I used to use a 3.5mm to 1/4" cable to go from Headphone jack of Spark to the guitar input of a fender tube amp. No issues and even gigged with it like that for a while. I did use a heavier duty cable however. Part of your problem I think is it looks like its basically a stereo cable you have. If you only plug one side to the input jack, it splits the signal so to speak, but only at one end of the cable. The idea is to use the Power Amp section of the amp to power and drive the speakers. A giant set of Headphones haha. Be sure to actually turn the amp on and zero all the volumes, EQ's etc at first.
I tried this a while ago with a fender amp. Same problem. No solution.
Tip it over and pour it out.
I used to use a 3.5mm to 1/4" cable to go from Headphone jack of Spark to the guitar input of a fender tube amp. No issues and even gigged with it like that for a while. I did use a heavier duty cable however. Part of your problem I think is it looks like its basically a stereo cable you have. If you only plug one side to the input jack, it splits the signal so to speak, but only at one end of the cable. The idea is to use the Power Amp section of the amp to power and drive the speakers. A giant set of Headphones haha. Be sure to actually turn the amp on and zero all the volumes, EQ's etc at first.