No bias in S-5 12K, 8L, 16L, etc? - diyAudio
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Old 19th March 2012, 01:45 AM   #1
plexus is offline plexus  Canada
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Default No bias in S-5 12K, 8L, 16L, etc?

A friend bought an 8L kit and it exhibits the usual lack of bass. I have a pair of Dynaco Mk3's with 6SN7 drivers and some tube amp experience but not a lot. I have been trying to determine what the lack of bass could be caused by. There are a lot of guesses on the net (mostly aimed at the output xformers) but I have yet to see a definitive "this will fix the bass problem". One clue is to compare the 12G/K design with the 8L and if you do, you will see they are the same design except for the different tubes used and some resistors on the pre-amp which I assume are changed to make the different tube operate properly. It seems suspect to me that the circuits are so similar yes the performance is so different - the 12G/K seems to not have the same bass cut off issues at the later 8/16L, at least from reviews.

There has to be a reason why the 8/16L amps sound so "bad" compared to the 12G/K. and it might not just be the output transformer because I would think with all the people trying different transformers, there would be more a resounding "yes this fixed the problem".

On thing I noticed is, on both amps, there doesn't appear to be any negative bias on the output tubes. is this correct? I am a newbie when it comes to tube amp design but I know on my dynacos that bias is there and is important. I have even modded them to allow discrete bias adjust (and that has since been build into the driver boards I recently installed). So why on these S-5 amps is there no bias or is there and I just don't get it? and finally, could this be a cause of the reduced bass in the L series?

Is there a definitive mod to the stock L series that fixed the bass problem? It seems suspect that the 12's and the L's share such similar circuitry yet sound so different. my instinct is telling me that they just took the 12K design and modified it as little as possible to work with the L series tubes (5670 and 6005) without really paying attention to what those tubes need to perform well. do you think there is any truth to that?
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Old 19th March 2012, 01:57 AM   #2
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too many questions - learn to edit.

Clearly the output tubes do have negative bias, otherwise they wouldn't work. I suspect you are not used ot seeing it as it is designed - they are cathode biased via the 300 ohm shared resistor.

With not too much effort you can calaculate what the bass roll-off of the circuit would be - any deviation from that is most likely the result of the titchy iron in the opts.
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Old 19th March 2012, 02:03 AM   #3
plexus is offline plexus  Canada
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Ok that's what I thought based on my limited knowledge of biasing techniques. i "get" fixed bias (as in the dynacos) but I don't quite understand how cathode biasing works.

Well, I find it odd that there are so many people wondering why the L-series amps have so much bass roll off yet no one has found a definitive fix for this. I guess it will be one of those things we'll never know...
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Old 19th March 2012, 06:28 AM   #4
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especially if you are all wandering around asking someone else to figure it out for you...
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Old 19th March 2012, 07:03 AM   #5
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This site has a good set of primers on the sort of thing you are asking about
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Old 19th March 2012, 09:48 AM   #6
DF96 is offline DF96  England
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Genuine lack of bass is likely to be caused by a poor OPT. A false perception of lack of bass when comparing two different amps could be caused by one amp having too much bass: either high output impedance so speaker bass resonance boost, or poor LF loop stability so an LF boost. It is easy to become accustomed to error and then to regard it as truth; genuine truth then looks wrong.

Too little bass could also be caused by using low values for coupling and decoupling caps but the modern trend is to make these too large so that is unlikely.
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