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Auto Self Bias

Sitka

Sitka

Active Member
I am new to tube amps. My current amplifier is a MP 301 mk3, it is a "self bias" amplifier. For me, this is a connivence, when rolling tubes, I don't need to use my meter.

But, is there a trade off when designing a circuit? Most things in life in general and electronics in particular involve trade offs when making basic design decisions. What are the pros and cons to a circuit that is self bias? When reviewing other tube amp manufacture sites, it seems that most require manual bias. Whats the truth?
 
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I believe this amp is a standard cathode-biased amp. Nothing complicated.

Cathode-biased topology is somewhat self-biasing (self-leveling), by it's inherent nature. :)
 
Where it might become a more complicated answer is that it appears this is listed as being compatible with a bunch of different tube types. Fair bet not all of them are biased at an optimal level.

if its designed around one particular tube type, the resistor can be chosen to bias that tube family correctly, and it works absolutely fine.
 
gadget73 said:
Where it might become a more complicated answer is that it appears this is listed as being compatible with a bunch of different tube types. Fair bet not all of them are biased at an optimal level.

if its designed around one particular tube type, the resistor can be chosen to bias that tube family correctly, and it works absolutely fine.
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No. I'm not sure why current manufacturers say those kinds of thing in their ads.

I supposed that running tubes in their "sweet spot" is no longer a target, for a lot of current engineering. :idea:
 
correction: the manufacturer states "Auto biasing design that eliminates manual adjustment".

I don't know if "auto biasing" is different from "self biasing". Whats the difference?
 
Sitka said:
correction: the manufacturer states "Auto biasing design that eliminates manual adjustment".

I don't know if "auto biasing" is different from "self biasing". Whats the difference?
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"Autobiasing" involves some circuitry that senses the tube's operating current and adjusts it for optimal performance. It may be a fixed-bias (meaning, counterintuitively, adjustable bias voltage on the grid) or self-bias (using a fixed-value cathode resistor) amplifier. Either way the circuit is supposed to ensure correct and balanced operation of the output tubes.
 
The Musical Paradise is just a typical self biasing amplifier nothing special in that regard, nothing wrong with that.

Paraphrasing a well known tube guy on another forum, different tubes will change the amplifier's sound, but they won't make the amplifier perform better.
Many tube amplifier buyers want to tube roll, so manufacturers tell them what they want to hear.

I think it is fairly obvious from some recent discoveries that tube amplifiers can perform well below optimal operating points and claimed specifications yet many listeners/owners still think they are the best thing they have ever heard.
 
zackthedog said:
I really doubt that there's any complicated circuitry in there. I think it's just a cathode-bias design that "automatically" biases the tubes without any need for adjustment. That's my guess, anyway.
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Exactly. This is a relatively low cost unit. I doubt it has any sophisticated auto-bias circuitry like more expensive amps.
 
CohibaJoe said:
Watch and read this as this explains Cathode Bias, Adjustable Bias "fixed bias" and PrimaLuna Adaptive AutoBias.

https://www.primaluna-usa.com/adaptive-autobias
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Informative article especially wrt to the benefits of "adaptive" autobias (primaluna's term for their version advanced bias control circuitry) versus those older/less sophisticated schemes. Luckily this great tech is now available for regular folks as well, ref:

https://www.erhard-audio.com/AutoBias.html

I used these to convert my flaky fixed bias monoblocks over to autobias and it proved a great move, ref:

Tube life - my power tubes are now lasting way longer. I've only had to replace a single power tube in past 5 years, versus roughly every 12 months prior. Savings well exceed what the boards originally cost. In fact I dont think Ill ever need to buy another power tube. Previously I also had to go with matched pairs so when one of the pair flamed out I was left with an orphan tube. The short life associated with fixed bias meant I quickly built up a stock of these orphans. But autobias now controls each tube individually, orphan adoption problem solved!

SQ - have to agree with PrimaLuna article (also in erhard link) wrt to providing noticeable SQ benefits

Status indication - boards come with leds letting me know they have biased to the set level. No more flying blind!

Worry free - just switch amps now and enjoy, no more frequent checks with DVM to see if bias is holding steady or has it drifted off again. Rock solid now!

I actually like my amps now!
 
Legrace said:
I've only had to replace a single power tube in past 5 years, versus roughly every 12 months prior.
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Honestly, you don't need autobias to get good tube life. I have self-bias amps from the '50s and '60s with the original output tubes still working well. If tubes only last 12 months, it's not the topology, something is simply defective or misadjusted.

Jack
 
TriodeLuvr said:
Honestly, you don't need autobias to get good tube life. I have self-bias amps from the '50s and '60s with the original output tubes still working well. If tubes only last 12 months, it's not the topology, something is simply defective or misadjusted.

Jack
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I easily rack up 3000 hours a year, what the mfgr of my amps states is about what you should expect with modern tubes. And they are right, that's about what I would get pre autobias. A 50's or 60's era tube amp with original tubes mullards etc everyone knows they were made from materials etc that arent even possible today. If you want modern tubes to last autobias is the way to go.
 
Surely you're not saying that a tube autobiased at e.g. 50mA will last longer than a tube that's fixed-biased or self-biased to the same current? None of my amps have autobias, and the USSR tubes I use last years and years.

Jack
 
interesting... I read the Prima Luna adds... they claim a sophisticated technology for auto bias. Is it a valid claim?

perhaps a pp amp is worth consideration? I'm considering a new amp, but is the Prima Luna a good match for my speakers? Altec Valencia, 97 db.
 
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