How do you know which pots to use ?

[Subzero]

Well, my SRV special is finished and the sound is awsome .

The only problem I have is that the Drive and Tone pots are working in a weird way.
The drive pot, which is a 1m0 Linear taper, reaches full gain at about 10% of it's turn. (It's planned to be an Audio taper of the same value, but I can't get here...)
The tone pot, which is 25k Linear taper, changes something only on the first and last 10% of it's turn.

The pedal is functioning properly and the pots are good as well (went thru 3 differant sets of pots...).

Could it be that the pots are of the wrong value ?

Thnx
Itsik

[Mark Hammer]

There are times when attention to pot taper is important, and times when it isn't.  What you describe is a circumstance where the resistance changes you are most interested in are crowded into a small region of pot rotation.  Depending on a variety of factors, that problem may be easily solved by opting for the same value pot in a different taper.

Sometimes, however, one of the "other" commercially available tapers (whether linear, log, reverse-log) does not accomplish what you want.  In those circumstances, one can either custom-taper a pot with additional resistors (see The Secret Life of Pots at www.geofex.com) OR you can decide to break up the full range of some control into several smaller segments, each of which offer sufficient dialability to be useful.  For instance, a phaser speed control that goes from, say, one complete sweep every 10 seconds, to 3 sweeps per second, may offer sufficient control over the various slow sweeps, but need a little more precision when it comes to dialing in faster sweeps that sync up with a song rhythm.  It is unlikely that one single pot value/taper could do that.  So, if the speed pot were 500k, the user might choose to install a switch that placed a 68k resistor in parallel with that pot to drop its maximum resistance value down to 60k (smaller resistances wold be accompanied by faster speeds).  With a control whose combined resistance is in the range where the faster speeds live, the user would now be able to dial in the faster speeds with a bit more precision.  Think of it like placing your cursor over a portion of your pot's resistive strip and clicking the "enlarge" icon.

[petemoore]

Could it be that the pots are of the wrong value ?
If they are not the values shown on the schematic or the schematic doesn't provide the 'right' values.
The value can be measured, it is the resistance of the potwafer, each end of this resistive wafer is attached to an outside potlug [1 or 3], apply DMM set to correct range and take a reading...[rarely the pot needs a lug lifted so the circuit resistance which might be there isn't being measured aslo].
  The [wrong] 'Taper' is what would fit the symptoms you describe...ie full range of control, but jammed up at one end of the potshaft's rotational range.
  Audio pots have a 'curved' or 'tapered' resistance, a larger resistance on one side than the other when in center position, resistance will rise slowly at one side, fast at the other end of it's range, [between wiper and 1 outside lug]...[the effect is the same but reversed when wiper and other outside lug are tried].
  Linear pots do a 'straight line' resistance..using a 100kL as example, when wiper is set to contact the center of the wafer [potshaft in center position], the resistance from middle to either outside lug is equal [50k / 50k]...the rise and fall rates of both sides rise and fall 'evenly' as one side goes up/the other goes down at about the same rate.

[rogeryu_ph]

Guys,
All pots I bought and on the store were less than the value maybe this is the problem. Sample,  5k pot mesured only 4.2k, 100k pot only 92k, 50k pot only 45k is there a way to compensate or to exact the value?

Thanks,
Roger

[GibsonGM]

That's OK, Roger, the pots are fine. They were just built to less tolerance, maybe 10%.  So a 100k pot might be 110K or 90K...I wouldn't bother add resistors to make them "exact" for this type of project.    One thing to look at might be:
"The drive pot, which is a 1m0 Linear taper, reaches full gain at about 10% of it's turn."

You could try turning the pot into audio taper (read "The secret life of pots" mentioned above"), would probably mean adding series resistance to bring it back to the right range.

Another area to look at:  is the pot in backwards? ie.,  Wiper going to the right (output) side?  Is the series resistance correct (that 51K in the feedback loop on the original TS 808 schematic, don't know what the "SRV" version has).   There might be an error in your build as opposed to the design - that is usually the case!!
Good luck )