Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on February 08, 2013, 05:25:23 pm
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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.
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IIRC, KOC and Merlin talk about it in 1 of their books.
I have never tried it myself.
Brad :icon_biggrin:
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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.
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The AX84 Blues Preamp uses two parallel triodes with different LEDs in place of cathode resistors.
Schematic (http://www.ax84.com/static/corepreamps/Blues/AX84_Blues_Preamp_Schematic.pdf)
A search over there for the development thread(s) might be helpful.
Cheers,
Chip
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I have built the AX 84 Blues preamp with the LED Bias. Sounds good to me.
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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
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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. :dontknow:
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Does it replace both the cap and the resistor? Will the LED light up?
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Yes it replaces both (LED gives full bypass like a big cap) . It does light up.