Purpose of 150K/180k resistor before OD level pot.

[tonelab2]

On some schematics it's a 150k, on others it's 180K. I take it that this resistor attenuates the signal somewhat, if not what other purpose does it serve? Why different values on schems/layouts, what is achieved by changing it's value?

[Darkbluemurder]

Indeed it acts as an attentuator before the volume pot. It also acts as a low pass filter. The higher the resistor the more attentuation and the more high frequency reduction you will get.

[Tdale]

But since there is a 100K trim pot after the resistor, the resistor value shouldn't really be that important. If you put in a 150K in stead of a 180K, you just adjust the trimmer to be 30K higher, and you get the same total resistance..?

Tommy

[Bob-I]

But since there is a 100K trim pot after the resistor, the resistor value shouldn't really be that important. If you put in a 150K in stead of a 180K, you just adjust the trimmer to be 30K higher, and you get the same total resistance..?

Tommy

Not exactly. You also have the other side of the trimpot, the side to ground. Adjust the trimmer 30k higher and you get 30k less signal dropped to ground.

If you look at the OD entrance on the HRM schematic you'll see some wacko stuff. He used a 470K bypassed with a 47pF cap into a 200K into a 25k trim with a 4.7k tail. Obviously HAD thought that tuning this OD entrance was critical to the sound of the amp.

[dogears]

It also effects the knee where the coupling cap rolls the lows. The 180k will give a little more bass.

Indeed it acts as an attentuator before the volume pot. It also acts as a low pass filter. The higher the resistor the more attentuation and the more high frequency reduction you will get.