With all the modeling going on these days I thought it might be of value to show the transfer curves of the tone stacks in Fender, Marshall and Vox AC30TB tube amps. I hear complaints that it takes too long to get a sound without twiddling for hours with the modeler. I think one reason people have so much trouble is the amp they are using is coloring the tone with its own tone shaping curves. To help alleviate that you need to understand the tonal shape of the amp you are trying to emulate and the tonal shape of the amp that you are driving with the modeler. Tone is in the ears of the beholder. I hope this will offer some help in finding that elusive tone. So here we go with an example. I use a Line 6 Pod XT Pro setup to emulate an AC30 TB. My tube amp is a Black Heart Little Giant with a 12ax7 and an EL84 and it has a Marshall Tone Stack as shown below. It is a great little straight up basic tube amp, not much headroom though. If I set my amp tone controls to the center position and drive it with the Line 6 AC30 emulator the sound I will hear is going to have the lows and highs boosted un-naturally, because the Line 6 is producing the tone shape of a AC30 with boosted lows and boosted highs. That boost is added to the boost my Marshall style amp produces if I have the tone controls on the amp set to the center. So to fix this I use the chart below that shows how to set the tone control position for a flat frequency response from a Marshall tone stack. I set my amp like that with Bass on 1 and Treble on 1 with Mid all the way up. Now it sounds more like an AC30 and I can work with it using mostly the controls on the Line 6.
The following graphs show you the real positions of the tone knobs to get a flat response. Look at the lower left corner of each graph and you will find the tone control position.
If you add the boost of the AC30 emulation from the line 6 to the boost of the Marshall tone stack it doesn’t sound very good. So I set my Marshall style tone controls to have a flat frequency response like the graph on the left.
Now the Line 6 is doing it’s thing. My amp is not coloring it all up and I have a pleasing sound that I can work with. In my case it works great and now I have a known starting position.
For your reference,you should notice the significant volume cut in the Fender and Vox flat positions.
Here are the tone control positions and frequency response of a Fender Tone stack.
Here are the tone control positions and frequency response of a AC30 TB Tone stack
The bottom line is, if you are using a modeler to emulate an amp you need to provide a flat frequency response in the amp it is driving or you are going to get some cut and boosted frequencies that just don’t sound right.
There are still many variables that are not taken into consideration such as the tone shaping circuits following the tone stack in your amp, phase inverter, output tubes, transformer, speaker etc. But the tone controls are your interface and provide you with 80% of what you need. Setting your amp flat first will give you a known starting position to twiddle from. And you should be able to get the tone you want much more quickly from a known starting position. Another thing to remember about modelers- they are not meant to give you the exact sound of the different amps out there- it is impossible to do that. You can’t make a Bassman sound exactly like an AC30 with Alnico Blues. But they do give you a lot of versatility and give you sounds that are in the ballpark. Stomp boxes are great too. All of these things have their place. Rock on!