Guitar Amp Headphone OUT Jack. What Pot to use?
To solve this with 100% certainty, you need to know what is inside the amp. They typically have a resistor in series (R1) with the headphone output. My old receiver has 150 ohms, your amp most likely has a series resistor in this ballpark. 

A low value pot could be used (without R2), but audio (log) taper pots with lower than 500 ohms are not that common, digikey has a 500 ohm log taper. With a 500 ohm pot, you need to add another series resistor (R2). 

I would experiment with values of R2 until I got the desired attenuation.

If you already have a 1k pot, it could be made to work also, R2 would need to be larger.







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