OD trim/pot with 346k - a must have!

[hans-jörg]

Hi Gents,

very, very interesting replys.
Dont hesitate ...

Thank You.

Hans-Jörg

[martin manning]

Another part of the equation to this would be when selecting a value pot for OD trigger...Consider where the wiper is set in the trace..Lower settings of the pot you have larger series resistance, which effects the high end frequency response...100k set 50k will be brighter than say the 346 set the same being darker or some might consider it smoother.. Thought I would mention..

Another good thing to consider... it's amazing how many implications a simple voltage divider can have, eh?

If I'm not mistaken it's the parallel combination of the series and shunt resistances of the 220k series resistor and the trimmer, plus the 68k grid resistor that you need to look at. In this case the result is similar, since the corner frequency is 10.6kHz for the 220k w/ 350k trimmer, using a 100p input capacitance for OD1, and it'd be 11.6kHz for a 220k w/ 100k trimmer, with both set at a "worst case" 100k to ground. A 100k trimmer set at 25k would be up at 17.5kHz.

edit: forgot the 68k grid resistor, so I corrected my figures... the series 68k really moderates the difference with the larger trimmer resistance, but brings the knee down into the upper audio range for the 100k to ground settings.

[lgehrig4]

Reviving this oldie

My 124 has a 500K pot and after reading this I temporarily connected a 1M across pins 1-3 and set the resistance to ground around 25K.

So far I like what I hear, but before soldering this in I was wondering if using a 1.2M resistor instead of 1M (which I currently have) would make a tonal difference, provided in both cases the resistance would be set to 25K?

If the only benefit is pot range (easier adjustment) I'm not concerned, but if using a 1.2M instead of 1M has tonal implications I would be interested in trying one.

[martin manning]

My 124 has a 500K pot and after reading this I temporarily connected a 1M across pins 1-3 and set the resistance to ground around 25K.

So far I like what I hear, but before soldering this in I was wondering if using a 1.2M resistor instead of 1M (which I currently have) would make a tonal difference, provided in both cases the resistance would be set to 25K?

If the only benefit is pot range (easier adjustment) I'm not concerned, but if using a 1.2M instead of 1M has tonal implications I would be interested in trying one.

There is more going on...
With 220k + 500k to ground following the 47n coupling cap the -3 dB roll-off of the clean 2 stage (in combination with Ck-Rk) is at 4 Hz. With a 1M//500k you have 220k + 333k to ground following the 47n coupling cap, the -3 dB roll-off is at 6 Hz. Not much difference there, but the reduced load rotates the AC load line clockwise about the bias point, which increases clipping on the cut-off end. My ODS (124-ish) has 220k feeding a 100k trimmer, which goes a bit farther down that path. In addition, the series resistance going into the OD1 stage affects the its HF roll-off, so when you decrease the size of the trimmer and then raise its setting to get the OD drive you want, you will get reduced HF roll-off.

[lgehrig4]

My 124 has a 500K pot and after reading this I temporarily connected a 1M across pins 1-3 and set the resistance to ground around 25K.

So far I like what I hear, but before soldering this in I was wondering if using a 1.2M resistor instead of 1M (which I currently have) would make a tonal difference, provided in both cases the resistance would be set to 25K?

If the only benefit is pot range (easier adjustment) I'm not concerned, but if using a 1.2M instead of 1M has tonal implications I would be interested in trying one.

There is more going on...
With 220k + 500k to ground following the 47n coupling cap the -3 dB roll-off of the clean 2 stage (in combination with Ck-Rk) is at 4 Hz. With a 1M//500k you have 220k + 333k to ground following the 47n coupling cap, the -3 dB roll-off is at 6 Hz. Not much difference there, but the reduced load rotates the AC load line clockwise about the bias point, which increases clipping on the cut-off end. My ODS (124-ish) has 220k feeding a 100k trimmer, which goes a bit farther down that path. In addition, the series resistance going into the OD1 stage affects the its HF roll-off, so when you decrease the size of the trimmer and then raise its setting to get the OD drive you want, you will get reduced HF roll-off.

Thank you Martin! I wanted to know if there would be a noticeable difference between 1M and 1.2M, and from what I am reading here I'm guessing no.