Tubesin said:I have read several issues of using LED in the cathode in place of a resistor. The purpose will improve the sound sonically? Does it require to paralell with a capacitor? Is it easy to control the bias voltage and be stable in the long run? Thank you for your informations.
Do a little search and you wil find quite a few references to this approach. Briefly:
"Improvement" is as much a matter of taste as anything, but it does eliminate any distortion that might be generated by a bypass cap. Which leads to your second question:
No, a bypass cap is not needed. Some have reported benefit with a small film cap, but given the low impedance of the LED, it's hard to make a technical case for that. It's a try it and see proposition.
As for voltage, it's a matter of selecting the right LED, or series string of LED's. You'll generally want at least 5-10mA current through the device (I've had good success with miniature low current LED's at about 2mA). Parallel strings can be used for higher current apps.
They are very stable.
Sheldon
Edit: A good thread for technical info: http://www.diyaudio.com/forums/showthread.php?s=&threadid=92207&highlight="led+noise"
That is why I don't see the big brands amplifier makers emply this device up to the moment. If led use high current to be stable, then tube selection is a priority. For preamp and voltage amplification in power amp. I think 6dj8, 12au7, 5687, 5842, 6sn7 should be appropriate. Very informative for further experimentation, I just like to improve the sound of my diy 2a3 amplifier....thank you all.
You can run lower current tubes with a LED. Sometimes it works just fine as is, but sometimes, you need to boost the current through the LED a bit. If that's needed, you can do it by bringing a resistor from the B+ rail to the LED-anode/tube-cathode junction.
The non-use of LED bias in big name amps is far more a matter of fashion than it is of performance, cost, or versatility of design.
The non-use of LED bias in big name amps is far more a matter of fashion than it is of performance, cost, or versatility of design.
More on led performance. Have you noted the sound stage dimension and dynamics in the music? And the musical tone is in the warm side... how expressive is the female vocals? I'm looking for red led now, which brand is best? I'll connect 2 in series for about 3volts bias because my present 12au7 is biased that way, will it change with the replacement or need adjustment afterward? Thank you
I built a current booster supply out of a LM1705. Works well, though i'm certain that a simple resistor will work just as well. In the circuit I used, with ECC88's, the additional current tightened up the sound, but this could have been because the supplementary current shifted the bias point slightly.
The simple RED Leds of which everyone talks of are getting increasingly hard to find. One local supplier (Maplin) only supplies high brightness LED's with a forward voltage drop of 3V. Even more normal Red LED's are typically having voltage drops of more than 2V. My point is read the specs carefully before dropping one in.
Shoog
The simple RED Leds of which everyone talks of are getting increasingly hard to find. One local supplier (Maplin) only supplies high brightness LED's with a forward voltage drop of 3V. Even more normal Red LED's are typically having voltage drops of more than 2V. My point is read the specs carefully before dropping one in.
Shoog
So then, what's more important? Keeping the bias *voltage* absolutely constant, or keeping the =average= cathode *current* absolutely constant even though the tube ages. Assuming class A of course.
For situation 1, have a led in the cathode path.
For situation 2, have a bypassed ccs in the cathode path.
For situation 1, have a led in the cathode path.
For situation 2, have a bypassed ccs in the cathode path.
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