Negistrem / Negistor problems

[oliphaunt]

In my ongoing search for a simple on/off tremolo I have been trying the Negistrem: 

I have read all about how finicky it is about the transistors, and I have had absolutely no luck getting it to do anything at all.  Honestly I don't know how to tell if the Negistor modulation section is doing anything at all.  Would I hear a pulsing in the background noise or something to that effect even if the JFET wasn't working well?   How can I test it?  I have tried the only 2n2222 I have and about 15 2n5088s and nothing obvious is happening so far.  I have tried a pile of J201s and a couple of MPF102s for the JFET. 

One (possibly) odd thing about the JFET is that the guitar signal passes through it whether power is applied or not, so I always hear the straight signal, but never any sign of modulation.  Is that normal?  I would have thought no signal would pass without power, but this an odd application...


I also did a search for Negistor looking for some idea as to what I should be expecting.  I found this schematic:


I have tried my 2n2222 and it modulates nicely but the 5088s don't.  Not sure what that means, except that my 2n2222 has the possibilty of modulating.

Thanks for any thoughts you may have.

[Cliff Schecht]

Azlitz will be posting my tremolo circuit soon with a full build report. I designed the unit to have a very wide range of sounds and have found that it does a very good on-off chop without a lot of bleedthrough or other problems associated with a square-wave based tremolo. I'm not sure where Azlitz is on his getting the device posted but I posted a schematic a while back. Give it a shot, it'll do what you're looking for without the need for any trimpots or other "tricks".

[earthtonesaudio]

The Negistrem/tron is picky about both transistors.  The 2n2222 is essential for the BJT, but certain JFETs work better than others.  I got it to work with (several) J113s, but couldn't get it to work with a J201.

If it's passing signal with no modulation, it means the negistor is not oscillating.  Decrease the 100k limiting resistor (to 10k or 1k even) or increase the supply voltage (not much higher than 15V though). 

[oliphaunt]

The Negistrem/tron is picky about both transistors.  The 2n2222 is essential for the BJT, but certain JFETs work better than others.  I got it to work with (several) J113s, but couldn't get it to work with a J201.

If it's passing signal with no modulation, it means the negistor is not oscillating.  Decrease the 100k limiting resistor (to 10k or 1k even) or increase the supply voltage (not much higher than 15V though). 

Thanks for the help, I'll try all that!

[oliphaunt]

I got three more 2N2222s to try out, and have not had an luck yet.  I replaced the 100K resistor with a pot so that I could try any value.   I aslo replaced the two trim pots with 500K resistors on each side, I figure that's not a problem.  Still no oscillation.

Question 1:  In the schematic below is the 2N2222 oriented with the emitter at the top and collector to ground?   I have tried both orientations, but am a little confused about the schematic.

Question 2: Is there a way test just the oscillation section so that I know for sure whether or not it is doing it's job?  If both tarnsistors are picky then it's a reall gamble tying to blindly find the combination of both of them that works.

[earthtonesaudio]

1: Emitter at top, collector grounded, no base connection
2: Make the "simple oscillator" shown in the second schematic you posted.  Listen to the output to determine which ones should work.  If it makes and audio frequency oscillator it should make an LFO.  Then just change out the "frequency capacitor" for a larger value to make an LFO out of it.

In my opinion the 1M rate pot is a bit too large for the Negistrem/tron.  At maximum resistance I seem to recall it just wouldn't oscillate.  It's possible that 100k for the limiting resistor is also too large.  I'd just copy Christian's schem and increase the frequency cap.  Also make sure your supply voltage is at least 9V.  12V would be better, but 18V might be too high.

The JFET is wired as a switch, so the values for the resistors don't really matter.  There's no buffering or gain, so higher value resistors will load the guitar signal less.  I used 1M on both the source and drain rather than a trimpot (I think the only reason the trims are there is for parts count, as that month's FX-X had to do with "10 parts or less").  With regard to the JFET being "picky," any JFET should work as long as the supply voltage is enough.  Even J201s ought to work, but perhaps only at 15V or more.  I liked J113s because they have an extremely low V_GS(off) value and will work at 9V.


I used metal can 2n2222a transistors for the oscillator, but I don't think that really matters.