LED in cathode

[Tubesin]

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.

[SY]

In order:

It depends on the circuit. A LED will generally have a very low AC impedance; that may or may not be a good thing, depending on the design.

Generally not.

Yes, very stable. The trick is to keep currents through the LED fairly high, 10mA or so for a normal LED.

[Sheldon]

Originally posted by Tubesin
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: Red LED noise.

[Tubesin]

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.

[SY]

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.

[Tony]

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.

hey SY,

do you get hate e-mails from tube purists?

[SY]

Oh, yes, but then again, I get hate mail from everybody.

[arnoldc]

Originally posted by SY
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.

I have used this successfully and is my preference now for biasing tubes with < 10mA current.

[SY]

FWIW, in my 12AT7/ECC81 and 12AX7/ECC83 distortion measurements, boosting the LED current above the idle currents (2 and 1 mA, respectively) made no measurable difference in distortion spectra. I wouldn't have expected that, but there it was.

[Tubesin]

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