J201 weirdness

[Bosco Birdswood]

Putting a resistor in series with the 22uF "bypass" capacitor - your 1k (R18) limits signal gain a bit, it lets the 22uF increase signal gain, but not by as much.

Not by as much as what? So presumably I could increase the gain of the pedal by removing or reducing the 1k resistor (R18) from all the transistors?



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[anotherjim]

Good spot from Jonathan - the gain control is drawn wrong.

Not by as much as what? So presumably I could increase the gain of the pedal by removing or reducing the 1k resistor (R18) from all the transistors?

Yes, lowering the 1k must increase signal gain. The signal voltage gain is limited by the ratio of the drain and source resistance. If they were equal, the gain is a little less than x1.

Ok, Bosco, what have you changed so far? Have you fitted variable resistors in the drains?

Setting for 4.5v drain voltage is a starting point for these things. 4.5v is an ideal for maximum output swing. However, -Vgs may not be biased for maximum control voltage swing at the gate.
So the next step after getting Drain at 4.5v is to tweak the drain resistor one way or the other and listen for the best and loudest sounding setting.

[Frank_NH]

Looking at the old (hand-drawn) schematic, I agree that removing the 1k source resistors in series with the bypass caps (and letting the caps bypass the entire source resistance) will increase gain.

One other thing - I noticed that the buffer (T3) before the tone stack has a 56K source resistor.  I would lower that to 10K to get more current going through the JFET.

Let us know how it turns out! 

[Bosco Birdswood]

Good spot from Jonathan - the gain control is drawn wrong.

Yes, lowering the 1k must increase signal gain. The signal voltage gain is limited by the ratio of the drain and source resistance. If they were equal, the gain is a little less than x1.

Ok, Bosco, what have you changed so far? Have you fitted variable resistors in the drains?

Setting for 4.5v drain voltage is a starting point for these things. 4.5v is an ideal for maximum output swing. However, -Vgs may not be biased for maximum control voltage swing at the gate.
So the next step after getting Drain at 4.5v is to tweak the drain resistor one way or the other and listen for the best and loudest sounding setting.

OK cool, I'll try removing the 1k resistors and changing the value of the 56k resistor. Would I measure the current between the drain and source of the J201s?

My methodology so far has been to remove the original drain resistors, temporarily install a 250k pot and use that to work out the best resistance to get 4.5v at the drain. This value varied wildly from 35k to 135k...

Then due to space constraints (I bought a pre-drilled enclosure to save time and the jack holes are way too high) I installed fixed resistors rather than trim pots. I'm dead on 4.5v (to one decimal!) at all drains now apart from at t3 which is supposed to be 9v

Maybe if time allows I'll have a play with the pot again in place of resistors now that it's essentially working. At least I'll now be able to hear what's going on. I also built an audio probe so that might be of use.

I'll have a look at the Gain stage. I probably just made a tracing error as with the output.

Tell me this, why are both the variable and fixed resistance output of the trim pot connected to the drain on the English Channel schematic? Is this to ensure a minimum resistance rather than allowing a 0 resistance setting?


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[anotherjim]

Tell me this, why are both the sweep and the fixed output of the trim pot connected to the drain on the English Channel schematic? Is this to ensure a minimum resistance rather than allowing a 0 resistance setting?

Actually, it's to ensure that if the moving contact of the control momentarily breaks contact, resistance won't become infinite, it can never be more than the full resistance.

As I said, don't get fixated on 4.5v drain. Since changing drain resistance also changes -Vgs by interaction, an individual FET could be happier with some other drain voltage.
An audio probe can be a very good idea, since you can hear how it is at each stage.

[Bosco Birdswood]

I quickly tried swapping out the 1k R18 with a jumper and swapping the 56k from the source of the buffer to ground with a 10k. Pedal now has no gain. I suspect the buffer resistor swap to be the issue but will investigate in the morning.

By the way, can you explain why the signal routes from the source and not the drain of the transistor before the tone stack? Seems like a phase inverter thing to me [emoji848]


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[anotherjim]

The one with the drain directly at + supply is a source follower. It has no gain itself, simply acts as a buffer. Tone stack works as it should IF it is supplied from a low impedance source. The source follower has a very low output impedance - which is the source impedance to the tone controls. If it had a drain resistor, the impedance would be pretty much whatever value that resistor was.

OK cool, I'll try removing the 1k resistors and changing the value of the 56k resistor. Would I measure the current between the drain and source of the J201s?

Don't remove the 1k - replace it with zero ohm - a wire.

I'm not sure that tweaking the 56k can directly improve overall gain. It might need to be adjusted if T3 source voltage is too far from 4.5v AFTER setting T2 drain in its happy place.

Find current by ohms law. Measure the voltage across the drain or source resistor. DC current though either resistor and the FET is the same, since they are in all in series. Current is the voltage across the part divided by the resistance of the part.
Example, 15k drain resistor with 4.5v across it is = 4.5/15000 = 0.3mA.
Current through FET and Drain resistor is therefore also 0.3mA.

[Bosco Birdswood]

Don't remove the 1k - replace it with zero ohm - a wire.

Sorry, I meant to say replaced 1k with a jumper. I cut the leg off a resistor and used that.

Thanks for the other info. Makes sense.



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[Bosco Birdswood]

Given my resistance is so high between the 9v rail and the drains compared to the original pedals should I be raising the resistance of the resistor between source and 0v to increase the Vgs, or is it unnecessary?

I've tried a few things but I just can't achieve the gain of the original. Maybe tweaking the drain voltage by ear as suggested would help. I've also ordered a load more J201s to try out to see if the issue just lies with my current ones.


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[anotherjim]

That fx you are copying, did it have it's FET's in sockets?

The easiest way to clone a J-FET based circuit, is to take each original FET out and find another one from a bag load that matches it. You can then copy it part for part.

[Bosco Birdswood]

No such luck :/


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[anotherjim]

Dang!

But you can measure it's drain voltages. So with the same resistances in yours, you can try different FET's until you have one reading nearly the same.

[Bosco Birdswood]

Dang!

But you can measure it's drain voltages. So with the same resistances in yours, you can try different FET's until you have one reading nearly the same.

Yeah my game plan is as follows:

I have 20x J201 en route from China from two different sellers. Given that I have also copied the layout of this board and as such have no room for trim pots, I'm going to make up a breadboard with drain and source trim pots as well as the electrolytic and 1k resistor to keep it as close to the pedal as possible. I'll maybe even hook up a signal chain so I can listen to the transistor as I vary the voltage.

I'm then going to take each J202 and bias for roughly 4.5 Vds, use that to work out the relative resistance of the J201, then adjust the source resistor to get the roughly 0.4v Vgs as per the original pedal.

The amount adjusted here then also gets added to the drain resistance to ensure you're adding the same amount of resistance to both sides of the effective voltage divider.

If the pedal still doesn't have the gain I'm expecting I'll just live with it. It's still doing what I want it to essentially. I'd just like it to be as close to the original as possible.

I suspect that because I have quarter of the Vgs voltage on my pedal compared to the original the J201 is entering the non-linear area too soon and compressing / attenuating the signal somewhat. That's my layman's prediction for the day anyway!


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[Bosco Birdswood]

Hi guys, so I have another issue with this build. I decided to replace all the J201s as it was easier to work out the bias resistors (Rd and Rs) with new JFETS on a breadboard, and then install them afterwards. Everything measures great on T1, T4, and T5, but the voltage is now fluctuating on T2 (realise this should be labeled Q2 but I'm referring to the labels as they are in my diagram). It will start at around 5v at the drain and then slowly reduces in voltage. I've tried different J201s, I've tried breadboarding the J201s to see if they're stable there, which they are. I've removed C10 and C9 to see if they are causing issues and that did not change anything. I've even removed the J201 to see if there is anything weird going on like a voltage at the gate that shouldn't be present. The source seems to fluctuate as well slightly and I can find no reason for this.

Is this an issue with dodgy JFETS even though they appear to be working in most places (haven't passed audio through the circuit yet) or have I screwed something up somewhere?

[duck_arse]

it sounds as though you've lost your gate bias resistor. is there one?

[Bosco Birdswood]

it sounds as though you've lost your gate bias resistor. is there one?

Sadly not


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[Bosco Birdswood]

Wait! Yes! C12 is in the wrong place on the schematic. The Gain pot is actually the 0v reference for Q2 and I don't have it connected. Problem solved - thanks @duck_arse! Bloody love this forum


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[duck_arse]

easy. no wukin furries.