anyone used an LED in place of the cathod resistor on a preamp tube/not trem

[phsyconoodler]

I saw a thread on another site where a guy used an LED in place of the cathode resistor/cap on the preamp tube of a 5 watt amp design.He claimed it was incredible but I've always wanted to try it.
  His U-tube sounds clips were hideous but the question remains whether there is any merit to his claim.
Sluckey used it to get better trem on the bias-vary trem circuit,but I want to try it on a preamp tube and see what it sounds like.

 Anyone try it yet? Just curious. Not really getting all freaked by it.

[Willabe]

IIRC, KOC and Merlin talk about it in 1 of their books.

I have never tried it myself.


            Brad     

[HotBluePlates]

I haven't tried it.

I saw a thread on another site where a guy used an LED in place of the cathode resistor/cap on the preamp tube of a 5 watt amp design.He claimed it was incredible ...

All it does it act like a perfectly bypassed cathode resistor, because for any current a preamp tube can draw through it (without melting), there will still exist the same on-voltage across the LED, and therefore at the tube cathode. Pretend it was a 10,000uF cap across your 1.5k resistor.

The catch is, whether the on-voltage is for that LED better be the operating point you wanted to run your tube at. For the most part, and with typical Fender-like circuits (~1mA and 1.5kΩ for ~1.5vdc), this seems to work fine.

I haven't researched whether this will shunt heater-to-cathode leakage hum the way a large bypass cap will (it very well might).

If you want to tailor bass response by tweaking a bypass cap, the LED is not for you.

[Fresh_Start]

The AX84 Blues Preamp uses two parallel triodes with different LEDs in place of cathode resistors.
Schematic

A search over there for the development thread(s) might be helpful.

Cheers,

Chip

[pullshocks]

I have built the AX 84 Blues preamp with the LED Bias.  Sounds good to me.

[G._Hoffman]

I tried it on one of my builds.  I didn't like it, but I didn't experiment with a lot of different LEDs, and of course different colors have different voltage drops, so very different sounds one assumes.  I found I was happier with a resistor.  I think you'd have to try a variety of colors, brands and such to find the right one.


Gabriel

[echuta13]

I tried it in a champ build.  I think it was a 1.6v one (red).  Plate voltage was around 150v (going off of fuzzy memory).  The sounds was clean and full.  I think it sounded best with a .0047 coupling cap.  I would think that it would be a good fit for some types of builds, but I'm not sure if I would use it much as I think doesn't have as much color/character as a cap & resistor.  Best way I could describe it is that it sounded "linear"  Don't know if that helps. 

[skeezbo]

Does it replace both the cap and the resistor? Will the LED light up?

[echuta13]

Yes it replaces both (LED gives full bypass like a big cap) .  It does light up.