J201 weirdness

[Bosco Birdswood]

Hi guys, I'm in the process of building a clone of a drive pedal based on J201 transistors. I'm cloning a friend's pedal which I'm using as a reference. The weird issue I have is that having ordered J201s from Banzai I built the pedal and noticed that voltages and resistances differ from the original. After a bit of head scratching and triple checking my work I have discovered that the J201s on the original pedal have a resistance of circa 700ohms between the output and ground pins, whereas there is no continuity on the ones supplied by Banzai. Is this normal behaviour for a transistor to have relatively low resistance between the ground and the output?

The long and the short is that on my build I get nearly 9v at the output pin (the official name escapes me - I'm a noob!) whereas on the reference pedal it's 4.5v. Something must be creating a voltage divider.

I've sketched the schematic to the best of my abilities. Forgive the unofficial resistor symbols. It all seems to make sense but I am totally stumped by this J201 issue.

The pedal passes sound but it's very quiet with no overdrive.


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[amptramp]

Welcome to the forum!

Although you say there are J201's in your circuit, you are using the symbol for bipolar transistors, not JFET's, so which of these devices are JFET's?  The J201 is sought after because of its low cutoff voltage, -0.3 to -1.5 volts max.  The normal variation in Idss and Vp may make some JFET's look open while others have some resistance but how are you measuring the resistance?  There has been a problem with counterfeit devices, so you may have some duds.  (I first became aware of counterfeiting when I got some 74LS74's that were 16-pin devices.  They should be 14-pin.)

If you have traced out the schematic, I can see some suspicious areas such as the zener diode across an electrolytic with the negative end going only to the capacitor.

As for the output voltage, it should be zero because the pot at the output is connected to ground at the bottom and a capacitor at the top - there is nothing there to introduce any higher voltage unless you have a component problem.  This may be another schematic tracing error.

[GibsonGM]

You can build it in sections, get each section working before tackling the next one, if that is easier. You may need to use a trim pot to set the drain of the J201's near 4.5V....that FET variation may mean the listed resistance won't work or will sound horrible with a different '201. 

[anotherjim]

Junction between C4, C13 and zener anode should, I think, be 0v. Zener probably 12v & acting as reverse polarity/overvoltage protection.
The rest of the drawing seems reasonable.

Unlike ordinary transistors, your J201's have normally conducting channels, so out of circuit or power off, can read low resistance and that reading different between in circuit or out of the circuit.

As above post, we usually have to see what the voltages are like.

[Bosco Birdswood]

Welcome to the forum!

Although you say there are J201's in your circuit, you are using the symbol for bipolar transistors, not JFET's, so which of these devices are JFET's?  The J201 is sought after because of its low cutoff voltage, -0.3 to -1.5 volts max.  The normal variation in Idss and Vp may make some JFET's look open while others have some resistance but how are you measuring the resistance?  There has been a problem with counterfeit devices, so you may have some duds.  (I first became aware of counterfeiting when I got some 74LS74's that were 16-pin devices.  They should be 14-pin.)

If you have traced out the schematic, I can see some suspicious areas such as the zener diode across an electrolytic with the negative end going only to the capacitor.

As for the output voltage, it should be zero because the pot at the output is connected to ground at the bottom and a capacitor at the top - there is nothing there to introduce any higher voltage unless you have a component problem.  This may be another schematic tracing error.

Thanks for the welcome

All transistors are J201s here. I looked into the Zener diode section and my understanding is that the Zener is there as a voltage regulator. It's connected to the 9v rail in the schematic. I'm guessing the electrolytic cap is there to smooth out the supply. I suppose my question with this is why only go to this trouble for the power supply to the last J201?

To clarify, when I said 9v at the output I was actually talking about the Drain of the J201s, but being lazy with terminology!

As for the J201s, I'm pretty confident they're legitimate only due to the fact that Banzai Music is a reputable place. I guess I just need to research the Idss and Vp issue a little more. Up until Monday of this week I knew nothing about this stuff, and just like the Blackface Princeton clone I built I kind of had to learn how it worked because it didn't work first time round!

I'm very limited in what I can do to the reference pedal as it's my friend's so I can't really start desoldering components! I measured the resistance between the drain and the pin that goes to ground (official name escapes me) with the J201 in situ. All were around 700ohms give or take.

So the question is, will varying the resistance between the 9v supply and the drain change the voltage at the drain, and therefore is this all I need to do to bias the J201s down to the desired 4.5v?

Please bear in mind I'm a total beginner with electronics!


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[Bosco Birdswood]

Junction between C4, C13 and zener anode should, I think, be 0v. Zener probably 12v & acting as reverse polarity/overvoltage protection.
The rest of the drawing seems reasonable.

Unlike ordinary transistors, your J201's have normally conducting channels, so out of circuit or power off, can read low resistance and that reading different between in circuit or out of the circuit.

As above post, we usually have to see what the voltages are like.

OK, that's really interesting. So I shouldn't be too worried about measured resistances being quite different between my clone and the reference circuit then, but more concerned about voltages at the drain which I presume I could alter by changing the R16 / R17 etc values? Maybe with a trim pot as suggested above?

So, (forgive me if this is a dumb question) is 9v at the drain too much for the J201?


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[Bosco Birdswood]

Interestingly, this pedal is based very closely on the Vox AC30 top boost circuit. If you compare the schematics they're very close.


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[rutabaga bob]

Welcome aboard!  You'll find lots of good (and knowledgeable!) people here.

-Love the name-

[Bosco Birdswood]

Welcome aboard!  You'll find lots of good (and knowledgeable!) people here.

-Love the name-

Haha thanks - it's actually my dog's name with my surname as mispronounced by my vet [emoji23]


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[duck_arse]

your vet, or your dog's vet?

[Kipper4]

What NO input cap?
Welcome.

[Frank_NH]

Your pedal appears to be a very similar to the ROG English Channel.  The fixed drain resistors were possible because he likely tested them beforehand for proper bias.  Anyhow, compare your schematic with the English Channel.  The latter gain stages of your pedal are a bit different, but you might try building the English Channel on a breadboard and see how it compares.   

http://www.runoffgroove.com/englishchannel.html

[anotherjim]

I'd forget thinking of the zener as a voltage regulator. As there is no series resistance in the power input, a zener regulator would probably fry due to excessive current. That's why I think it could be a 12v one  - with the correct 9v supply it will never act. If Excessive voltage or reverse supply is plugged in, it is meant to cause that power supply to either shut down or blow its fuse. There is no guarantee that will happen - instead the zener could be blown open circuit and be no use at all or fry completely and permanently short circuit. Some designers do like to fit this form of protection as it's so easy to add. For reasons I just gave, I don't think much of it! There are better ways to protect the power input.

J-FET transistors can usually handle at least 25v, so no, 9v is not excessive.

C4 is the power filter capacitor, so it must be wired between +9v and 0v. C16 is a low pass filter with the 15k R11 so C16  must go to 0v.

[Bosco Birdswood]

C4 is the power filter capacitor, so it must be wired between +9v and 0v. C16 is a low pass filter with the 15k R11 so C16  must go to 0v.

Hi Jim, you are completely correct - I missed a 0v tie on the schematic that sits between C4 and C16. Good spot! Since re-drawing this part of the schematic what you are saying regarding the zener diode makes a lot more sense to me as it highlights that the zener is completely separate to the signal chain. So basically what you're saying is that if the voltage were to exceed 12v (supposing that was the rating of the zener) it would in effect short the power supply out, protecting the circuit?

If I could pick your brain further then, am I right in saying that, taking the T1 J201 as an example, R17's purpose is to set the voltage at the drain, R19 is maybe to regulate the current going through the J201, and I'm guessing that the purpose of C14 / R18 is to smooth the signal going to 0v making it DC to stop it from modulating the circuit elsewhere. Is that correct?

C13 then decouples T1's drain DC power supply from the Gate of T2, and C10 is a low pass filter after the gain stage.

Is that all correct?!

[anotherjim]

Let's put these two together.


The English Channel has trimmer controls for drain resistors. Why? Because the FET transistors have large variations, even in the same production run. At bottom left is an instruction to adjust the trimmers for 4.5v at the drain. It is possible to select the FET's to suit fixed resistor values, the builder of the original you have traced may well have done that - or wished for good luck.

The Drain resistor alone sets the voltage gain IF the Source resistor is worth zero. But the Source resistor also helps to achieve the right bias by raising the Source voltage relative to the Gate voltage which is 0v. Note that the Gate has no positive voltage supply, it is "tied" to 0v by some resistance.

The FET channel (drain to source) is NORMALLY conducting and to be controlled, the Gate voltage needs to be some amount below the Source voltage to turn the channel towards a high resistance, a condition called "pinch off" or "cut off". The amount of voltage needed is called -Vgs in specifications. You cannot set the Gate any lower than 0V, so the Source voltage must be raised above 0v. Raising Source resistance to achieve a good bias will subtract from the available gain.

A 22uF capacitor has low impedance at guitar frequencies. In parallel with the Source resistor it performs a magic trick. DC bias is not changed since the 22uF does not pass DC. However, the Source resistor is bypassed by the AC signal through the capacitor - so gain is increased for AC signal.
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.

[Bosco Birdswood]

Great, this is starting to make more sense. Does current matter between drain and source? On the original pedal (Menatone top boost in a can) I measure 0.5mA with no signal going through, but only 0.1mA on mine.

Also, would have drain voltages of 8.5v rather than 4.5v  be the cause of my pedal being quieter than the bypassed signal with no break up at all? If so why?


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[anotherjim]

It's a balancing act. If you change drain R you change the current through the channel toward 0v. That changes the voltage produced across the source R and therefore the Vgs changes, therefore the resistance of the channel changes and that changes the current again.
So yes, current very much matters. Increasing current tends to increase -Vgs, which tends to drive the device to cut-off, which reduces current. So, the thing settles to a state of equilibrium. For a good amplifier it helps if this state is in the middle of the linear operating region of the FET. Setting drain voltage to half of supply means the AC signal can swing up or down with decent "headroom" before clipping the top or bottom peaks. That does not mean it is super clean undistorted because the "linear region" is maybe not in quite the same place.

Getting a perfect setup is a whole level of messing with all the resistors or selecting FET's. But we are not necessarily aiming for hi-fi are we?

[PRR]

> J201s on the original pedal have a resistance of circa 700ohms between the output and ground pins, whereas there is no continuity on the ones supplied

Sounds like different pinouts. Not all are pinned-out the same. Learn to know your G from your S from your D and how to identify the parts in your hand.

A JFET with no other bias will read 100-1000 Ohms Drain to Source. Infinity (>>1Meg) suggests you found the Gate, not Drain or Source.
______________

> What NO input cap?

Using JFETs (despite the NPN in the drawing) a to-ground no-cap input is acceptable.

[Bosco Birdswood]

@anotherjim you are a legend! Pedal is now working almost perfectly. My final (maybe...) question is how to increase the gain of the pedal. Compared to the original pedal it doesn't break up as much and has a little less boost. Which resistors would you recommend changing / removing?


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[jonathanfdean]

Your gain control is hard wired to Max with variable highs

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