12b4a current
hey guys,
i need your expert opinions on how much current to give the 12b4a tubes to have them perfrom at their best sonically.
i have also came across many other schematics for the linestage.
which circuit brings out the best in the tubes?
this was contributed earlier in another thread by Brian Beck.
Thanks!
hey guys,
i need your expert opinions on how much current to give the 12b4a tubes to have them perfrom at their best sonically.
i have also came across many other schematics for the linestage.
which circuit brings out the best in the tubes?
this was contributed earlier in another thread by Brian Beck.
Thanks!
Attachments
another schematic
i found a well documented project in a korean webpage.
http://blog.joins.com/media/folderListSlide.asp?uid=notek&folder=46&list_id=8891061
triode dick mentioned his cleo6 preamp project that driving a tube to its rated current does not necessarily mean it will perform its best.
what do you guys think?
Thanks!
i found a well documented project in a korean webpage.
http://blog.joins.com/media/folderListSlide.asp?uid=notek&folder=46&list_id=8891061
triode dick mentioned his cleo6 preamp project that driving a tube to its rated current does not necessarily mean it will perform its best.
what do you guys think?
Thanks!
1. If not using 2x 3.3k carbon on pins 2,7 most 12B4A will simply oscillate.
2. Mikvous has done all those B+ and bias points with resistor anode loads and the LM317 cathode CCS one. We have listened to them all. 330V B+, cascode mosfet anode load, and 15mA with 1k2 cathode resistor bypassed, is surely the best we experienced. Opinions may vary but this is the best advice I can give you in my experience.
2. Mikvous has done all those B+ and bias points with resistor anode loads and the LM317 cathode CCS one. We have listened to them all. 330V B+, cascode mosfet anode load, and 15mA with 1k2 cathode resistor bypassed, is surely the best we experienced. Opinions may vary but this is the best advice I can give you in my experience.
Well, Salas got me the bug to build one!
As I start collecting bits, I would like advise as to which transformer to buy for this pre. As I understand, I need one with 300V, 5v and 6v secondaries for the heaters. Which one is the best choice soundwise?
It is going to be tube rectified and shunt regulated with a mosfet CCS.
Any advise is welcome.
P.S I already ordered some tubes to get motivated!
As I start collecting bits, I would like advise as to which transformer to buy for this pre. As I understand, I need one with 300V, 5v and 6v secondaries for the heaters. Which one is the best choice soundwise?
It is going to be tube rectified and shunt regulated with a mosfet CCS.
Any advise is welcome.
P.S I already ordered some tubes to get motivated!
12b4 BIAS POINTS
It can be that I am completely wrong, but when I look at triode characteristics it seems to me that a triode becomes less linear when the cathode is made more negative (at a given anode current). And also that a triode becomes more linear at higher anode currents (at a given grid voltage). From that point of view, why would one bias a 12B4 at -9V, or even at -18V like Salas does? For preamp use, normally a maximum output swing of 10V RMS is more than enough, so there is no risk off drawing grid current.
My test setup has a variable current source in the anode, and has also the opportunity to experiment with different cathode voltages. Soon I will perform some listening tests and report what my ears think about it.
Sparkle: I agree with you that a constant current source in the anode gives more gain in sound quality than in the cathode. I have tried a LM317 ccs in the cathode and was not very impressed with the results. It was not bad either, but not better than a Riken resistor.
Peter
It can be that I am completely wrong, but when I look at triode characteristics it seems to me that a triode becomes less linear when the cathode is made more negative (at a given anode current). And also that a triode becomes more linear at higher anode currents (at a given grid voltage). From that point of view, why would one bias a 12B4 at -9V, or even at -18V like Salas does? For preamp use, normally a maximum output swing of 10V RMS is more than enough, so there is no risk off drawing grid current.
My test setup has a variable current source in the anode, and has also the opportunity to experiment with different cathode voltages. Soon I will perform some listening tests and report what my ears think about it.
Sparkle: I agree with you that a constant current source in the anode gives more gain in sound quality than in the cathode. I have tried a LM317 ccs in the cathode and was not very impressed with the results. It was not bad either, but not better than a Riken resistor.
Peter
After many tests and circuit configurations my recommended point is where we got our best sound and most silence with a Mosfet cascode anode load. Systems and preferences may vary as I wrote earlier. It will be very interesting to know your forthcoming review of various bias points too.
Attachments
Bias Point Listening Impressions
As promised in my last reply, hereby some of my listening impressions.
I have listened to a lot of different bias settings. Initially the anode CCS was set at 20mA. The cathode voltages I tried are Uk=4.5V, 7V, 9V, 11V, 14V, 18V and 20V. I started at 4.5V and every step up gave an easy to distinguish improvement in sound quality. The easiest way to describe the sonic improvement is that the soundstage opens up when the cathode voltage is raised. Music comes more and more alive and more details can be heard. The bias point Salas prefers is a very good one (Uk= 18V, Ia=15mA), and 20V is even a little better. I did also try 20V at 15mA (instead of 20mA). At 20mA the music sounds a bit more relaxed, but also a little less alive and the bass is a tiny bit fatter. Here it comes down to personal preferences or what suits your system.
The 12B4A’s I used for testing are RCA’s pulled from an old Tektronix scope. Don’t know how good/bad they are. At Ia=15mA and Uk=18V the anode voltage is about 160V. That is higher than the datasheet indicates, so probably they are quite old. Salas: What is your anode vlotage at this setting?
I am looking for NOS 12B4's. Which are preferred?
Peter
As promised in my last reply, hereby some of my listening impressions.
I have listened to a lot of different bias settings. Initially the anode CCS was set at 20mA. The cathode voltages I tried are Uk=4.5V, 7V, 9V, 11V, 14V, 18V and 20V. I started at 4.5V and every step up gave an easy to distinguish improvement in sound quality. The easiest way to describe the sonic improvement is that the soundstage opens up when the cathode voltage is raised. Music comes more and more alive and more details can be heard. The bias point Salas prefers is a very good one (Uk= 18V, Ia=15mA), and 20V is even a little better. I did also try 20V at 15mA (instead of 20mA). At 20mA the music sounds a bit more relaxed, but also a little less alive and the bass is a tiny bit fatter. Here it comes down to personal preferences or what suits your system.
The 12B4A’s I used for testing are RCA’s pulled from an old Tektronix scope. Don’t know how good/bad they are. At Ia=15mA and Uk=18V the anode voltage is about 160V. That is higher than the datasheet indicates, so probably they are quite old. Salas: What is your anode vlotage at this setting?
I am looking for NOS 12B4's. Which are preferred?
Peter
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