Have you rebiased the 6V6 for the very low voltage you are running at? You should have 40-50mA of plate current at least...if you only have 20-30mA this could be a problem. If you have rebiased, then it's a bad tube, return it to the vendor & ask for a replacement.
Further to that...I'm not sure how you have managed to get less dc voltage after the rectifier, than the AC supplied by the PT? You do have a filter cap going to ground at pin 8 of the rectifier, don't you? I would expect 300vdc-ish for a 260-0-260VAC PT.