Adjustment of FET bias in Procrastinator

[jlm1948]

The title almost says it all. I have set the bias so that the signal is greatly attenuated, but I would like to know if there is a more scientific procedure. I have a scope, gen, millivoltmeter. The trimmer is very near the top of its travel.
Thanks

 

[zgrav]

how does the sound change when you alter the setting on the trim pot? with your current setting, what voltage are you getting on collector for Q6? Usually a trim pot for a transistor will set the voltage at about half of the supply voltage, and changing that amount affects the compression or amplification from that transistor. For an effect that tamps down the attack on your picking, I think you would want a more compressed signal.

 

[jlm1948]

Q: "how does the sound change when you alter the setting on the trim pot?"
A: Not much, except at the very end of the trimmer rotation, where it seems to act as expected, but not very convincingly. That's my concern, since I think there may be a better operating point if the control range was extended. I'll experiment with a lower value for R23.

Q: "with your current setting, what voltage are you getting on collector for Q6?"
A: Without signal I get about 4.7V. With signal, it goes down to a voltage between 0.7 and 2.7V, depending on the trimmer's setting, which is expected.

Q: " Usually a trim pot for a transistor will set the voltage at about half of the supply voltage"
A: That is true for a stage that operates in its linear region, but here Q5 & 6 are used as a switch so their state varies between open and closed.

Eventually I got somewhat more elaborated answers from forums, but basically it ends up being "tweak until you're satisfied with the result".
I don't think that's how it was done at Boss when they made the Slow Gear. It may be a suitable procedure for a boutique manufacturer, but not for an industrial product.

 

[zgrav]

I think you are right that Boss would have had a recommended setting, but even that would have been based on someone's decision about what setting made the effect work the way they wanted it to. The settings for using the pedal also must have a narrow sweet spot. I watched a youtube video for the Boss pedal where the person had adjusted the pedal before recording the video and he did not change the settings on the pedal during the video. For a similar pedal, here is a link to the PAIA Gator build docs that includes a short discussion about using a noise gate for envelope shaping: https://www.paia.com/manuals/docs/5730pages_200.pdf

 

[TeraWales]

Hello...remove the output MOSFETs. Don't connect any load. With no output transistors and no load the circuit will balance itself anyway and we will see if everything is fine or not relatively safely;Set VR1 pot to the maximum R position.
Connect the V-meter (DC) to the output (R17 / R15 junction);
Now switch the power on. You would see something close to zero (less than 100 mV) at the output. If you see higher DC - something is wrong. Solve the problem until you see less than 100mV.

printed circuit board and assembly

 

[zgrav]

Perhaps I should have asked this earlier -- when you describe the trim setting working as expected but "not very convincingly" what do you mean? It sounds like you are wanting more out of the pedal effect than you are getting at any point during that trimmer setting.