A Question about system design

[pbman1953]

You all have helped me with tech support so I know you can chime in here on my question. The bass control on the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound.


My Sunn 2000s single volume is right after the input. Looking at the Traynor YBA-3,same thing. The volume is after the input but before the v1 input. The master volume is way down the chain right before the power tubes



Shouldn't the volume be the only item in the middle of the tube and have the bass control after the tube output? I wonder if anyone moded that?

[AlNewman]

I think it's because both the bass and mid controls are wired as variable resistors.
You could wire it like a Marshall tone stack with the wipers making voltage dividers, and it would react like a Marshall tone stack.

The cool thing about that amp is the active eq.

[pbman1953]

What would be the benefit of having this tone stack?

[AlNewman]

Perhaps more versatility...
And it's always fun to forget your tone knobs are down and think your amp is broken.

[HotBluePlates]

... the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  ...

The Volume control is hooked to the output (wiper) of the Treble control.

... on the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  My question is why ...

You may be focused on these two models, but that tone stack (and the Volume control's position right after the Treble output) is the same on nearly all Fender amps from the mid-60s onward.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound.  ...

Look up a "Bridged-T filter."  Its function is to impose a mid-scoop on a sound, while leaving the higher & lower frequencies less-reduced.

The standard Fender tone stack is sort of a modified Bridged-T filter, with some parts made variable to adjust the mid-scoop and overall sound of the amp.

The Bass control is a variable resistor, and sits between 2 caps whose other end are connected.  When you "turn the Bass up" you really make the 0.1µF cap appear in the circuit as though it were a 0.147µF cap.  That's because the resistance between the caps falls to zero, the caps are placed in-parallel, and the total capacitance goes up.



One form of a Bridged-T filter would have a cap-to-ground.  But sometimes the user doesn't want to "scoop all the midrange out" so they add a resistor between the cap & ground.   That's the "Middle" control in this circuit.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound. ...

What would be the benefit of having this tone stack?

If you were playing guitar rather than bass, it would be more obvious.
The standard Fender tone stack is not much use for bass, though even with guitar turning down the Middle and Bass controls (and having the Treble control below half) just about zaps all the signal.  But that's not really intentional, but just a byproduct of this circuit being arranged for convenience, low cost and low parts count.

[pbman1953]

... the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  ...

The Volume control is hooked to the output (wiper) of the Treble control.

... on the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  My question is why ...

You may be focused on these two models, but that tone stack (and the Volume control's position right after the Treble output) is the same on nearly all Fender amps from the mid-60s onward.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound.  ...

Look up a "Bridged-T filter."  Its function is to impose a mid-scoop on a sound, while leaving the higher & lower frequencies less-reduced.

The standard Fender tone stack is sort of a modified Bridged-T filter, with some parts made variable to adjust the mid-scoop and overall sound of the amp.

The Bass control is a variable resistor, and sits between 2 caps whose other end are connected.  When you "turn the Bass up" you really make the 0.1µF cap appear in the circuit as though it were a 0.147µF cap.  That's because the resistance between the caps falls to zero, the caps are placed in-parallel, and the total capacitance goes up.



One form of a Bridged-T filter would have a cap-to-ground.  But sometimes the user doesn't want to "scoop all the midrange out" so they add a resistor between the cap & ground.   That's the "Middle" control in this circuit.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound. ...

What would be the benefit of having this tone stack?

If you were playing guitar rather than bass, it would be more obvious.
The standard Fender tone stack is not much use for bass, though even with guitar turning down the Middle and Bass controls (and having the Treble control below half) just about zaps all the signal.  But that's not really intentional, but just a byproduct of this circuit being arranged for convenience, low cost and low parts count.

After I asked the question, I researched more and yes, this tone stack was very common to Fender.  Thanks for the detailed write up!