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So I have been trying alot of different tubes lately in my 18 watt, both NOS and NP, and I was curious about the bias on the power tubes. At the moment I have several brands of NOS EL84's that I want to swap in, but I have read that the cathode resistor value may need to be different for NOS. The resistor I have in it now is 150ohm/5 watt, and so far I have not noticed any redplating with the couple of different sets of power tubes that I have tried. Is it true that the value may need to be changed for NOS vs NP to get the best life/sound out of the older tubes?

You should be fine with the 150. It was increased a couple of years back to provide some increased safety margin. you can always take some measurements and calculate the power dissipation if you are concerned.

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Stephen
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Thanks for the quick response Coco! Everything seems fine so far but I just wanted to check with you guys and get your opinions.