Vico Vibe... uneven at higher speed

[Elijah-Baley]

Hello.
I built a Vico Vibe tremolo. I just add a boost at the end to set unity volume, and it works.
I found just a bit too much fast, so I replaced C4 with a 330nF to slow the tremolo, but I replaced also R6 with a 2k to recover the max speed lost because C4.



I adjusted the chopping with the 1M trimmer. Everything's ok, except I got a weird effect at higher speed. At about 9/10 it seems sound still ok, very fast tremolo. At 9.5/10 it sounds faster, but kind of uneven (instead "ta ta ta ta ta ta", something like: "ta tata ta tata ta", if it makes sense). At the max, this weird thing make it sounds like it was slower, how if two oscillations are overdub, I can't explain it better.

Are the transistors need to be matched? That could be maybe a clue. Both are 2N5088, but different production, the labeling is different, I guess I bought in different shops or one of them is much older.
When I turn the Depth pot the tremolo stops a moment.

I started from a verified veroboard layout and modified it a bit, but I think that's ok, I'll check it again, later, but except that it seems work.

[antonis]

It sounds like C1 doesn't keep up fully discharging..

[R.G.]

Twin Tee oscillators (this is one) get really funny if the ratios of the element values stray too far from the "ideal". If you have a scope, I bet that the output sine(ish) waveform has a shift  in DC level or symmetry at the high end, or that it's going into ultrasonic oscillation bursts at the high end.

If you don't have a scope, you could try putting a 10 to 100 ohm resistor in series with the oscillator transistor's base lead and a 100pf or so cap across the base-emitter of the oscillator transistor. Maybe also stick a smallish resistor in series with the emitter to lower the device's open loop gain a bit. Oh, heck, just throw in some more RF-suppressing stuff like a 0.1uF ceramic cap across the 9V power supply near the oscillator transistor, stuff like that. If any of this changes the uneven-ness at all, you've proved that it's going into parasitic oscillation bursts.

The transistors have no need of being matched in any way, so their provenance and age don't matter.

[Elijah-Baley]

It sounds like C1 doesn't keep up fully discharging..

If that's the problem I don't have any reason to use a 220nf cap, for example. Nevermind if I loss a bit of speed, that will be anyway a bit more slower than stock schematic and I can set better faster setting.
What I did when I built the cricuit, to see if that cap to slow the rate, I tried to put a 100nF in parallel to C4, then I choiced to exagerate soldering a cap 330nF. I also try to jumper the 3.3k, the speed was really fast.
Indeed, just after I soldered the 330nF, I noticed the problem, indeed, jumpering the 3.3k I didn't like it, anymore. I searched just some lower value, until I used a 2k.

Twin Tee oscillators (this is one) get really funny if the ratios of the element values stray too far from the "ideal". If you have a scope, I bet that the output sine(ish) waveform has a shift  in DC level or symmetry at the high end, or that it's going into ultrasonic oscillation bursts at the high end.

If you don't have a scope, you could try putting a 10 to 100 ohm resistor in series with the oscillator transistor's base lead and a 100pf or so cap across the base-emitter of the oscillator transistor. Maybe also stick a smallish resistor in series with the emitter to lower the device's open loop gain a bit. Oh, heck, just throw in some more RF-suppressing stuff like a 0.1uF ceramic cap across the 9V power supply near the oscillator transistorstuff like that. If any of this changes the uneven-ness at all, you've proved that it's going into parasitic oscillation bursts.

The transistors have no need of being matched in any way, so their provenance and age don't matter.

Thanks R.G..
So, a 47R between C3 and Q2; a 100pF on Q2 between base and emitter; a "smallish" resistor (100R? smaller?) between emitter and the ground, on Q2 again.
I have as noise filter in the 9v a 100uF electrolytic, but I can add a 100nF ceramic. Not phiscally so close to the Q2, but if I move a jumper I could place that cap very close.

Actually, those are all mod I could try on my layout. Of course, replace just the 330nF is easier.

[PRR]

The "safe" path is to change all the caps the same ratio.

C2 C3 C4 from 100nFd to 150 or 220nFd.

This concept also implies that a one-pot (resistor) speed control will not give a wide stable range. In the RCRCRC phase shift oscillator, three Rs, one pot control makes frequency range about third-root of R, so a 31:1 pot range (100k+3.3k) makes only 3:1 frequency range. If the R gets too low or too high, increased loading makes the oscillator drop-out (or cleverly, dropout every other cycle), or demands a real amplifier (and AGC) not a single transistor.

[Rob Strand]

Normally on a twin-T oscillator the cap to ground (C4) is double the others (C2=C3).
You could try adjusting the R6/R7 to tweak the speed.

[Rob Strand]

After a bit of stuffing about with spice models I could get the oscillator to produce an odd oscillation pattern.   Things that seemed to promote it were:
- high gain transistor
- small value for the adjustable timing resistor
- 200n instead of 100n for the grounded twin-T tap.

I also found putting a resistor across the 100k pot could stop the odd oscillation, say 4.7k
When resistor is added the oscillator swing increases since the output isn't throttled
by the poor bias point.

You would have to compensate for that by backing off the depth pot level and also
adjusting the other trimmer (the one on the zener ckt).

The circuits are all entangled.  You can't change something without compensating/readjusting.
It makes me think what is normal anyway.

[Elijah-Baley]

Ok, all this make me think is not so easy mod a tremolo effect changing one or two parts individually.
I have this stripboard populated, and almost ready to box it up, so I don't want to mess up with ti too much.
Anyway, maybe worth to have all the those three 100n caps the same value. Indeed, I have reported in my notes, read somewhere «Whatever the value, it's important that these three caps are the same or you get weird results.»
Too bad, I don't want to use the only three 330nF cap I have, but I have enough 220nF.

[ElectricDruid]

Too bad, I don't want to use the only three 330nF cap I have, but I have enough 220nF.

Since it's only an LFO and not part of the audio path, this is the kind of place where cheap ceramics are an excellent choice! Save the nice film caps for the audio path!

[Elijah-Baley]

I don't have any ceramic caps with those values, but thank you for the advice!

Anyway, I think I can get slower tremolo even increasing one or both the 560k. That could be even convenient.

[Rob Strand]

Anyway, I think I can get slower tremolo even increasing one or both the 560k. That could be even convenient.

It's going to be a case of experimenting to see what works.

While earlier I mentioned the common cap ratios.  Typically the resistor to ground is only half the value resistor in arms of the other T.  In the case you this circuit 560k/2 = 280k but the variable resistor is much lower than that.

Some years back I started a thread on an inductorless wah based on a Twin-T circuit which had one transistor.  In that circuit I found the adjustable resistor to ground was better when it was less than R/2 value.   Same sort of thing as this circuit.    The thing is though you can make also the grounded resistor too small.   By making the 560k's larger and leaving the 100k pot +3k3 to ground the same it might be equivalent to making the 3k3 too small.  Think of the resistor ratios.

If you increase the 560k's by some factor then the 100k pot + 3k3 needs to be increased by the same factor.   Since the standard pot values jump from 100k to 220k you would have to try 1M to 1M5 resistors, a 220k pot and a 6.8k resistor.  Then all the cap values could be reduced.

[Elijah-Baley]

Ok, thanks.
I'll do some experiment this week.

[Elijah-Baley]

I soldered three 220nF caps (C2, C3, C4) and jumpered the 3.3k resistor (R6). I got a good rate range.
I Adjusted the trimmer to have a good chop effect and a better range of the Depth control.

Not the best tremolo I have, but it sounds fine, and it is into a small box.

Thanks.

[Rob Strand]

Not the best tremolo I have, but it sounds fine, and it is into a small box.

At least it works and is useable.  You can't expect miracles when the modulation is done by simple transistor circuit!