Elliot is correct. These tubes are small, they last multiple thousands of hours. Particularly when you first get into tubes it is a little disconcerting. It is power amps that have shorter a tube life. But with modern equipment even these are getting longer. It is likely you will not face any tube problems soon. Personally, I would either not worry about it and just order a full complement if something fails or order a full complement and put in the closet. I have quite a pile of tubes I ordered just in case and traded the equipment before a tube went out.
TUBES
Hello 1&All i have not had any tube equipment since the 90s with that said i just upgraded my PreAmp with a NAT - Symmelical All Tube PreAmp just purchased on Audiogon not knowing the age of this unit or who manny hours i have been playin since 10;00am when it arrived till 6;00pm i am very happy with this preAmp Replacing my Plinius 16 which now goes 4 sale soon asking 1.400 .00 any way i need advise on buying / replacing all the tubes such as 6n30p-dr ,6n2p-ev,6n1p-vi , 6x4wa, oA2Wa Looking for Really good QTLY Tubes any Help PLEASE & thank you G