Ohms Law confirmation

[deuce42]

Hi guys

I'm sorry Im still a newbie but am hoping you could confirm if I'm correct with how to calculate resistor values in parallel using the ohms law formula.

I want to make a modified PPIMV. The traditional Lar Mar uses a 250k dual gang pot with 2M2 resistors for each gang. I understand this is supposed to replicate the 220k PI-Power tube bias resistors in most circuits. So for my application of this PPIMV I want it for a Mark Knopfler mod to a K60. I understand the bias resistors are changed to a 100k in the Knopfler mod rather than 220k.

So if resistors in parallel for normal 2M2 ohms resistors and 250k pots = approx 224k, and I need would need to be around 100k for the Knopfler bias resistor values, and I right that the 2M2 resistors should change to roughly 165k? ie 165k in parallel with the 250k pot using ohms law should be roughly 100k?

I'm just trying to make sure I understand

Thank you

[martin manning]

I think that will be fine. Note that as the PPIMV is turned down the resistance to ground at the input (as seen by the PI) will increase towards 250k. That will bring the bass up a bit, but I don’t think that’s a bad thing.

[deuce42]

Great thanks Martin

[sluckey]

I consider those 2.2M resistors as safety resistors. They insure you don't lose bias voltage to the output tubes in the event that the MV pot/s fail. The large value is chosen so as not to affect the value of the pot very much.

With that in mind, if I were to replace 100K grid bias resistors with a MV pot, I would use dual 100K pots and still use 2.2M safety resistors.

[martin manning]

I considered that too, Steve, but some increased bass at low volume would be desirable (Fletcher-Munson effects), and then there is the added safety of only 165k to the bias voltage instead of 2M2 if there is a pot failure.