Author Topic: Microtubes B3K - JFET hiss noise  (Read 663 times)


Microtubes B3K - JFET hiss noise
« on: July 20, 2018, 05:59:53 PM »
I build a Microtubes B3K from but i used BF245B instead of J201. Yes, I checked the pinout and turn JFET 180 degrees.
Effect works but I'm strugling with hiss noise on JFETs.
I used audio probe to detect problem and i think is JFETs. I cant find better JFET in my local stores, and im wondering what should i do to get rid of this noise.
If I play my bass hard, on higher notes that hiss noise gets quieter, so maybe lower resistanse could help?

I read about that using proper resistance values on source should work? but i cant figure it out.

Please help me with this, what value on witch component should I change.


Re: Microtubes B3K - JFET hiss noise
« Reply #1 on: July 20, 2018, 08:33:20 PM »

You don't yet know to blame the JFET; could be many things.

Measure and post Voltages. Let's start with both ends of R2 R3 TRIM1 (as numbered on the schematic found here.

Related thread:


Re: Microtubes B3K - JFET hiss noise
« Reply #2 on: July 21, 2018, 05:23:20 AM »
Thank you for your response.

Ok, first of all lets get the same schematics. This is the correct one for this project.

So the line is splited into two ways. A clean lina and a drive line and blended together at the and.
The clean line is good, no hiss, no noise. Just fine. The problem must be in the drive line.

I meassured voltages through the first part of drive line.

please tell me yours conclusions.


Re: Microtubes B3K - JFET hiss noise
« Reply #3 on: July 21, 2018, 10:09:29 PM »
> i used BF245B instead of J201.

They do not seem to be BF245B. data Your reading of 486mV, 0.486V, across 3.3K, says 147uA or 0.147mA. The upper device, flowing the same current, seems to have Vgs of just 0.06V (corrected for meter loading). A BF245B at 200uA should show 1.6 to 3.8V. I think what you got has Vgs much lower than BF245B. In fact closer to what a J201 should be.

Nevertheless, the biases around the two JFETs should allow it to work, though maybe not at best gain or lowest hiss.

I am suspicious of the low Vgs on the top part. Are you very sure there is not a short around Q2 or C8?


Re: Microtubes B3K - JFET hiss noise
« Reply #4 on: July 22, 2018, 04:18:02 AM »
I checked the Vsg ones more time and it was the same and I checked for shorts twice and I'm pretty sure there is none.

The signal is going trough the JFET stage as you seys, but its get "dirty" in bad way. I can explain it that way. If I probe the 9V line it sounds like a little hiss or bzzz. Then after JFETs the guitar signal is mixed with this 9V hiss sound, and after first opamp it gets ever worse.

I founded that, when the signal i strong enough the noise disapears. Like, when i pluged my phone through the headphone jack and play some music everything seems to be ok, but when i pluged my guitar the sound i dirty with this hiss. But when i play hard D or G string the noise is getting quieter while note is played. I think you get the point.

Later I'll post some audiosamples, so you can better understand my problem.

This is some photos of my project, and as you can see on photos JFETs are BF245B. Its very interesting what you said about them.

You said that this configuration gonna work but not in the best way. Could you tell my what i suposed to do, to get it better?


Ok, I just recorded a sample. Here it is:

The sound of bass is preety good, but this hiss noise is very annoying. Hope this help. :)

« Last Edit: July 22, 2018, 12:24:27 PM by Altarin »


Re: Microtubes B3K - JFET hiss noise
« Reply #5 on: July 23, 2018, 04:45:05 PM »
Ok, I figure it out!

It was noise from cheep dc adaptor.  :icon_redface:

I build simple filter circuit from this page:

And the noise just gone!  :icon_cool:

Thank you anyway for helping me.


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Re: Microtubes B3K - JFET hiss noise
« Reply #6 on: July 23, 2018, 06:21:07 PM »
I was just about to advise you to use a battery or filter the power supply ;)  VERY common problem!
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...


Re: Microtubes B3K - JFET hiss noise
« Reply #7 on: July 23, 2018, 08:09:17 PM »
I played a lot with Jfet distortion with the pedals.
The simplest option is to install a capacitor between the gate Q1 and ground.
The capacity of 220p-330p should be sufficient. But with a decrease in noise, high frequencies also decrease.
This will definitely help if the problem is in mu-amp, it helped me out, but the noise reduction depended on the number of stages of distortion.


Re: Microtubes B3K - JFET hiss noise
« Reply #8 on: March 15, 2019, 01:33:25 PM »
Hi guys, I've built a couple of these thinking it was a good idea and I've come up against the same issue with both, after eliminating the issue of the one of my IC's were buffered and the other unbuffered, i followed on to rebuild one and both of these pedals now have the most irritating hiss.. I lose most of the volume when turning the 'level' A100 Pot half way, and the diodes and Jfets have all been replaced. The clean side of the blend seems to have none if any effect an i can hear it is actually working. but it just ups that hissing.

did u ever figure out what it was?? 
Because going on 3 days i've really started to lose hope that its the schematic and not me. (I'll up a picture and sound clip when i figure out how to)



Re: Microtubes B3K - JFET hiss noise
« Reply #9 on: March 15, 2019, 02:22:07 PM »
I don't know if it will help but if anyone can shed some light on what it is, and i'm using a regulated 9v+ DC on my Soldering station to power the circuit..


Re: Microtubes B3K - JFET hiss noise
« Reply #10 on: March 19, 2019, 07:57:05 AM »
Just rebuilt this, twice, for a third time.

Oh my god, please someone heeelp, What is this awful hiss i am getting and tremolo on full ?? sounds like a waterfall on 3/10 level and then oscillates with distortion. It must be a wrong value resisitor..

I also have this feeling the switch going to Grunt 1-2-3 pads are not labelled correctly or possibly the pots legs aren't also. The hiss i can understand might be from the power supply but The pots aren't working as they should.  (C100 Drive) - (A100 level) - (B100 Blend) are these correct?? I am reading all over the show people are changing these up to different arrangement or using just A100 pots across the board.
It works to a extent but not quiet enough to use, what am i doing wrong.... I asked on another forum but no reply yet. If anyone can help me please do, i'm desperate after 3-4 days of troubleshooting. im ready to give up. today is the last stab at this before i jog it on


Re: Microtubes B3K - JFET hiss noise
« Reply #11 on: March 20, 2019, 01:49:22 PM »
I had some what of a revelation today, i replaced the 2 x 100uF caps and boom its not hissing and not oscillating. Still finding the pots are still acting just like volumes and not really changing much of the sound and the middle position on the switch is awfully quiet in comparison to the rest of the settings which grunts and sounds beefy as hell now.

The C-100 Drive pot doesn't seem to do much apart from drop the volume, not finding its changing the gain much and tapers off to 0 volume, the A-100 Level pot which I'm guessing is what would act as a volume is working and the B-100 Blend Pot is iffy at best. It works a bit on some switch selections and doesn't on others or doesn't do anything and seems like its just another volume. I'm still working on it but i have a somewhat working pedal now which I'm very happy about.

One other thing, does this have more drive to give? I find it's a bit weak on the drive side of things. The grunt is present but the growl is not.. It's enough to get by and add a certain amount of grit but not 100% to the original. It comes very close though.

I'm still going through the PCB to get this final bit out the way but for now its working. Thank you to the lad who emailed me. appreciate that :)
« Last Edit: March 20, 2019, 02:05:49 PM by Jimblez »


Re: Microtubes B3K - JFET hiss noise
« Reply #12 on: March 20, 2019, 10:59:02 PM »
Would you be able to measure the voltages on the IC pins and the JFETs and post your results here?
It may help with diagnosing the problem.