Tell me about Tone controls in tube amps?

Most amps have tone controls of some sort (a notable exception is the venerable tweed champ--nothing but a volume!).  These vary from a single control to full tone stacks and some amps even have Presence and Resonance controls. So what effect do the controls have on the sound and an amp’s versatility?   Can tone controls be easy added to a design?

Types of Tone Controls:

Single Tone Control: these generally work but rolling off the treble as the knob is turned down.  They tend to be highly interactive with the volume control. This type are simple enough that they do not cause much signal loss so they do not need additional gain stages to drive them.  These types of controls may have less versatility but have little signal loss so an amp with this configuration has great pure tube tone.  Every guitar really sounds like itself.  Generally only lower gain amps use this type because the more gain the more necessary full tone controls become.

Bass, Middle and Treble or Bass and Treble Controls:

They are a bunch of different circuit types here that differ in how effective they are but basically they all allow the user to shift the frequency response of an amp somewhat. Curiously the Bass and Treble configurations often use a fixed resistor in place of a Middle pot and not really different from a Treble, Mid and Bass setup. These full stacks are more versatile but cause greater signal loss so it takes an additional gain stage to make up for the loss. This results in less tonal purity and touch sensitivity than a simple tone control. Interestingly you’ll see some amps have a pot or switch labeled RAW. These controls simply allow the player to bypass the tone controls for a purer sound. So a tone controls cannot simply be added they require a tube gain stage which means adding a tube!

Where the controls are placed in the circuit has an affect on the effectiveness of the controls.  For example a Blackface Fender has the controls early on in the circuit so the controls are very effective. With the controls all the way down there is no volume at all.  In many Marshall amps the tone stack is after every thing else in the pre-amp. Users often wonder if the tone controls in a Marshall are working because they have a smaller affect on the sound.

 Tone control placement has an effect on the sound of an amp that has lots pre-amp distortion is present. If the tone controls are early on in the amp the controls with affect what the signal looks like that gets distorted. If they are after the distortion is generated they are instead affecting the already distorted signal. This sounds different. Generally amp with controls later on sound harder and meaner with those with the controls earlier sounding smoother and less aggressive. 

One more thing to consider is how they are implemented. With going into gross detail some tone controls are plate fed and some use what is called a cathode follower. The cathode follower variety adds more harmonics and give the amp a crunchier sound. This is the type found in most Marshall's and the plate fed type is more common to cleaner amps like a Fender Twin Reverb.

Another thing to consider is how effective tone controls will be when the amp is overdriven.  With amp with abundant pre-amp distortion the tone controls are very important and effective in getting a good distortion sound.  In amps that get more of the overdriven tones from power tube break up the controls are less effective.

Presence and Resonance Controls:

Presence and Resonance Controls work on the same principle only Presence is a type of treble control and Resonance is a type of Bass control. They are both only found in amps with negative feedback. Negative feedback is when a small part of the amps signal is fed into the output. This smooths out certain frequencies and increases bandwidth but reduces gain.  These control simple adjust how much bass or treble content gets fed back.