Adding a potentiometer to a circuit

[Govmnt_Lacky]

I am playing around with adding a Depth control to the Phase 90 from Tonepad. This may sound noobish but, I have no idea how to "properly" input it into the circuit. Also, I don't know which method is the best to use. So I guess this is a request for information as well as a poll.

Here is the link:

http://www.tonepad.com/getFile.asp?id=42

Here are the versions that I have come across:

1) Just a 1M pot in place of the 1M resistor coming off the wiper of the trim pot.
2) A 500K log pot in series with a 470K resistor.
3) A 500K linear pot in series with a 470K resistor.
4) A 500K log pot in series with a 1M resistor.

Also, I am confused on what is the best way to incorporate the series resistor. Where would the resistor go on the pot? Would it be soldered directly to the pot (between lugs 1&2 or 2&3) or would it be soldered in-line to a lug? (lug 2 or 3)

Thanks for the opinions and help. 

[Mark Hammer]

You have to consider what the usable range of any control might be.  Yes, there might be 1M there now, but what happens if the 1M shrinks down to 10k or, heaven forbid, zero ohms?

So what you often have to do is consider a pot that only captures some of what's there, by considering what yo want the pot to do.

Case in point.  The Ross/Ropez phaser has a 150k terminating resistor.  Whether that value is "required" is something I can't determine, but I did want an output volume pot since I had built a bit of gain into the output stage on one of mine.  Finding a 150k pot is unlikely, and tweaking a 250k or 500k with parallel resistors would be a PITA.  Realistically, I only wanted the control to give me a bit more volume than bypass (for solos), or a bit less (for rhythm), so I went with a 100k pot and a 51k resistor from the ground lug to ground.  Problem, solved with commonly available parts.

A sensible way to turn the 1M resistor into a variable control is to pick a fixed resistor that doesn't drop below an acceptable degree of current limiting, and augment it with a pot value that applies a reasonable amount of current limiting.  I think I went with a 1M pot in series with a 330k fixed resistor.  So, you end up with a bit more current limiting than stock, and a bit less.  Nothing blows up, everybody's happy.

[Govmnt_Lacky]

Thanks Mark  I will definitely incorporate your suggestion.

Now, would the 330K resistor be wired between the board and Lug 2 or Lug 3? Or, would it be wired directly on the pot between lugs? (2&3 or 1&2)

By the by.... that CE-3 is still giving me fits! 

[Mark Hammer]

Thanks Mark  I will definitely incorporate your suggestion.

Now, would the 330K resistor be wired between the board and Lug 2 or Lug 3? Or, would it be wired directly on the pot between lugs? (2&3 or 1&2)

Depends how you want the control to work.  My own preference in such matteres is to:
a) use a 1/8 resistor,
b) solder it directly to the pot lug in question,
c) put some heat shrink over the resistor and lug to secure and reinforce them,
d) run wires from the resistor free end and the other pot lug to be used, to the pads for the 1M on the board.

By the by.... that CE-3 is still giving me fits! 

Set it aside until the fits subside.  I can't begin to count the number of things I have racked my brains over, trying to trouble-shoot, and it was something simple and dumb that I couldn't see because I was too close to it.  A bit of distance can sometimes by a DIY-ers best friend.