[Electronic M]

If you did it the way oldtvnut described and there is still distortion the next thing I'd try is connecting the cartridge to a different amp and checking for distortion. I'd look for a tube based device as those usually expect a ceramic cartridge on their phono input.
If distortion is still present on another amp I'd say it's the cartridge and get another one. If the distortion is gone with the same cart feeding a different amp, then the amp in the Sylvania is suspect.

Ironically I could have bought a rough one of these Sylvanias sans speakers yesterday for a buck in the WARCI swapmeet donation auction... I've got enough phonos and too little space so I passed.