Tone Control Wiring question

[PCollen]

With some amps (Asteroid, for instance) the bass control center tap is shorted to the 'top' tap to create the variable resistor..the upper portion of the pot is shorted out when the wiper is rotated. On other amps (Express) the bass pot 'top' tap has no connection, same as the middle control pot, and so the upper portion of the pot is not shorted out when the wiper is rotated, but is effectively not there. So, what 's the difference between doing it one way or the other ? The results are the same, aren't they ?

[Ken Moon]

The difference only occurs if the bass pot fails (wiper open is the most likely failure of the pot).

In the first instance, the amp will act like the bass control is all the way up, and the treble and middle controls will still work.

In the second case, if this failure occurs, there is no path to ground for the treble or bass pot, and only the middle control will work (don't know what this would do to the sound).

So it's a fail-safe precaution, and I don't see any reason to leave the "top" open.

edit: looking at a couple of schemos, the effect will be different if the tone stack is followed by a gain pot or grid reference resistor, or if it goes straight to the grid of another gain stage. The explanation is still the same, but the effect could be different (if the bass pot fails) depending on the amp.

[PCollen]

The difference only occurs if the bass pot fails (wiper open is the most likely failure of the pot).

In the first instance, the amp will act like the bass control is all the way up, and the treble and middle controls will still work.

In the second case, if this failure occurs, there is no path to ground for the treble or bass pot, and only the middle control will work (don't know what this would do to the sound).

So it's a fail-safe precaution, and I don't see any reason to leave the "top" open.

edit: looking at a couple of schemos, the effect will be different if the tone stack is followed by a gain pot or grid reference resistor, or if it goes straight to the grid of another gain stage. The explanation is still the same, but the effect could be different (if the bass pot fails) depending on the amp.

Thanks, Ken. However, with the mid-pot having the "top" connection N/C, the same risk exists as with the bass pot ..no path to ground in the tone stack if the wiper opens. I just don't understand why it's done one way on the bass pot, and the other way on the mid. The Express does it the same on both..the "high risk" way with the top connection open on both mid and bass pots.

[Ken Moon]

with the mid-pot having the "top" connection N/C, the same risk exists..no path to ground if the mid-pot wiper opens. I just don't understand why they would do it one way on the bass pot, and the other way on the mid.

I don't understand that either. One possibility is that most builders/designers just don't think about it.

Having worked in test engineering for over 30 years, I always think about how circuits fail, not just how they work. When we get new engineers out of college, it takes a while for them to grasp the concept.

[PCollen]

with the mid-pot having the "top" connection N/C, the same risk exists..no path to ground if the mid-pot wiper opens. I just don't understand why they would do it one way on the bass pot, and the other way on the mid.

I don't understand that either. One possibility is that most builders/designers just don't think about it.

Having worked in test engineering for over 30 years, I always think about how circuits fail, not just how they work. When we get new engineers out of college, it takes a while for them to grasp the concept.

I just reviewed several BF Fender amp layouts (AB763/AB568), and find that Fender always left the 'top' of the pot connection open on the bass pot, and on the mid pot if the amp had one. Ken Fisher implemented the tone stack wiring in his Express and Liverpool the same way. If a bass or mid wiper opens, that wiper sees infinite resistance and the path to DC ground is lost. With the top and wiper shorted on both mid and bass pots, the open wiper sees that the pot is turned to 0 (max pot resistance), but the path to ground in the tone stack is maintained through the pot resistance.

[Ken Moon]

When you look at a TW schematic, it's pretty obvious that he copied the first two stages and tone stack from the typical Fender preamp.

Nothing wrong with that - please don't attack me for saying it

This little detail probably never crossed his mind.

[TUBEDUDE]

Is it possible that by connecting the end of the pot to the wiper you are creating a parallel path that, effectively changes the taper of the pot?

[Ken Moon]

Is it possible that by connecting the end of the pot to the wiper you are creating a parallel path that, effectively changes the taper of the pot?

Nope.