Virgil Overdrive substitutions…

[Bricksnbeatles]

I don’t have any 120pf caps on hand, but I have plenty of 100pf and 200pf. Looking at the FuzzDog schematic of the same circuit, it looks like the 120pf cap forms a LPF with a 1k resistor, which if my admittedly rusty math is correct, would put the cutoff at around 1MHz, which seems awfully high. Even a 200pf cap would be around 700kHz which is many many octaves above the threshold of human hearing. Should be able to swap it out with absolutely no impact on tone right— it’s just filtering potential rf noise, right? In that case better to sub for a significantly larger value instead of a slightly smaller value (100pf would cut off at 1.5MHz), right? Just want to make sure I’m not missing something obvious that would put the impact into the audio range.

http://pedalparts.co.uk/docs/NaughtyMonkey.pdf

On another note, would this circuit be a good candidate for subbing the j201 for a J113? I have an absolute ton of J113s and with the scarcity of J201s, it seems I’d be better off saving my 201s for circuits that need it if this one can do without.

 

[music6000]

I find that some 100pF ceramics caps I use always measure up around 150pf so measure to to suit.

 

[swyse]

I think that's just there to stop radio stations from being picked up, I would think lower frequency is better to catch the AM stations too so I'd use the 200pf, I personally use a 330pf in the same set up on one of my designs because AM goes down to 540khz in the US and I didn't notice any difference in sound, but I wasn't having any radio station issues before either.

 

[Bricksnbeatles]

I think that's just there to stop radio stations from being picked up, I would think lower frequency is better to catch the AM stations too so I'd use the 200pf, I personally use a 330pf in the same set up on one of my designs because AM goes down to 540khz in the US and I didn't notice any difference in sound, but I wasn't having any radio station issues before either.

Great! That’s what I thought. If my math is right I could technically go up to 6n8 (23kHz) without it interfering with the audio spectrum at all, but I’ll probably just throw a 2-300pF cap in.

 

[lrgaraujo]

About the fet, I think you can replace the j201 with a J113, since it's acting as a source follower



Your math is correct, but not entirely: the cutoff frequency of a 1k-6n8 lpf is indeed ~23khz.
However, a 1st order lpf begins attenuating before said frequency. If I'm not mistaken, the cutoff frequency is the frequency at which a 3db attenuation occurs. You can see this on this ltspice plot:

 

[benny_profane]

would this circuit be a good candidate for subbing the j201 for a J113?

That stage is a source follower with the gate biased to Va (4.5v? The power section is a bit odd.). The Vp spec for J201 is -0.3 to -1.5V; the Vp for J113 is -0.5 to -3.0V. I'd suggest measuring the J113 devices you have on hand and pick one closest to the min of the spec. You may have to adjust Rs (R4) to account for the increased Idss of the J113. I think you'd be fine with the sub otherwise.

JFET Biasing - part 1

I spent a fair amount of time trying to make this as concise and readable as I could. Intro In theory, setting the bias on JFETs should be easy. In practice, not so much. Here are the details on why that is and what we can do about it. Let's review some theory and get the math out of the...

forum.pedalpcb.com

 

[Bricksnbeatles]

About the fet, I think you can replace the j201 with a J113, since it's acting as a source follower

Awesome! Thought so, but never hurts to check.

Your math is correct, but not entirely: the cutoff frequency of a 1k-6n8 lpf is indeed ~23khz.
However, a 1st order lpf begins attenuating before said frequency. If I'm not mistaken, the cutoff frequency is the frequency at which a 3db attenuation occurs. You can see this on this ltspice plot:

View attachment 49030

Ah yeah, my bad. I know it refers to the -3dB point, but I was thinking the attenuation is less than half a dB just half an octave before that cutoff point, which would be imperceptible (1dB is often considered to be the minimum perceptible difference to humans) which would make a 23kHz cutoff be imperceptible below 20kHz, and even that‘s a generous upper threshold for human hearing. Looks like the 1dB point is around 10k in that plot, which would be fairly noticeable. Wasn’t planning on putting a 6n8 cap there anyway, but figured I’d jump into the hypothetical rabbit hole.

 

[Bricksnbeatles]

That stage is a source follower with the gate biased to Va (4.5v? The power section is a bit odd.). The Vp spec for J201 is -0.3 to -1.5V; the Vp for J113 is -0.5 to -3.0V. I'd suggest measuring the J113 devices you have on hand and pick one closest to the min of the spec. You may have to adjust Rs (R4) to account for the increased Idss of the J113. I think you'd be fine with the sub otherwise.

JFET Biasing - part 1

I spent a fair amount of time trying to make this as concise and readable as I could. Intro In theory, setting the bias on JFETs should be easy. In practice, not so much. Here are the details on why that is and what we can do about it. Let's review some theory and get the math out of the...

forum.pedalpcb.com

so much of that goes over my head haha
I really should get my hands on a DCA75— maybe I’m best off just socketing and seeing what works best, since I don’t have a JFET testing rig set up on any of my breadboards (even though I *typically* don’t love socketing transistors)