I have extra photomultipliers to sell. I can offer both the extended red version(R928) and the standard one. As per my write up on the Aminco Bowman PMT.... If you are not receiving a signal with any instrument that contains a PMT, it might seem like an obvious part to change to fix the problem but please do not rush to do this. PMTs are highly reliable, I read one fluorometer service manual that stated they have 100000 hour life expectancies and that all other possible options should be ruled out before replacing the PMT