Please help me out - debugging my Ruby Bassman amp

[Rutger]

Hi,

This is my very first build, a Ruby Bassman amp from runoffgroove.com Ruby.
Well, stubborn as I am, I gave it a view extra’s:

- 2k gainpot instead of the 2k resistor
- tonepot like the one on the Noisy Cricket
- mastervolume right before the headphone/speaker jack

So far, no sound/noise or whatsoever comes out of the amp. I checked and double checked my schematic, layout and perfboard artwork but I can’t find anything, so I hope you can help me out.

I did some measurements on the JFET and the IC. The measurements are done with the tonepot disconnected.

JFET
D= 9,2V
S= 8,4V
G= 8,4V

IC
P1= 2,9V (gain on 10)
P2= 0,4V
P3= 0V
P4= 0V
P5= 0,7V
P6= 9,2V
P7= 0,4V
P8= 4,6V (gain on 10)

The only thing I can tell from these measurements is that things go wrong, but I don’t have any clou…

[loudthud]

First off it looks like the JFET is not installed correctly or it is defective. Do you have the Gate connected to +9V where the Drain should be?

Next, on the 386 pin 5 should be about 4.5V and pin 2 should be closer the 0V. Do the voltages change when the volume control is rotated to the ground side?

[Rutger]

Thanks for your reply!

Today I had some time to figure things out. The JFET is installed correctly but I measured the JFET and I think it's broke (I measure 0 ohm between drain and source), so I need to get a new one (my spare one is defect as well...). Actually I wonder how...?

When I turn the volumeknob voltages change indeed, with the volume at max P2 becomes 0V. P5 stays around 0,7V.

[Rutger]

I installed a new JFET but I still have 0.7V on P5. Does anyone have an idea what it could be?

[loudthud]

The JFET should measure something around 200 ohms from Drain to Source. This can vary from part to part but it shouldn't measure a dead short. The Gate should look like a diode to either Drain or Source, low resistance one way, reverse the leads, very high resistance or infinite.

JFET
D= 9,2V
S= something between 1V and about 4V.
G= 0,0V

The JFET should have no effect on the LM386 unless you have a bad capacitor that connects the JFET with the volume control.

The only thing I can think of that would cause problems is a bad IC, bad capacitor or a wiring error. Try removing the capacitors one at a time or all of them to see of the voltages look more like:

IC
P1= 1,2V
P2= 0V
P3= 0V
P4= 0V
P5= 4,5V
P6= 9,2V
P7= 5V
P8= 1,2V

[Rutger]

Thanks Loudthud!

I have my LM386 problem fixed. Quite embarrasing actually... I soldered the layout looking at the back of the perfboard, so that the layout is mirrored when you look at the parts on top. That's no problem for most of the parts, but you can't mirror the IC ofcourse... so I reversed the legs of the IC and placed it back into it's socket, now it's working like it should! I measure the right voltages and it actually works when I skip the JFET.

Only thing now is the JFET. I checked everything over and over, solderjoints, parts, layout, etc... I still measure 9,2V 8,4V and 8,4V at D, S and G with a new JFET. Maybe the configuration of D,S and G differs from standard configuration...? I'll test the JFET's again tonight.

I keep staring at the darn thing, sometimes it's so obvious that you keep overlooking it.

[loudthud]

Carefully check the JFET with an ohm meter out of the circuit. The GATE will look like a diode to the DRAIN or SOURCE. (Diode= low resistance one way, reverse the leads, very high resistance)

Just get the Gate hooked up and the other pins are interchangable!

[Rutger]

Thanks loudthud, I checked the JFET's and they're (still) okay.

Well, this mirrored soldering thing kind of freaked me out... turns out that I made the same mistake with the JFET as with the LM386 over and over... I had it reversed.
It seemed right when I looked on my layout and at the back of the perfboard, but I should 've looked at the top of the perfboard to see I was doing it the wrong way.
The pots same thing: they are all reversed so they work anti-clockwise instead of clockwise

Personal learning point: never, NEVER solder you layout from the back of the perfboard! It messes with your head.

Well, the positive thing about all this is that I learned how to debug a circuit and check everything in detail befor I do anything stupid. It's my first project ever, and it's nice to know that there's nothing wrong with my electronics or soldering skills. Only with my common sense, haha! It gives me confidence to go further to a next project

Now it works, and works very good. The noise level is very low. It does what they tell about it. Cleans are really nice, it's quite responsive, the overdrive is okay. Just a typical stompbox kind-a overdrive, nothing tube-like really. I prefer the 2N5457 over the MPF201, just a subtle difference. I placed a 33k resistor right before the JFET for a tighter overdrive and it seems to do it's job nicely. The tonepot actually works quite nice as well, far more subtle than most toneknobs in stompboxes.

Thank you very much for your tips and patience!

[Rutger]

Ruby farts...

Hi,

Since I made the Ruby work it had been laying around in a box since I haven't had the time to build it in a housing untill now. Everything is installed now and since my mastervolume didn't work, I replaced it with a switch to ground so I can switch in a little more overdrive.

But now it farts, that arteficial overdrive kind of noise on top of the guitarsound that appears when you pick the strings a little harder, then stays on when the notes die out and finally suddenly dissapear. It farts in every setting, clean and overdriven, especially the low notes. First I thought I had to look at the last change I made (the switch) but bridging the switch doesn't solve the problem. I have no clue how to solve this, so can anyone tell me where I should start looking...?

[Rutger]

anyone?

[loudthud]

Hard to say what the problem is without having the amp right in front of me. Are you running from a battery? Try a fresh one. What kind of speaker are you using? Have you tried a different one?

ICs like the LM386 have a problem if they get too much signal on the input pins. Try installing a 47K resistor between the volume pot and pin 2 if the IC. If it fixes the problem but there is too much signal loss, try a 22K or a 10K.

[Rutger]

Thanks

I run it with an adaptor, which is fine. I'm using a brand new 8" speaker, haven't tried something else.

I did find a loose solderjoint and fixed it, from input to earth. Didn't fix the problem but gave me a very sensitive input so it picks up every bit of noise and a German radiostation...
Seems like every time I get my hands on this little build some new problem appears

Measured the voltages of the JFET and IC again and found a slight difference on pin 7 of the LM386: 4,6V instead of the 5,2 I've measured before. I must have overlooked it last week. The rest is still fine.

[Rutger]

This weekend I had some time to take a look at my little project again.

I swapped out the 'gain'switch for it wasn't very useful. I hoped that would do something good about the weird buzz but it didn't. There is obviously something wrong after pin 5 and/or 7, for the cap and resistor to ground are there to prevent such noises. But didn't figure out what yet... any idea how to measure the quality of a cap with a simple multi-meter?

I placed a shielded wire between input and the perfboard to eliminate the noises and radiostations. But again that didn't help much. So I decided to let the electronics for what it is and finished the speakercab for the 8 incher I was working on as well. I installed the speaker in the cab and did a little test run with the ruby connected... noises and radio broadcasting belongs to the past...! No idea how but apparently the speaker had something to do with it.

Now that the speaker is installed the little buzz isn’t that noticable anymore. Actually the little cab is quite convincing, I made it closed back and ‘oversized’ and that is noticeable, it isn’t boxy like many practice amps are. It sounds very warm and the speaker still needs to break in. With the ruby it is a joy to play with! The overdrive of the ruby a lot more confincing than through headphones.

I recommend installing the toneknob of the noisy cricket, with the ruby this simple circuit works very subtle and effective. Much better than on an avarage stompbox.

Maybe a little weird to be that enthusiastic about a project like this. But the fact that the project really does what you had in mind before you started building is very satisfactory.
I will use this one as a practiceamp for my students.