Some old models of amps have 6. 35 millimeter inputs, which match 1/4 inch headphone adaptors. These won’t accept standard 3. 5 millimeter auxiliary cables.
If you’re working with a 6. 35 millimeter input, the large end of your cable should plug into the headphones jack in the front of the amp. Make sure you’re plugging your cable into an input slot, not an output slot.
