Sunn Beta Lead preamp sounds too gated/spitty decay

[valveandsound]

Hello all! I have been building a clone of the Sunn Beta Lead pedal and have it all together and working. I noticed that with any level of moderate drive, the output sounds gated, like a transistor stage that is not biased properly. With higher gain settings, the overdrive sound loses a lot of sustain - if i play a chord/note, i can hear almost what sounds like a gate like sound or a sound where the sustain drops off very quickly.

I swapped in another CMOS IC and different charge pump (using an LT1054) but no change. Confirmed i am using the same values in the schematic below as well.

I am using the CD4069UBE CMOS and Schematic as shown in post below. I'm not terribly familiar with fine tuning CMOS based overdrive circuits. I've read that you could increase the series resistance to the gate(s) input to change the schmitt trigger level, or even add a bias control with a 1Meg resistor/pot tied between the 9vDC and the input of the first stage. Does anyone have any thoughts they could share about either of these methods?

[valveandsound]

Schematic as used:

https://imgur.com/a/U1rHTaK

Here are the voltages for the 4069UBE (no input signal, at idle):
1. 3.9v                8. 0.057v
2. 3.66v              9. 9v
3. 3.8v               10. 0.085v
4. 4.5v               11. 0.9v
5. 4v                  12. 0.45v
6. 2.4v               13. 4.9v
7. 0.001v           14. 8.2v

If I had to guess, the inputs for each gate (pins 1/3/5/13 respectively) look to be biased around half of the 8.2v supply being fed to the chip, so maybe safe to say it's getting more or less the appropriate bias. But when I listen to demos of this pedal online it seems like people are having more sustain before the gating effect kicks in.

[Rob Strand]

Here are the voltages for the 4069UBE (no input signal, at idle):
1. 3.9v                8. 0.057v
2. 3.66v              9. 9v
3. 3.8v               10. 0.085v
4. 4.5v               11. 0.9v
5. 4v                  12. 0.45v
6. 2.4v               13. 4.9v
7. 0.001v           14. 8.2v

Pin 12 is off.  It's either a typo or perhaps check the soldering on the feedback resistor and IC pins.

[valveandsound]

Rob, thanks for the follow up. I did indeed confirm pin 12 is 0.45v.

I checked the 470K feedback and the 100K in series upstream of it, both measure correct and show continuity to each other and to the feedback pins.

What should this voltage be?

[valveandsound]

I did swap the CD4069 with another one that I had, no change in voltage or performance.

[Rob Strand]

I did swap the CD4069 with another one that I had, no change in voltage or performance.

Pin 13 is a little high and that could be forcing pin 12 to be low.
Both pin 13 and pin 12 should be around 4.5V.   DC-wise the circuit involving pins 13 and pin 12 is the same as pins 1 and pins 2, which are at 3.9V and 3.66V.  So the voltages should be something from 3.6V to 4.5V.

It's possible the 2.2uF cap C10 is faulty.

The voltage on output pin 6 is a little low as well.

If you want you can put a high-valued resistor (relative to the feedback resistor) from the gate input to ground and that will make the gate bias to a higher voltage.

If the circuit is oscillating then it could be causing havoc with the DC voltages.   That would require a completely different set of solutions.

[valveandsound]

I did swap the CD4069 with another one that I had, no change in voltage or performance.

Pin 13 is a little high and that could be forcing pin 12 to be low.
Both pin 13 and pin 12 should be around 4.5V.   DC-wise the circuit involving pins 13 and pin 12 is the same as pins 1 and pins 2, which are at 3.9V and 3.66V.  So the voltages should be something from 3.6V to 4.5V.

It's possible the 2.2uF cap C10 is faulty.

The voltage on output pin 6 is a little low as well.

If you want you can put a high-valued resistor (relative to the feedback resistor) from the gate input to ground and that will make the gate bias to a higher voltage.

If the circuit is oscillating then it could be causing havoc with the DC voltages.   That would require a completely different set of solutions.

I'll try shorting AC signal to ground with a .1uF cap before the CMOS stages to see if that changes the voltages

[Rob Strand]

I'll try shorting AC signal to ground with a .1uF cap before the CMOS stages to see if that changes the voltages

One thing that doesn't makes sense is pin 12 is 0.45V and pin 13 is 4.5V.  In that circuit how can pin 13 be 4.5V, as pin 13 get the voltage from pin 12 via the resistor.    If pin 12 was shorted to ground then pin 13 would follow and be some low voltage.  If pin 13 was shorted to something at 4.5V then it would make more sense.

[valveandsound]

OK, so I think maybe whats going on here is oscillation..

I replaced C10, no change. But if i check DC voltage at the CMOS chip (IE touch black probe to chip pin 7/ground) and then check voltage at pin 12, it jumps up to like 1.5vDC. If I bring my hands close to the chip itself when measuring this way, i can get the voltage on pin 12 to jump up to like 4vDC.

If i have my finger close to the chip and i bring the gain control up and down, it will change pin 14 voltage from .5v up to around 4vDC at max gain. if i dont have my hands/fingers close to it, then it stays pretty consistent at 0.4vDC regardless of gain pot sweep.

[valveandsound]

Well, i think that is a dead end. I built two of these pedals, using the exact same parts. One of them is still in the enclosure and one is out of the enclosure. The one inside the enclosure stays stable at around 0.47v regardless of gain pot. The one outside of the enclosure is the one that freaks out when i put my hands near the chip/gain pot.

[valveandsound]

So i compared the aionfx schematic with a copy of the original Sunn Beta lead schematic and it appears that the 2.2uF and 100K coming off the gain pot to the last stage of the CD4069 are swapped around. Would that cause anything weird? I wouldn't think so.

[Rob Strand]

So i compared the aionfx schematic with a copy of the original Sunn Beta lead schematic and it appears that the 2.2uF and 100K coming off the gain pot to the last stage of the CD4069 are swapped around. Would that cause anything weird? I wouldn't think so.

Shouldn't make a difference.

Your last three posts make me think it could be oscillating as well.

The question is:  Is is oscillation around the whole unit due the high gain, or, is it some local oscillation problem around the internal feedback loop: R15, IC2A, IC2B, IC2C.   The components C8 and R14 might be there to stabilize the local feedback loop. 

As an experiment you could tack some caps across C3, C5, C8. Try paralleling the same value that is already there - that will effectively double the cap value.    See if you can get rid of the oscillations first.  After that you might need to reduce some of all of those caps in order that the sound isn't affected.   Many times you only need to tame one point.   C3 and C5 deal with overall oscillation whereas C8 deals with local oscillation - you might need to tweak R14 as well.

[valveandsound]

Good idea. I need to order some more caps anyways so i'll do a few decades' worth to have extra on hand   Thanks so much for sticking with me!

[valveandsound]

OK so some progress here!

I went ahead and ordered a few new CMOS chips. The ones in the pedal are older, and from the same batch from the looks of the date codes on them.

Anyways, dropped a new one in and now Pin 12 voltages look correct. Both pin 12 and 13 are closer to 3.5vDC! Gonna box it back up and give it a listen but I feel like this might be it!!

[valveandsound]

So can confirm, with new IC in it sounds quite a bit different. Note naturally decays much better.

As an experiment, i did drop in a few others from the batch i recently bought. Some of them will measure weird on pin 12/13 and sound spitty - so maybe can chalk this up to some sort of tolerance issue OR perhaps the oscillation is still there. Either way both seem to be working now. Thank you Rob for your help!

[Rob Strand]

So can confirm, with new IC in it sounds quite a bit different. Note naturally decays much better.

As an experiment, i did drop in a few others from the batch i recently bought. Some of them will measure weird on pin 12/13 and sound spitty - so maybe can chalk this up to some sort of tolerance issue OR perhaps the oscillation is still there. Either way both seem to be working now. Thank you Rob for your help!

Great result.   You expect some variation but your original parts seem further off the mark than expected.   I'd have to go hunting for old measurements I've done in the past.