Fixed Distortion Plus Volume Knob

[aviherman5]

(Repost because I did it in the wrong section  )

Hi all,

I want to make a one knob distortion plus and keep the volume knob fixed at 12:00.

Assuming a pot of 10k (value is arbitrary really), can I just put a a 5k resistor to ground or do I need a voltage divider of 2x 5ks?

Here is the schematic I'm going off of:


Thanks!

Avi

[duck_arse]

you need two resistors for the voltage dividing, but 5k//5k won't give you 12:00 on a log taper pot ......

[aviherman5]

you need two resistors for the voltage dividing

Thanks!

but 5k//5k won't give you 12:00 on a log taper pot ......

Yes, I was going to play with the values, Just wanted a proof of concept

[Mark Hammer]

The 10k volume pot on early models was really an error in design.  Many have found that higher values result in greater output.

Consider that the 10k resistor, just before the diodes, and the 10k pot,  yield the equivalent of a 20k pot that can never be turned up past halfway (think of the diodes and cap as simply "taps" along the way).  Use of higher-value (50k, 100k) volume pots will allow for more of the clipped signal to come out, instead of being attenuated.  MXR corrected this in later issues, from what I understand.

[antonis]

MXR corrected this in later issues, from what I understand.

Shouldn't also correct the 3 X 1M bias configuration..??

[Mark Hammer]

Personally, I think that was one of those "happy mistakes",like the one that gave us the Proco Rat.  According to textbooks, the 3 x 1M biasing may be suboptimal.  But it  may be what gives the humble 741 in that application its character.

[GibsonGM]

It "makes it gritty".... 

[FiveseveN]

How did you determine that 12:00 is the exact setting you need now and forevermore? A trimpot would probably be more useful.

[Keppy]

Consider that the 10k resistor, just before the diodes, and the 10k pot, yield the equivalent of a 20k pot that can never be turned up past halfway.

Also consider that the voltage divider Mark describes is setting the amount of signal fed to the clipping diodes. If you replace the pot with a single resistor and you don't use the same value, then you're changing the clipping characteristics. If you want to keep the character of the original, use two resistors that add up to 10k, or use a trimpot.