MXR Phase 90 clone - trim pot question

[kerryandray]

Hi, all.

I have a mostly built p90 from GGG (clone of the MXR Phase 90).  The plans indicate it requires a 250K trim pot.  They don't mention what it does or how to set it.  Another topic here (http://www.diystompboxes.com/smfforum/index.php?topic=63754.msg503096#msg503096) indicated it "sets the amount of back bias on the phasing JFETs to get the best phasing sound" and that you just set it based on what sounds best.

Unfortunately, I don't have one on hand (I am waiting on one in the mail), but I do have a 500K trim pot.  Is there any reason I couldn't get it to work using that?  Would there be any risk to doing this?  I would think it would be fine and that the 500K pot would just basically get set somewhere on the first half of it's rotation (i.e. somewhere between 0 and 250K ohms, thereby acting like a 250K pot does).

Thanks in advance for any advice folks can offer.  :-)

Ray

[Kipper4]

While waiting for the trimmer.
If you wanted to try it. You could solder some wires on the 500k off board and tack a 220k from lug 1 to 3.
Parallel resistance......
Will probably give you something in the parish. Close enough for an experiment.
Rich

[kerryandray]

Hey, Rich.

Good point.  I'm pretty sure I have plenty of 220K resistors laying around.  I'll try that.  With the way the board sits in this particular pedal, it will be pretty easy to access the trimpot without having to disassemble/unwire anything. So why not?  Thanks!

Still, though, I'd like to know A) whether or not using a 500K in this case would pose any risk to the JFETs being biased and B) whether it would basically work as a 250K pot in any case if I just used the first half of the 500K pot's rotation.

Ray

[Kipper4]

Using a 500k might give you an even small range of rotation to bias the fets is why I suggested that.
Feel free to try it.
Have fun.
Let us know how it goes..

[Mark Hammer]

The trimpot does two things.  It reduces the bias voltage coming off the zener, and also provides some current-limiting of that bias voltage to the JFET gates.  But the 1M resistor tied to the trimpot wiper also reduces the current feeding the FETs.  In fact, you can play with the setting of the trimmer, AND/OR the series resistor to move the range of the phasng around.  So, yes,you can use the 500k trimmer, but you will likely need to reduce the value of the 1M resistor .  Assuming the trimmer is set to its midpoint, the zener voltage has to pass through 125k then 1M.  So, given that a 500k trimmer can produce the very same bias voltage, but at a lower current, you'll want to aim for comparable current-limiting.  That would suggest something like an 820k series resistor instead of 1M.  I will suggest a 680k resistor and a 250k pot to adjust the range.  Set the pot for max resistance, and then set the trimmer for gurgliest sweep range that sounds reasonably full (i.e., it does't "choke" at any point).  Now you can move the sweep range up or down between gurgly and swirly.

[kerryandray]

The trimpot does two things.  It reduces the bias voltage coming off the zener, and also provides some current-limiting of that bias voltage to the JFET gates.  But the 1M resistor tied to the trimpot wiper also reduces the current feeding the FETs.  In fact, you can play with the setting of the trimmer, AND/OR the series resistor to move the range of the phasng around.  So, yes,you can use the 500k trimmer, but you will likely need to reduce the value of the 1M resistor .  Assuming the trimmer is set to its midpoint, the zener voltage has to pass through 125k then 1M.  So, given that a 500k trimmer can produce the very same bias voltage, but at a lower current, you'll want to aim for comparable current-limiting.  That would suggest something like an 820k series resistor instead of 1M.  I will suggest a 680k resistor and a 250k pot to adjust the range.  Set the pot for max resistance, and then set the trimmer for gurgliest sweep range that sounds reasonably full (i.e., it does't "choke" at any point).  Now you can move the sweep range up or down between gurgly and swirly.

Once again, thanks for the informative reply!  This is why I asked.  What I take from all the above is, by changing the 250k to 500k, it affects the circuit in such a way I'll have to change other components to get the circuit to perform as expected. Since I just wanted to see if i could easily test it out with the 500k i have on hand (which is not exactly the case), plus i have the 250k ordered already...I think I'll just wait for the 250k. 

Thanks to everyone for your help!

Ray