Blown B+ fuse or fried cathode resistor if no fuse in circuit. BTW, there is matching and there is matching. I would recommend matching plate current under "realistic" conditions; ie, B+ and neg. bias similar to your amp. Guys on ebay who use a tube testor that measures transconductance to match output tubes at a plate voltage of 200-300V are a joke IMHO and the results next to useless.