Cecil B Posted January 10 Share Posted January 10 Hi Tube Amp people, looking for those that are experienced with this stuff for opinions. Which is your preference self-bias (auto or cathode bias) or fixed bias (user adjustable bias). I understand that auto bias will bias the tubes to a certain level (not optimum level) to give longer tube life? Whereas fixed bias allows user to bias the tubes to their optimum or user preference level. Which is better and why? Cheers. Link to comment Share on other sites More sharing options...
RoHo Posted January 10 Share Posted January 10 22 minutes ago, Cecil B said: Hi Tube Amp people, looking for those that are experienced with this stuff for opinions. Which is your preference self-bias (auto or cathode bias) or fixed bias (user adjustable bias). I understand that auto bias will bias the tubes to a certain level (not optimum level) to give longer tube life? Whereas fixed bias allows user to bias the tubes to their optimum or user preference level. Which is better and why? Cheers. It's horses for courses, really, neither is "better". Self-bias is simpler, both circuit and operator-wise, as there is no need for a bias power supply and there are no user adjustments. The designer will set the desired bias current which can be high or low or anywhere in between. The cathode resistor inherent to SB also acts as a current limiter in case of tube failure. Fixed-bias is the opposite with user adjustment required but this gives the ability to try different bias levels. The main PS voltage can be lower as you are not dropping a set % across the cathode resistor. And if you have a separate bias control on all tubes you can adjust for tube mis-match. 3 Link to comment Share on other sites More sharing options...
xlr8or Posted January 10 Share Posted January 10 @RoHo has hit the nail with his reply. I'll just add that fixed (adjustable) bias may be preferred in a PP amp running a quad or more set of EL34's or KT88's in AB mode to deliver more power overall. For the given plate voltage the bias can be adjusted accordingly to run tubes in the preferred operating range. Self bias would be preferred in a low powered DHT SET amp running for example 45, 2A3 or 300B tubes. The importance of using matched tube pairs in the latter application is more critical. 2 Link to comment Share on other sites More sharing options...
Cecil B Posted January 11 Author Share Posted January 11 Thanks Roho for straight forward explanation. It was what I was thinking but wasn't sure. Thanks xlr8or for answering what was going to be my next question about SE and PP amps, very perceptive... Cheers. Link to comment Share on other sites More sharing options...
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