trimpot on phase 45...

[Joecool85]

Do you have to use a trim pot or could you just throw in a regular pot of the same resistance and then change it on the fly?  I like the idea of having 2 knobs not one, plus I think it might give a lot of sound options.  Course, I won't be sure till I build it lol.

[petemoore]

  Do you have to use a trim pot or could you just throw in a regular pot of the same resistance and then change it on the fly?
  That's how I did it.
  A very small portion near the middle of the pot travel makes any phase tones, and it does allow variations..tweeky adjustment..you could 'narrow' it...
 

[Joecool85]

How would you narrow it?  Like...instead of having a 250k (I think thats what it called for) pot, you would have a 100k pot and a 150k resistor?

[petemoore]

 
  Adjust the 250k to middle of sweeping portion.
  Draw the connections on paper for reference.
  Remove without disturbing setting.
  Measure from wiper to pin 1,  note resistance value
  Measure from wiper to pin 3  note R value
  using say a 50k pot, attach a resistor values from during note 1 to pin 1
  attach resistor value to emulate note 3 value to pin 3 of the 50k
  These two value resistors should add to = 250k or whatever the actual measured value is of your pot [lugs 1 and 3 = pot value]
  basically you're building a fixed value resistor divider, to mimic most of what the pot was doing be placed where the pot was.
  This'll get you close. you can further trim out one side or the other, adjusting values, or add a fixed resistor to the 50k pot, across lugs 1 and 3 to narrow the amount of tuning function more.
  I doubt you'd need a 100k, 10k might be the value you want.
  See the secret life of pots at GEO...
  That may take some fine tuning.
  Another way to fine tune it would be to get it close to what you want as far as amount of usable control, then use another pot or two to find the 'outside the pot' and 'across lugs 1 and 3 of the pot' resistances for optimum knob control.
  basically you're setting up a smaller variable resistor divider adjustment [~50k vs 250k] between another divider. If you subtract 50k, equal % ages from the two outside resistors you'll the seriesed circuit [the pot between two resistors] will add to 250k or so, probably doesn't matter, but to keep with the shown values, staying at 250k for the divider adjustment...
  The pot is necessary to make it possible to adjust for circuit inconsistancies due to tolerances, it could be just two fixed resistors, if they were valued perfectly, I've built a couple 45's, a 100k would probably do there, 250k insures wide enough variance to overcome unlikely but possible tolerances.
  The math probably would help, a little confusing ... I kind of got over my own ability to explain this... for the one sides resistor, you can use a pot to find it's value to setup, so the adjustment pot is closer to what you want, then replace the 'R value finder pot] with a fixed resistor.
  Think of it this way....
  100k pot 50k pot 100k pot = 250k
  The two 100k pots on the outside are variable resistors [wiper and an outside lug], the 50k pot wired as variable *Divider [*three lug connection]
  set the two outside 100k pots so the 50k pot adjusts sweep mainly, possibly just past where the sweep fades out on either side [to taste].
  So now you have a -wire-100k-50k-100k-wire-...connect a wire to the 50k's wiper, using the three wires, put that where the 250k was and tweek...then replace the measured values of the 100ks with fixed resistors. reduce the 50k for less sweep by paralleling a fixed resistor to it's wafer [lugs 1 and 3].
  put yet another pot across the 50k to quick find what value you want across the 50k's wiper [adjusted by paralleling resistance across it's wafer, lugs 1 and 3].

[petemoore]

  Your'e setting a smaller value variable divider inside two resistances to add up to the 250k...not sure if reducing the value across the 250k really matters here, that's part of what made the darn post so complicated.
   say you have 50k-10kpot-50k
   turn the pot all the way CW you have
  50k wiper to rightside lug / 60k wiper to leftside
  all the way to the CCW and you have
  60k wiper to right side / 50 wiper to left side
  Hopefully I havne't totally confused you, if so, just use the find it with the pots method...two temporary 100k variable REsistors [pots] for each outside lug of a 50k pot wired as variable DiVider. Smaller divider value narrows sweep adjust.

[Joecool85]

That sounds really confusing...couldn't I just find where the range is that it actually changes tone, lets say it changes tone from 150k to 200k.  Shouldn't I be able to wire in a 150k resistor with a 50k pot?

[R.G.]

That sounds really confusing...couldn't I just find where the range is that it actually changes tone, lets say it changes tone from 150k to 200k.  Shouldn't I be able to wire in a 150k resistor with a 50k pot?

You're missing the point, I think. That trimpot is not a tone control.

The trimpot sets the JFETs so they have a good change of resistance with the LFO drive on the gate. Get it off to one side or the other and the circuit doesn't phase. 

[Joecool85]

Right...not tone.  I know that, but it does change the sound of the circuit from what I understand, here is why:

"This is an MXR Phase 45 clone. It is built per the layout at www.tonepad.com except for substituting the FETs with J201s, using a 200K trimpot instead of 250K, and using 0.047uF caps instead of the 0.05uF caps specified on the schematic. This circuit has a very nice sound that can sound much like a flanger depending on how the trimpot is set. One mod that could be done to this circuit is to make that trimpot a regular pot that could be adjusted from the exterior of the enclosure. (Now I think of this?!)"

http://www.diyguitarist.com/DIYStompboxes/MyStompboxes.htm

I know it isn't tone, but would it be a worthwhile mod anyway?

[petemoore]

 Â  Basically I'm trying to provide a way so you can visualize it.
 for even numbers take a 100k pot as example...and 'oclock' as potshaft position referencer.
 the shaft rotates from 7 to 5 oclock, but the only portion of that travel you can use is very small, between 12 and 12:15, and you want to 'find' 12:03 but the knob is so touchy it's hard to get it to settle at exactly 12:03.
 So now we pretend to cut the 100k pot wafer into three pieces, the first section is useless to have as variable...7 to 12 oclock, the middle is just 12 to 12:15, and the last is useless to have variable...12:15 to 5 'oclock'...put them back together and they still add up to the same value.
 MEasureing the first section of wafer you get 50k, measureing the second [middle section] you find 10k, and the 12:15 to 5 portion measures...40k.
 You make the middle portion a 10k pot, and the first and last sections fixed resistors  50k>10k adj.Divider> 40k
 Now when you adjust the pot, instead of 100k 'worth' you have only 10k worth of adjustment [sandwiched between] two other resistances that still add to 100k.
 So it's like taking your 250k pot and cutting the wafer in three...first section takes up the first half of the wafer and is replaced with a fixed R of same value, second middle section [the section you want as variable Divider] takes up only 30k, and the thirs section is the last 'small half'.
 So you take your 250k pot, subtract a small portion of the R value to be made up of a smaller Variable Divider [say 50k], the 'leftover' 200k is distributed somehow to either Both Sides of the 50kpot..to 'float' it in the middle...
 The phase will only work when that divider is setup for that unit. this divider could be 2 fixed resistors. In fact, if you knew the exact resistances, and had them as fixed R's, you could probably just throw a 10k pot in the middle of them and have your phase adjust knob that goes through what results in phase [fully Cw/to CCW] and no useless adjustment range to it.
  Probly end up being something like 100kfixed 50k VDivider 100kfixed...but...you may have to 'rebalance' the 100k's ... like 120k on one side of the pot and 80k on the other, that's the 'trick'....finding the 'setup' values for either side of the smaller pot that put it's smaller resistance right in the pocket of what the phaser responds to in terms of sweep, if you're resistances are set up perfectly, the smaller pot with chosen resistances on either side will adjust through the small range of resistances that are in the 'sweep zone' [ie make it phase sweep].

[Joecool85]

Now that makes sense!  Here is the real question, to those of you that have built the phase 45, does adjusting the trimpot actually change the sound of the pedal enough to bother doing this?

[petemoore]

 Â Set your 250k pot for center of sweep.
 Keep pot set at that position.
 Measure wiper to lug 1 resistance
 Measure resistance between wiper and lug 3
 pick a fixed resistor, or build one to replace the resistance between wiper and lug 1.
 Pick another fixed resistor to replace of wiper and lug 3,
 *put resistor '1' where wiper and lug 1 went
 *put resistor 3 where wiper and lug 3 went.
 *splice a pots lugs 1 and 3 into the chosen divider resistors you replaced the pot with.
 resistor '1' goes to lug 1 of a 50k pot, R'3' goes to lug 3 of a 50k pot.
 now instead of a 250k pot, you have a 50k pot 'floated' between two resistors, the added value of the replacement parts should add to anything around 250k over or under some won't matter.
 Then adjust as necessary. Took me a moment to boil it down.
 
 
 

[petemoore]

Now that makes sense!  Here is the real question, to those of you that have built the phase 45, does adjusting the trimpot actually change the sound of the pedal enough to bother doing this?
  Well it was hard to hear the added wobble on the smaller amp I use for testing, having the 250k sweep pot on the box was kinda cool, I think the different sounds are worthwhile for a phaser to have, that way one can demonstrate the importance at least of having that pot set perfect.
  At 250k, the sweep portion is only about 1/8'' turn of the pot.
  It's hard to tell exactly where the sweep starts changing anyway on this thing, change starts very subtly, soon after loss of sweep depth is noticed, also a slight overall tonal change, max wobble is hard to decide a setting for...not a bad knob to have on the top of the box, I like having it so I can mess with it while I'm checking it out playing it.
  So the pot only does much for it over an 1/8'' section of the wafer, near middle of range. It'd be nice to cut the wafer in three pieces, the middle piece being the 1/8'' section I 'use' getting replaced by a Variable Divider [pot] of it's measured resistance value, fixed resistors replacing the parts of the wafer [at their values] I'd never use.

[Joecool85]

Sounds like something I might want to experiment with after I build it then.

[slacker]

Now that makes sense!  Here is the real question, to those of you that have built the phase 45, does adjusting the trimpot actually change the sound of the pedal enough to bother doing this?

I did what you're talking about on my phase 45, built from torchys layout, and using a bias pot is definately worth doing. I used a 10k pot with resistors either side which makes the pot usable over it's entire range. I did pretty much what petemoore has said. Theres a post about it in this thread
http://www.diystompboxes.com/smfforum/index.php?topic=37677.msg265737#msg265737
The pot sound like it alters the frequency range that gets phased, at one end of it's rotation the phase is very deep and at the other you get a nice subtle wobble.