Modded TS9, sound fizzles out

[tao_of_drew]

So a friend of mine modded my TS9 with the two 808 resistors in the output stage and put in two new diodes for the clipping, a red LED and a 1n34a germanium.
The pedal sounds awesome, louder and fatter. But the sound tends to fizzle out on a sustained chord.
The best way I can describe the sound is my amp made a similar sound when the inputs were dirty and needed to be cleaned. The sound is there and then it just drops dramatically in overall volume on long sustained notes.
We checked the solder joints and everything is solid.
I'm wondering if there is something amiss or it's just a result of the germanium diode in there as they can give a fizzling splaty sound to my ears. Also wondering if it might need two 1n34a's in series to match the red LED.
Anyone got any ideas?

[Mike Burgundy]

The point of clipping for each half of the wave is determined by the LED's threshold voltage - below that it won't conduct, above that it will (since it's in a feedback loop the waveform will be clipped off at that point). The transition isn't a perfect hard knee.
That threshold voltage is different for different types of LEDs. Regular Si are typically around 0.7V, Ge typically 0.3V, LEDs are all over the place, from 2V to over 4V. Higher intensity and more exotic color is usually higher voltage.
The way they behave at the knee also differs between types.
Nothing wrong with a bit of a mismatch for asymmetrical clipping (this is often wired in as an option with a switch, adding one Ge to one of the existing Si diodes for example) but one Ge vs LED might be a tad much.
edit: GEOFEX link upper left corner, find "technology of the Tubescreamer"

[Kesh]

After much experimenting and a bit of math, it's my belief that germanium diodes fizz out/gate in diode feedback circuits like the TS because their leakage. As I've posted before. This leakage occurs on the side of the wave that the LED is clipping.

Try a schottky diode of similar forward voltage like the BAT 43

[tao_of_drew]

Thanks for the info.

@Mike Burgundy: Do you suggest then putting two germaniums in series to try and better match the LED's output? (Even though it will still be off)

@Kesh: How do BAT 43s sound as clipping diodes?

[Pyr0]

Thanks for the info.

@Mike Burgundy: Do you suggest then putting two germaniums in series to try and better match the LED's output? (Even though it will still be off)

@Kesh: How do BAT 43s sound as clipping diodes?

I think the BAT43's sound great for clipping diodes, put two of them in series instead of the 2 Ge's

[Seljer]

It may just be that the germanium+led combination is too asymmetric. When only one side of the waveform is getting clipped it tends to have that fizzy quality. At least thats been my experience when breadboarding and checking waveforms with the scope

[Electron Tornado]

I've never gotten a good sound when trying germanium diodes in a configuration like the Tube Screamer. The LED can be a problem as well. Not all red LEDs clip the same.

[Kesh]

Tonight, sparked by this thread, I managed to get a great germanium clip sound on the TS I built by putting a compressor (a dod 280a build) before the TS. Other diode was an IR LED (sounded same as a 1n4148 though), drive pot was 1 meg, everything else was by the book TS808.

The compressor is holding the signal above the sputtering out threshold.