Cecil B Posted January 10, 2024 Posted January 10, 2024 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.
RoHo Posted January 10, 2024 Posted January 10, 2024 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
xlr8or Posted January 10, 2024 Posted January 10, 2024 @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
Cecil B Posted January 11, 2024 Author Posted January 11, 2024 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.
krazywaffle Posted April 15 Posted April 15 On 10/1/2024 at 1:28 PM, xlr8or said: Self bias would be preferred in a low powered DHT SET amp running for example 45, 2A3 or 300B tubes. Can you please elaborate why self (auto) bias would be preferred for low powered SET amps? Would this outweigh the convenience of being able to correct for imperfectly/not matched tubes? Thanks.
Spinspin69 Posted April 15 Posted April 15 You can also get add on auto bias boards that you can fix a bias and the board will maintain that throughout the life of the tube. 1
xlr8or Posted April 15 Posted April 15 38 minutes ago, krazywaffle said: Can you please elaborate why self (auto) bias would be preferred for low powered SET amps? Would this outweigh the convenience of being able to correct for imperfectly/not matched tubes? Thanks. It's a personal preference. Firstly, you are able to bias much hotter. Secondly, the tubes often age the same without the need of manual bias every few months or so. Thirdly, matched tubes for plate current and mutual conductance running at the same operating parameters as the SET amp excel in such a set up. 2
krazywaffle Posted April 15 Posted April 15 Thanks heaps for the insightful replies. Thinking autobias is the go for the 6L6/EL34/KTXXX monoblocks being designed/built by Eric McChanson. My only lingering concern is that NOS/used matched tubes are not always available (aiming to use all AUS-made tubes) so the ability to adjust bias to compensate for this could be ideal; however, as per your points, autobias might have more upsides for these low powered SET amps.
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