LED bias?

[Blair]

Say I wanted to replace the cathode resistor of a preamp with an LED to try it. I have a fist full of LEDs to try.

Can I just measure the cathode voltage and drop LEDs or strings of them until the voltage at the cathode is the same, or how do you determine the "right" led/s combination?

Thanks!

Blair

[dcriner]

The forward voltage drop depends upon the color of the LED. In series, they add up.

But, you'll need a ballast resistor in series too to limit the current through the LEDs. I think you'll need a capacitor bypass around the whole string to keep the LEDs from switching.

[Geek]

But, you'll need a ballast resistor in series too to limit the current through the LEDs. I think you'll need a capacitor bypass around the whole string to keep the LEDs from switching.

Maybe and nope.

If the total current is under 15-20mA or so (he said preamp, so I assume so), no ballast resistor is needed.

Capacitor will worsen performance. The intrinsic impedance of a forward biased diode is lower than a reasonable sized capacitor.

Morgan Jones did research into this.

Now, if the current is small, say on the order of a few hundred microamps (typical of a 12AX7), the current may be too small to keep the LED in the linear region. So what one does is add a resistor from the junction of the LED anode and tube cathode and send that to B+ for some bias - LED's are turned on harder in their linear region and no operating point changes because they act as constant voltage devices

Cheers!

[azazello]

You can listen new schematic a week, than You can back old with resistor and I think that old will sound better with more deep scene, IMHO, note pls.

[soundbrigade]

I've even seen a 6T9 (Sixtynine ) PP with Zeners.

There are many amps out there with LEDs, so I guess it works perfectly. At least you get a precise bias voltage.

[soundmasterg]

There are lots of people using LED bias in guitar amps. It works fine with no added resistor or cap. It also gives equal gain at all frequencies.

Greg

[burnedfingers]

I'm going to have to go on on record and say yes I have tried them and no I don't like them. They may be pretty to look at and give precise bias voltage but in my opinion some of the tube magic has been lost. Give me a resistor and I'll be a happy guy.

[Geek]

I'm going to have to go on on record and say yes I have tried them and no I don't like them. They may be pretty to look at and give precise bias voltage but in my opinion some of the tube magic has been lost. Give me a resistor and I'll be a happy guy.

With that said, they do sound different.

Try both and see what you like, Blair.

Cheers!

[EWBrown]

I've used LED biasing in preamp, linsetage and driver stage designs, with good results.

The Agilent (formerly HP) HLMP6000 LED's forward voltage drop is 1.57 VDC, most cheap genric3mm or 5mm red LEDS are 1.6 to 1.7 VDC, Yellow / Amber are approx 1.8VDC, and the old type "slime" green LEDs, 2.0 to 2.2 VDC.

The new white, blue and true green 5mm LEDs vary between 3.6 and 4.2 VDC, and the SMD 5050 white LED chips operate at 2.9 to 3.2 VDC. These contain three separate LED chips inside the device, it is best to connect all three of them in parallel, and stack 3 or 4 LEDs in series for EL84s with 300-325V B+, and 5 LEDs in series for 350-380VDC. These will need minimal heatsinking, to drawoff the exces heat. In the EL84 cathode biasing application, they willnerate LOT of bright white light.

/ed B