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For anyone reading this, the Decware Sarah 300B amp is a choke filtered power supply with a 4 uf capacitor.  It is kinder the Western Electric 300B that Western Electrics own 300B amplifier.  Additionally, it is an AC heated, cathode biased amp, around 60~70 mA for most 300B's so there is no way to abuse the tube or set anything wrong.  The B+ is below 400V so as far as 300B amps go, I can't think of anything that would be kinder to a 300B tube.  I can't imagine why Elrog would say that.

That makes some sense, but it is the opposite of the WE300B which lasts longer with AC than with DC.  My honest guess, not to discredit Elrog, is that the heater design is hard to balance making cancellation of the AC hum difficult.  They don't want a reputation for tubes that hum.  With DC there is no need for hum balancing and there is no hum.  The majority of the 300B amps on the market today have DC heaters, so they should be in good shape.  Thanks for bringing this information to the forum!

Steve : )