Zenith K731 help

[tacitapprova]

I have a Zenith K731 that has recently started losing some sound quality after playing for 10 or so minutes, before this, it played wonderfully. For the first 10 minutes, it sounds great, then the signal changes and becomes slightly clipped, losing the full frequency and introducing some distortion. It has been recapped and the selenium rectifier swapped out. I am assuming a tube is on its way out, but I don't have a tester. Anybody have any recommendations on where to start looking?