A diode biasing question.

[pinkphiloyd]

In my next project I was thinking of giving diode biasing a shot. I'm tempted to do it in place of any preamp tube that's biased in the normal cathode resistor/bypass cap method. It seem like I remember reading somewhere, though, that there really wasn't much point in using it beyond the input stage. But now I can't find that reference, if it even exists. I may have dreamed it.

So, is there any benefit to using widespread diode biasing? (I know, "let your ears be the judge." Just trying to decide how far I want to go with it. Any place to actively avoid it?

[roberto]

Avoid in saturating stages.

[pinkphiloyd]

Will do then. Thanks.

[Ken Moon]

Yes, the LED biasing seems to work best on the input stage.

The key is to get a bunch of different diodes (cheap), and try a few that are in the voltage range you're going for.

I found some teeny little red ones that give a very stable voltage of 1.2 volts on the cathode with two LEDs in series, and have used them several times.

There was an awesome hi-gain amp builder years ago on AX84 who posted several schematics that showed a paralleled 12AT7 for the input stage, with a shared plate resistor but different value/color LEDS on each cathode.

The sound clips he posted were very impressive.

[jjman]

Audio amps use them to avoid a cap on the cathode. While a cap is taboo in that world it is not taboo in a guitar amp. I use one in a trem cathode to indicate speed on the front panel of an amp.

[Littlewyan]

Early Mesa Boogie amps used them on the Cathodes of the first two stages but still bypassed them with a capacitor.