I now use a 56ohm emitter resistor, 50k on the LED string, no bypass on the cathode, no gridstopper and put the CCS's on seperate heatsinks. Hisss is much less, heat is only 32°C on the heatsinks. Previously the resistors on the end of the LED strings went directly to each other then to ground, now they go seperately to star ground. I cannot think of something else I would like to change, it is almost perfect - I say "almost" only because I do not know what perfect sounds like 
Thank you to all who contributed and helped designed this linestage, great job
This is the PSU I am using, with compliments of Triode Dick :
No humm, very clean, very stable.
Thank you to all who contributed and helped designed this linestage, great job
This is the PSU I am using, with compliments of Triode Dick :
No humm, very clean, very stable.
Attachments
Hi,
I am having a lot of problems with the 12b4 bias falling from +10 volts or even dropping to zero volts within about 2 to 24 hours after firing up the circuit.
My B+ is about anywhere from 250 to +300v depending on the PS I am using.
RL is anywhere from 5k to 7k, RK is 500 ohms.
I have never had so many tubes go bad with other tube pres I've built, so I wonder if anybody else out there has had problems with defective 12b4s. I've gone thru about 2 dozen of them so far, and I'm desperate to build a consistently working 12b4 preamp.
Any and all feedback is welcome.
I am having a lot of problems with the 12b4 bias falling from +10 volts or even dropping to zero volts within about 2 to 24 hours after firing up the circuit.
My B+ is about anywhere from 250 to +300v depending on the PS I am using.
RL is anywhere from 5k to 7k, RK is 500 ohms.
I have never had so many tubes go bad with other tube pres I've built, so I wonder if anybody else out there has had problems with defective 12b4s. I've gone thru about 2 dozen of them so far, and I'm desperate to build a consistently working 12b4 preamp.
Any and all feedback is welcome.
Hi,
About 15 years ago we already regarded a 12B4 or 6S4A as something that needed handpicking by careful testing and matching.
I.e. buy 20, use 2....
Those were originally TV tubes so you can expect specs that matter for audio use to vary wildly from source to source.
Nowadays I'd rather consider a tube such as a 6C19S for similar service and still run it through the same selection process.
Cheers,
About 15 years ago we already regarded a 12B4 or 6S4A as something that needed handpicking by careful testing and matching.
I.e. buy 20, use 2....
Those were originally TV tubes so you can expect specs that matter for audio use to vary wildly from source to source.
Nowadays I'd rather consider a tube such as a 6C19S for similar service and still run it through the same selection process.
Cheers,
Hi
Same problems with me.I've used may be 20 tubes and all giving the same problem.It seems that the operating point is affecting the situation.Haven't tryed other tubes but with a pair of Jan Sylvania is working good with Ra 7,5K, Rk 1,5K bypassed with 470uf cerafine and B+ 340V.This gives around 18ma and pretty linear load line.
Same problems with me.I've used may be 20 tubes and all giving the same problem.It seems that the operating point is affecting the situation.Haven't tryed other tubes but with a pair of Jan Sylvania is working good with Ra 7,5K, Rk 1,5K bypassed with 470uf cerafine and B+ 340V.This gives around 18ma and pretty linear load line.
OK,
So some respond by confirming that they have had problems getting good 12b4s (and caveat emptor), also one person says try using CCS in the cathode.
I'll keep plugging away at the 12b4, because I think it's hands down the best extractor of audio I have ever heard.
Another question: the 12b4 is a single triode, with pins 2 and 7 as grid connections. Does this mean I should connect BOTH 2 and 7, or can I get away with connecting the grid at 2 OR 7? I suddenly thought that if one grid pin is unconnected, that could be a source of problems(?). I've been connecting the grid only at pin 2 up thru now.
Thanks to everybody for the quick responses. For a while there I really thought I didn't know how to put together this simple circuit. It's some consolation when others say they have had problems getting the 12b4 to work for them.
So some respond by confirming that they have had problems getting good 12b4s (and caveat emptor), also one person says try using CCS in the cathode.
I'll keep plugging away at the 12b4, because I think it's hands down the best extractor of audio I have ever heard.
Another question: the 12b4 is a single triode, with pins 2 and 7 as grid connections. Does this mean I should connect BOTH 2 and 7, or can I get away with connecting the grid at 2 OR 7? I suddenly thought that if one grid pin is unconnected, that could be a source of problems(?). I've been connecting the grid only at pin 2 up thru now.
Thanks to everybody for the quick responses. For a while there I really thought I didn't know how to put together this simple circuit. It's some consolation when others say they have had problems getting the 12b4 to work for them.
12B4
Although it is not exactly germane it relates to this thread. I did a lot of testing of 12B4's vs 5687 in a choke fed design as the driver tube for a guitar amp. These two tubes were hands down better than the usual (12AU7, 6CG7, etc.) in this task. The sonics were identical until the driver voltage exceeded 50V P-P, then the 5687 was superior. A choke fed 12B4 is an awesome driver for 2A3, but I wouldn't use it for a 300B.
I also did testing of CF for various tubes, and again the 12B4 whomps the 12AU7 by a huge margin. 5687 is also wonderful for this job, as is the 6CG7. THe 12BH7 is one of my favorites if I want a little darker sound from my vinyl for muscians like Pink Floyd, Buddy Guy, or Glen Gould.
I didn't make a 12B4 line stage, but if I did I would use a current sourced design with a single 6CG7 supplying the CS for 2 Grounded Cathode 12B4's and two MJE 340s as th CS for the other two CF 12B4's. I would have dual outputs (direct CF and GC) as the 12B4 has a low enough plate resistance that the output impedance is already very low in a grounded cathode design. You only need the CF for really long cable runs, and a CF does take a little majic out of the signal no matter what you do.
I know I'm running against the current here, but I don't like the CCS. The sound (to my ear-YMMV) is inferior to a direct triode current source design. I also don't hear any improvement by running at 25ma vs 10ma, unless driving a set of headphones or some other high current need.
My office line stage can switch between 6CG7, 12AU7, and 12BH7, but I find I leave the 6CG7 in most of the time. Not terribly surprising as the 6CG7 is a 6SN7 in a 9 pin package--and with better microphonics.
Although it is not exactly germane it relates to this thread. I did a lot of testing of 12B4's vs 5687 in a choke fed design as the driver tube for a guitar amp. These two tubes were hands down better than the usual (12AU7, 6CG7, etc.) in this task. The sonics were identical until the driver voltage exceeded 50V P-P, then the 5687 was superior. A choke fed 12B4 is an awesome driver for 2A3, but I wouldn't use it for a 300B.
I also did testing of CF for various tubes, and again the 12B4 whomps the 12AU7 by a huge margin. 5687 is also wonderful for this job, as is the 6CG7. THe 12BH7 is one of my favorites if I want a little darker sound from my vinyl for muscians like Pink Floyd, Buddy Guy, or Glen Gould.
I didn't make a 12B4 line stage, but if I did I would use a current sourced design with a single 6CG7 supplying the CS for 2 Grounded Cathode 12B4's and two MJE 340s as th CS for the other two CF 12B4's. I would have dual outputs (direct CF and GC) as the 12B4 has a low enough plate resistance that the output impedance is already very low in a grounded cathode design. You only need the CF for really long cable runs, and a CF does take a little majic out of the signal no matter what you do.
I know I'm running against the current here, but I don't like the CCS. The sound (to my ear-YMMV) is inferior to a direct triode current source design. I also don't hear any improvement by running at 25ma vs 10ma, unless driving a set of headphones or some other high current need.
My office line stage can switch between 6CG7, 12AU7, and 12BH7, but I find I leave the 6CG7 in most of the time. Not terribly surprising as the 6CG7 is a 6SN7 in a 9 pin package--and with better microphonics.
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