# Got a question about pot

Berndizzle440
December 2nd, 2011, 10:36 AM
hey guys quick question. in know that a potentiometer is just a variable resistor but can you replace most any resistor with a potentiometer? the circuit im working with is just a basic lpb-1 circuit and was thinking about maybe adding a pot in place of the 10k resistor coming from the collector. heres the diagram im using, http://fuzzcentral.ssguitar.com/schematics/lpbschem.gif the only change i have made to this is im using a 2n5089 transistor instead of a 2n5088. thanks for any feed back :D im actually learning alot about electronics with this mini build and its really cool i never knew electronics was so interesting!!:shock:

FenderLover
December 2nd, 2011, 06:49 PM
Most common pots are rated at 1/2 watt, so anywhere you'd use a 1/2W resistor can sub in a pot.

Interesting?... Indeed. Enjoy the ride!

limbe
December 2nd, 2011, 07:48 PM
I was thinking the same thing,but looking at the schematic I realized the problem here is that you can give the collector + 9 volt easy.I think you should learn how a transistor stage works and how to bias it.(Among other things.)

jefrs
December 2nd, 2011, 08:11 PM
A potentiometer is not just a variable resistor (a rheostat),
The potentiometer is a voltage divider. it uses all three connections on the control. The applied voltage is in the two end connections and the voltage (signal) is divided out to the output by the position of the wiper. The guitar volume control is a potentiometer.
A variable resistor is just that, a resistor that is variable, it uses just two connections on the control, one end and the wiper. The guitar tone control is a rheostat.
It may be the same type of device but the way it is connected and its method of operation is fundamentally different electronically.

The 10k resistor in the circuit diagram can be carrying no more than 9/10000 < 1mA i.e. not more than 9mW. Any pot can handle this, it is what trimmer pots are for. However if you turn this pot to zero then there will be the full power of the battery applied, I would add a fixed ballast resistor to avoid frying the transistor.

The circuit is very similar to a Dallas Rangemaster Treble Booster, which does substitute a 10k pot, as a potentiometer, for the fixed resistor. http://fuzzcentral.ssguitar.com/rangemaster.php
http://fuzzcentral.ssguitar.com/rangemaster/rangemasterschematic.gif
This uses a geranium transistor, what was once the common as muck OC71.

tjk3052
December 2nd, 2011, 09:53 PM
Pick up a breadboard and try out different components. Great way to learn.

limbe
December 2nd, 2011, 10:59 PM
I wasn´t worried about the potentiometer,I was worried about the transistor connected to a 10k pot connected as a variable resistor.Thank you for the explanation and the circuit.I´ve still got some OC44s and OC71s.

jefrs
December 3rd, 2011, 04:57 AM
If you simply replaced the 10k with a rheostat (to avoid confusion) then the transistor might fail, but a potentiometer would work fine - as used on Rory's old Rangemaster Treble Booster, which seems to be the original version of this circuit

The OC71 is a delicate transistor and the glass bodied types are often photosensitive because a spot of paint will have worn off, recover with paint or shield from light.

This circuit is so simple it doesn't actually require a circuit board, it can be done as a 3D spider's nest hanging on the back of the pot.

justin.ray
December 3rd, 2011, 11:11 PM
Cool idea. You can use a pot in place of a resistor, but it's an expensive substitution (comparatively). You can get a pretty good stock of resistors cheap and sub them in until you find the value you want. If you really want to be able to adjust the forward bias voltage, you might think about putting a resistor in series with the pot to have a minimum resistance, you'll find that there will be a point where the potentiometer's range is not useful otherwise.

I don't think using the pot like is shown is the rangemaster schematic from jefrs will be quite the same for an LPB-based design, since the rangemaster is a PNP transistor and uses a 'positive ground' arrangement (the LPB is a negative ground NPN transitor arrangement). If you wanted to have a control similar to the rangemaster's control, you would want to substitute the pot for the 390 resistor. You'd probably want to go with a 500 ohm pot (in place of the 390 resistor), which may be easier to find as a trim pot. You'll probably have to make some other adjustments to get this to sound good over a decent range of the pot.

If you use the resistor where you are considering, it might respond a bit more like the SHO's 'crackle okay' knob because it will adjust the forward bias of the transistor, this causes a crackle because the change in DC voltage causes some electrons to shift plates on the output DC filter capacitor.

The LPB circuit is a great place to start with effects building because of its simplicity. Using the circuit as a basis, you can really learn a lot of things about gain-based effects, especially about the limitations of certain aspects of a single transistor amplifier. If you can stomach the more scientific side of things, this website is a great place to get a good foundation of transistor-based amplification circuits: http://www.electronics-tutorials.ws/transistor/tran_1.html.

jefrs
December 4th, 2011, 10:13 AM
There are NPN versions of the Rangemaster.

Those two circuits are so similar that I'm surprised they didn't have a fight over infringement. Otoh it is probably some basic one-transistor amp from the Mullard standard circuit book.

justin.ray
December 4th, 2011, 11:16 AM
Exactly, they'd have a hard time legitimizing an infringement. Most of these circuits are slight adaptations based on application notes from transistor manufacturers. Which I think is really quite interesting, as the dominate benefit (saturation of the transistor) in our application was undesirable in the original design. Pretty cool how things like this end up having uses that the inventors didn't foresee (not that they weren't geniuses).

Berndizzle440
December 4th, 2011, 11:13 PM
Thanks for all the feedback guys!!! one question though if i was going to replace the 390 ohm resistor with a pot how would i do it? any diagrams??? im a serious noob at this stuff :P

jefrs
December 6th, 2011, 03:31 PM
Thanks for all the feedback guys!!! one question though if i was going to replace the 390 ohm resistor with a pot how would i do it? any diagrams??? im a serious noob at this stuff :P

I would do it precisely as shown in the diagram, use a 390k or substitute another fixed resistor (see Rangemaster), that is the wrong place for a pot.

limbe
December 7th, 2011, 12:12 AM
A 390 kohm?

jefrs
December 7th, 2011, 09:32 PM
A 390 kohm?

My bad, that's a typo (I'm good at them) should be 390R = 390 ohms, the Rangemaster uses a 3k9.

The actual value depends on the transistor and is roughly equivalent to the cathode biasing on a pre-amp valve.