modded my TS-808 with LEDs for clipping and...

[mordechai]

So -- as I mentioned in an earlier post, I lowered the 4.7K resistor down to 3.3K after putting red LEDs in the circuit instead of the typical silicon diodes, and this helped recover some of the drive while retaining good output.  The end result of that was a somewhat more "natural" sounding clipping, but the circuit still had something akin to the typical midrange "bump" one expects from a TS-808.

A few minutes ago, I made one further mod -- I soldered together two LEDs in series and inserted them in, so that I had asymmetrical LED clipping...and the character of the overdrive changed.  It sounded really, really good, but the "bump" wasn't nearly as pronounced as with symmetrical LED clippers.

I'm very pleased with this modification, but would like to understand why the corner frequency has either shifted or has been reduced in terms of sonic prominence.

[Mark Hammer]

I suspect it is because there is only clipping via the 1-LED path, so you have less harmonic content.

[mordechai]

Why would the additional diode work against the clipping in this way?  I am still getting clipping, but the tone is more transparent.

[Mark Hammer]

You're getting clipping on one half cycle of the signal, but most likely nothing on the otherhalf cycle...or at least nothing that crops up for more than a few miliseconds here and there.  A pair of LEDs in series represents a forward voltage that could easily be in the 3V+ range.  A stock TS applies a maximum gain of 118x.  Divide our hypothetical 3000mv threshold by 118 and at max gain that half cycle of the signal would need to be at least 25mv (i.e., p-2-p > 50mv) in order to be clipped.  Not that many guitar signal remain above that point for very long.  True, they CAN produce peaks much greater than that, but those peaks don't really last all that long during the entire lifespan of the picked note.

[mordechai]

If I went for asymmetrical clipping by using one LED and one stock silicon diode, would this restore some of the harmonic content then?

[mordechai]

Actually, let me answer my own question, as I just tried it.  Yep...the "honk" came back, even if the character of the asymmetrical clipping is a bit different than the stock diode arrangement.  I like that in a TS-808 as-is, but for the purposes of what I'm tinkering with now, I preferred the more subtle frequency response. 

So, against one red LED, I tried subbing in a blue LED, and then an ultraviolet LED.  The resulting tone was very nice -- there was more harmonic depth and complexity, but not as mid-rangey as with the silicon diode.  The blue LED still had a bit of a "bump", but it was more subtle.  The ultraviolet also had a touch of a bump, but for some reason there was more bottom end.  It was nice, clear and tight, though -- rich and deep but well defined.

So, bottom line: with a series resistor of 3.3K and red/ultraviolet LED clipping diodes for asymmetrical clipping, the end result sounds great.  A lot more natural sounding, with just a bit of its own character to flavor my amp's own tone (amp = USA-made Fender Hot Rod Deluxe), and plenty of drive and volume on tap.  After so much testing, I have decided to call this thing the Test Drive :-)

Really happy with this -- give this a shot with your own TS project and chime in on whether you like it too!