRIAudio]

Almost all guitar tube amps use a set of passive filters that progressively dump frequencies to ground as a tone stack because they are relatively simple and allow for some tone shaping without adding substantial cost or complexity to designs. Truly understanding how they work requires a lot of background but this article is a good place to start:

https://robrobinette.com/How_The_TM... of the tone stack,Switch or Tone Cut" works.
If you have an opportunity to play an Ampeg V4 you can see what a tube amp with active tone controls sounds and feels like. Its a very different playing and tweaking experience.

 

[rstites]

The Ampeg V4 used a Baxandall Tone Control, which is the standard control used in HiFi audio, channel strips, etc. On the Tone Stack Calculator it's called the James. Same circuit. That'll let you take everything full circle between both recommendations above.

Yeah, the Baxandall is a bit different in a guitar amplifier since it can be a flat response, which traditional TMB controls don't do.

 

[BSRIAudio]

Tone stack calculator has multiple versions of the Bax and James both active and passive. The mid control on the V4 is the really compelling feature and is outside the scope of those shelving circuits. The B15 and a bunch of other ampegs are straight James though.

 

[Chubbs]

Outside of what others have said about the TMB tonestack you also have two other things to consider. For instance a mesa mark has its tonestack placed very early off of v1A with multiple gain stages after it. As compared to a Marshall 2203 which has its tonestack placed as late as possible on v2b right before the phase inverter.
The second thing to consider is a cathode follower vs plate follower tonestack. For instance a mesa and some other fenders use a plate driven tonestack where the signal to the TMB is taken off of the plate, compared to Marshall’s which take its signal for the TMB off of the cathode of tone stack tube. Which will effect the character of the EQ to a large degree.

 

[MistaGuitah]

If you want to play around with this, download the Tone Stack Calculator/Emulator from Duncan's page: https://www.duncanamps.com/tsc/

You can see the natural response of traditional amplifier tone stacks and then move the controls around and see how the frequency response shifts. Ask if you have questions. I can elaborate on anything you want out of it.

Neat tool

Almost all guitar tube amps use a set of passive filters that progressively dump frequencies to ground as a tone stack because they are relatively simple and allow for some tone shaping without adding substantial cost or complexity to designs. Truly understanding how they work requires a lot of background but this article is a good place to start:

https://robrobinette.com/How_The_TM... of the tone stack,Switch or Tone Cut" works.
If you have an opportunity to play an Ampeg V4 you can see what a tube amp with active tone controls sounds and feels like. Its a very different playing and tweaking experience.

This helped a lot. Thanks.

 

[rstites]

Tone stack calculator has multiple versions of the Bax and James both active and passive. The mid control on the V4 is the really compelling feature and is outside the scope of those shelving circuits. The B15 and a bunch of other ampegs are straight James though.

I'll go look at it again. I missed anything different from the standard issue Baxandall in the schematic when I looked at it. Sounds interesting. I've played one of those a few times, years-and-years back, but don't recall getting around to do much with the tone contorls.

 

[mnemonic]

Web version of the tonestack calculator is quite useful also, no install required.

https://www.guitarscience.net/tsc/info.htm
Most modern amps use some variation of the Marshall tonestack