Rat filter question

[msurdin]

hey everyone.

I have been working on building a rat for a bit now and I am now wanting to see if it is possible to change the Tone/ Filter.
I see how its supposed to be different then a muff tone for example, but is it possible to make it more like a muff tone?
It is very muddy and I guess what i will call "open" is past 70-80%
I would want to make the tone less muddy when down and be able to get brighter. I hope that makes sense.

I also notice when the tone and gain is turned all up it feedbacks easier. A really high pitched feedback.

I am using the GGG layout..

Thanks guys

Matt

[Mark Hammer]

The filter has the properties that the frequency at which it begins trimming treble is set by the formula F = 1 / [2*pi*R*C], where R is in megohms and C is in microfarads.  The stock cap value is .0033uf.  The stock R value ranges from .0015 meg when the pot is set to minimim resistance, to .1015 meg when the pot is set to max resistance.  The resulting range of where the treble cut is applied to goes from a low point of 475hz to a high point of over 32khz.  Clearly you don't need to move the entire range since 32khz is more than "open" enough.  What you probably want to do is simply reduce the overall range such that the lowest point is moved up a bit.  If you stick a 100k fixed resistor in parallel with the 100k Filter pot, that will bump the lowest point up to 936hz, which will make it a little less muddy and also improve the "dialability" a bit for you.

[the recluse]

Are you using the log pot suggested in the GGG layout?  You may want to try replacing that with a 100K linear pot.  That gives you a much more useable range throughout the travel of the pot, rather than just the dramatic shift at the end.

[Elektrojänis]

Are you using the log pot suggested in the GGG layout?  You may want to try replacing that with a 100K linear pot.  That gives you a much more useable range throughout the travel of the pot, rather than just the dramatic shift at the end.

Or the log pot can be wired backwards (reverse the connections to the outer lugs) for even more useable range. The only problem is that it will work backwards then too, but isn't that how it works on original Rats too?

[Mark Hammer]

Or the log pot can be wired backwards (reverse the connections to the outer lugs) for even more useable range. The only problem is that it will work backwards then too, but isn't that how it works on original Rats too?

Yes, which is why they called it "Filter" rather than "Tone".  That may well be the source of the problem.

[msurdin]

How do i wire the 100k resister to the filter pot? how much difference does it make?

[Mark Hammer]

How do i wire the 100k resister to the filter pot? how much difference does it make?

The pot is being used as a variable resistor, which means only two lugs are needed.  So, solder that extra 100k resistor between them.  That's it.

One detail, though.

Because the wiper on pots can sometimes make poor or intermittent contact with the resistive strip, leaving an open circuit and a potential noise source due to that intermitten contact, it is considered to be a best practice to use all 3 lugs on a pot, even though only 2 are truly needed to provide a single varying resistance.  So, you will often see it drawn and wired up with one outside lug connected to the wiper.  This means you have a varying resistance on one side of the wiper, and on the other side of the wiper is a zero-ohm path (the wiper and other lug linked) in parallel with whatever resistance is on the other side of the wiper.  Since 0 ohms in parallel with as many megohms as you can find to throw at it is still always zero ohms, the result will behave as if you are only using two lugs, but you will never have an open circuit since, if the wiper lifts you are still connected to the other outside lug.

So, assuming you used that approach, solder the 100k between the two outside lugs and everything will work as planned.

[msurdin]

Stupid question.... keep all the wired how they are? just add the 100k to the outside lugs.