achieving desired voltage

[MmmPedals]

You can get to a given voltage using a voltage divider (two resistors dividing the current), zenner diodes or voltage regulator just to mention 3.
Any reason to use one over the other?
Why do the many designs i have seen for emulating tubes with FETs (dr. boogie, umble etc.) use trim pots as apposed to a voltage divider? Wouldn't that make for a simpler build?

[potul]

well... these trimpots are basically voltage dividers that can be adjusted. It is normally needed to adjust the bias of the FET. Once adjusted, they could be substituted by equivalent resistors.

Potul

[JKowalski]

(two resistors dividing the current)

Remember it's two resistors dividing voltage only here, they are in series so they share the same current.

Voltage dividers are not friendly towards any decent sort of load (they are high impedance), unless you use very low value resistors to make the impedance low and at that point you are wasting alot of power in the voltage divider before it even gets to the load. Voltage dividers are for situations where you don't need very much current at all (like reference voltages).

Zeners are good references, they are low impedance kind of dividers, they will not fluctuate much with load current, as long at the current through the diode is kept higher then a certain amount (to keep it ON). They are also useful because they will not change output with changes in supply voltage (voltage dividers will).

Regulators are best for supplying power to circuits, since standard ones can do 100mA to a couple amps without appreciable change in voltage. For power a regulator is what you want. Overkill for high impedance reference voltages.

Why do the many designs i have seen for emulating tubes with FETs (dr. boogie, umble etc.) use trim pots as apposed to a voltage divider? Wouldn't that make for a simpler build?

Generally sticking trimpots everywhere is kind of a sloppy design strategy. Most cases there is no need for the control and you should just put those resistors down and not mess with them again. In my mind having a ton of trimpots just makes the design MORE complicated.

[MmmPedals]

well... these trimpots are basically voltage dividers that can be adjusted. It is normally needed to adjust the bias of the FET. Once adjusted, they could be substituted by equivalent resistors.


Potul

I meant to say it would be a neater build to do the math for correct value resistors for a voltage divider and leave out the trim pots. in the circuit as they are in the above mentioned designs they are in series with the fets so you need the trim pots.

(two resistors dividing the current)

Remember it's two resistors dividing voltage only here, they are in series so they share the same current.

Voltage dividers are not friendly towards any decent sort of load (they are high impedance), unless you use very low value resistors to make the impedance low and at that point you are wasting alot of power in the voltage divider before it even gets to the load. Voltage dividers are for situations where you don't need very much current at all (like reference voltages).

Zeners are good references, they are low impedance kind of dividers, they will not fluctuate much with load current, as long at the current through the diode is kept higher then a certain amount (to keep it ON). They are also useful because they will not change output with changes in supply voltage (voltage dividers will).

Regulators are best for supplying power to circuits, since standard ones can do 100mA to a couple amps without appreciable change in voltage. For power a regulator is what you want. Overkill for high impedance reference voltages.

So if i understand you the practical difference is the current they can supply. To keep the current up use zenners or even better regulators. is that right?

[R.G.]

I meant to say it would be a neater build to do the math for correct value resistors for a voltage divider and leave out the trim pots.

It would indeed be a neater build.

Unfortunately, there is no amount of math that can be done to make simple JFETs come out that predictably. They simply vary too much.

So if i understand you the practical difference is the current they can supply. To keep the current up use zenners or even better regulators. is that right?

Close. The practical difference is the amount of variability in the current they can supply and in the voltage which drives them.

A resistor divider is just fine as long as (a) the voltage feeding it does not change and (b) the current it supplies does not change. The voltage out of a resistive divider changes by the same percentage as the voltage feeding it. It also varies linearly with the changes in output/input current from it. They are, after all, resistors and Ohm's law says they're linear with voltage/current. You can only stabilize it by making the current through the resistor divider much, much larger than the variation of current into/out of the divider. This gets very inefficient indeed.

A zener diode has a breakover voltage which varies only slightly with increasing current. From the voltage at which it begins reverse conduction to the point where it burns up from overdissipation, this may be only a 5% change for excellent zeners, more for sloppy ones. So you can make a zener voltage reference vary by only the zener voltage change by ensuring that (a) it always has a current through it of more than X to keep it conducting, and (b) it never has more than Y current through it so it will not burn up. The minimum current is ensured by  predicting the minimum input voltage and the maximum output current, and choosing the series resistor to it so that it has some current under those conditions. The maximum current is ensured by predicting the maximum input voltage and zero (or a minimum if you can) current out, and selecting a zener which can dissipate that much power when the resistor from (a) is in place. This can be a very tricky design problem as the current  variation gets bigger.

An active voltage regulator accepts a variable input voltage and outputs a more-or-less constant output voltage with only the conditions that (a) there is enough input voltage to keep it running (b) there is not so much input voltage and output current that it burns up. Generally these conditions are easier to predict than for a resistor or zener reference.

The differences are
(a) the range of input voltage variation
(b) the range of output current variation