Beginner building the Ruby amp...

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

In this post http://electro-music.com/forum/post-62184.html dnny recommends using Maplin's 2N3819 instead of the MPF102 (which doesn't seem to be available in the UK). So that may not be the problem.

asafnetzer · Joined: Jun 16, 2007 Posts: 112 Location: Israel

Hi, It's really hard to see whats going on the board Very Happy

If you can post pictures of the lower side it could help more.

In the meanwhile try to check the connections again, look for solder bridges or cold solder points.

Best regards,
Asaf

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Well I just compared the coloured resistor bands on my resistors to the ones on the Ruby website and they are different! This must mean my resistor values are wrong, surely?

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Resistor bands are universal, aren't they? I've just checked the colours on my resistors with this Graphical Resistor Calculator and mine have very different values!

If I go into Maplin and specify colour bands, I should come out with the correct resisitors, right?

Uncle Krunkus · Posted: Thu May 08, 2008 3:50 am Post subject:

If you've got a multimeter of some kind, check the resistor values which you're unsure of.
Some resistors have a 3 band code, some have a 4 or sometimes 5 band code.
For example, a 27K resistor could be
Red, Purple, Orange (2,7 and 3(0s)) or
Red, Purple, Black, Red (2,7,0 and 2(0s))

Get in the habit of checking what you're putting in somehow. On a larger project, realising that you've soldered in 30*10K when they should have been 100K is a real pain in the arse. Getting two values swapped could lead to a fault that you never find. Checking them might seem tedious, but having things work first try will put a smile on your dial every time. Very Happy

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Ok, so I think I have 4 band resistors with a 5th for tolerance. They are:

My 1.5M is brown, green, black, yellow + brown = 150 000 = 1.5M which is correct

My 3.9K is orange, white, black, brown +brown = 3900 = 3.9K which is correct

My 10R is brown, black, black, gold +brown = 10.0 which is correct.

So it looks like my resistors are ok after all. I was comparing mine with the 3 band ones in the Ruby example.

asafnetzer · Joined: Jun 16, 2007 Posts: 112 Location: Israel

If you are a Mac user there is a nice free widget called resistulator, very nice for a quick check.

resistulator.jpg

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LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Cool, thanks for that - just installed it!

I've had a look on the underside and I can't see any cold solder joints or bridges. I may have to get a magnifying glass to have a proper look though.

Wild Zebra · Posted: Thu May 08, 2008 8:51 am Post subject:

Make sure you cut the trace on the board as well. Thats the red square with the red dot in the middle on the layout by the cap in the top left. You'll have to use a drill bit or something to break the copper so there is not a connection. Got me??

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Yep, I've broken it there and in between the pins of the 386. A friend thought that the biasing might be wrong on the 386 and it's getting too much negative feedback, but if I've got the right resistors in then I don't think that should be a problem.

Uncle Krunkus · Posted: Thu May 08, 2008 4:09 pm Post subject:

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

After work commitments meant I had to take a break from this project, I started on it again with renewed vigour today. I doubled checked for cold solder joints and shorts etc. to no avail. Problems that I have now fixed are:

It's still now working, however! It still oscillates but at a slightly higher pitch, and turning the gain pot determines how far I can turn the volume pot before the oscillation kicks in! I'm at my wit's end!

I'll try and post a picture of the underside tonight, but I think it might be easier to start again!

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

I've just replaced the 386 in case I had blown it with the bad connections - same thing unfortunately, is just makes a loud square wave sound.

I've taken a couple of pictures of the solder side of the stripboard in case anyone can see anything immediately obviously wrong (apologies for the terrible image quality):

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

If you have a potentiometer with the lugs pointing upwards and you're looking at it from the back (i.e. with the spindle pointing away from you), are the lugs numbered 1 2 & 3 from left to right?

Uncle Krunkus · Posted: Tue May 20, 2008 7:58 am Post subject:

It doesn't really matter as long as the middle connection (2) goes to the right place. Swapping the outside two just reverses the effective rotation.

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

So, in this case, the volume pot will be the wrong way round? Turning it clockwise will make it louder?

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

I went to modman's site where he has built himself a working Ruby amp. He has a list of correct pin voltages on there which are:

Uncle Krunkus · Posted: Tue May 20, 2008 4:54 pm Post subject:

It's either a dead FET or connection errors, and I think you've ruled out errors in the build, so I'd replace the FET and see what happens. I'm still not 100% sure that the FET you're using is a good substitute, but wait till you've tried another one before you go down that path. Smile

Uncle Krunkus · Posted: Tue May 20, 2008 4:59 pm Post subject:

When you take the FET out, check what voltage you have at the Gate connection. (Where the input comes in) With no input connected this should be at 0V.
I was just wondering as the figures you took could point to the FET being saturated somehow.

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Uncle Krunkus · Posted: Wed May 21, 2008 1:06 am Post subject:

I mean check the voltage at the point where the gate would connect. The junction of the 1M5 resistor and the input.

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Is the FET the wrong way round on the stripboard layout?
Posted Image, might have been reduced in size. Click Image to view fullscreen.

According to asafnetzer's picture here:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

The FET pinout is 1 = drain, 2 = source & 3 = gate

According to the Ruby diagram here:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

The input goes to the gate, but in the stripboard layout the input is going pin 1, the drain. The FET needs to be turned round, I think. Can someone confirm?

Uncle Krunkus · Posted: Wed May 21, 2008 11:45 am Post subject:

If you were using an MPF102 then the gate would be pin3 (according to that pic) and yes the layout looks the wrong way around. If you're using the 2N3819, the gate is pin 2 and you need to get that pin to the input. (The source and drain are often interchangeable, but for this design I'd try and get them right. Source to +ve)
What FET are you using? I thought you weren't using the MPF102.

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

Yes, I am using the 2N3819, but I soldered it onto the circuit board using the stripboard as a guide not the circuit diagram (i.e. I have the gate pin on the 2N3819 into 5a, not 5c as it should be). I'm back from work now so I'll be able to test that I'm getting 0v at the gate connection and I'll also be able to put a new 2N3819 onto the board correctly.

Thanks for your help!

LetterBeacon · Joined: Mar 18, 2008 Posts: 454 Location: London, UK

I've put a new FET in with the correct pins in the correct holes and I now get a buzzing from the speaker as you'd normally expect from a guitar amp - progress! Unfortunately when I plug my guitar in I don't get a sound although the buzzing gets louder when I tap the guitar strings, so I guess this must mean I've got some connections right!