Fetzer-ruby Amp Questions

[Tony Enamel]

Hey all,

I'm a complete newb when it comes to electronics, HOWEVER, I am going to build myself a Fetzer-Ruby amp circuit to use with my sustainer experiments. Here's the schematic:

http://www.runoffgroove.com/fetzer-ruby.png

A couple questions:

1. I went to the store to get components (caps, etc) I found 100n and 100u caps but the all have different voltage ratings. What is the appropriate voltage rating?

2. I'm in Ontario and Circuit City (no Radio Shack) is the local electronics shop but they don't have J201, 2N5457 or MPF102. Where can I find these?

Thanks in advance. When I actually start building this thing there will be many more questions!

Dezz

Edited July 4, 2006 by Tony Enamel

[lovekraft]

1. Any voltage that is well above the supply voltage - for a 9 volt supply, 15 volt caps should be fine, although I occasionally use 50 volt or even 63 volt caps if they're less expensive or easier to get. You could actually use 630 volt Orange Drops in this circuit, but it would make the board huge, so i can't recommend it.
   
2. Other JFETs that might work fairly well include BF245A, 2N5484 or just about any GP audio device, although the J201 is probably about optimal in this case. You may have difficulty finding any JFETs whatsoever at Circuit City - check to see if there isn't a store in your area that does radio repairs and such, they usually stock a few parts, and might have what you need. If not, I'm sure somebody in Toronto has something suitable, and there are several internet sources, notably Steve Daniels at Small Bear Electronics. If none of that gets you what you want, PM me - I'm sure we can come up with something workable to get you started.
   

[psw]

Hey there Dezz...

I recall a place called hobbytron in canada that was selling kits, some even made up...some of LM386 amp modules and if I recall, possibly something with a preamp...

Would you believe, they won't supply outside of canada...so they would not sell me one at all...perhaps we could do something with one of these...give me a bit to look into it...maybe you could supply me!

Thanks a lot for the assistance LK, there are a few people having some troubles with the circuit...I am still currently sans soldering iron...grrrr

good luck... pete

ps feel free to email me

[Tony Enamel]

I did a search at Hobbytron for LM386 and found a few amp kits there. Have a look at this page: http://www.hobbytron.com/Amplifier.html Do you think any of these will work for my sustainer experiments?

Edit:

I got a hold of the fellow from Small Bear and ordered everything I need. For anyone else interested, he has a parts checklist for the Fetzer-Ruby amp so you can be sure not to miss anything. I ordered the components to build 5 F-R amps and it cost about $45. That's less than $10/amp! Not too shabby. Why 5, you say? I learn by trial and error. And on the off chance that I'm succesfull the first time, I can use the other parts for some different projects (ie. stompbox boost, practice amp, etc.).

Edited July 4, 2006 by Tony Enamel

[lovekraft]

...Why 5, you say? I learn by trial and error...

Gotta agree with that! I usually buy at least 10 of any component I pick up, not only because I break/fry/lose them, but the quantity discounts are often too good to pass up! I'm glad to hear that Steve hooked you up - he's one of the (few) good guys in DIY supply, and he's building a well-deserved reputation for going the extra mile to help his customers, so don't hesitate to buy from him if he's got what you need.

[psw]

That sounds great and a good strategy. Really the sustainer project should be pretty cheap in the form I am proposing. The amps can be used for practice amps and the like so it wont go to waste and you will have something to practice on. My guitar is set up so that I could swap in different modified circuits easily to the driver ans switches if I wanted to to test other circuit ideas...if something else comes along or you think you make it neater or smaller, you will have the parts at hand...good one. pete

[Tony Enamel]

OK. I got all my components for my Fetzer-Ruby. At the risk of embarassing myself, I have a few questions:

http://www.runoffgroove.com/fetzer-ruby.png

1. on a polarized capacitor, how do you tell which lead is + and which is -?

2. on my trim/vol/gain pots there are 3 poles (ccw, cw and sweeper). How do I tell which is which on the schematic?

3. I'm using an MPF102 which has 3 poles (ground, source and drain?). Which is which on the schematic?

4. I have to "bias drain (MPF102) to 4.5v". How do I do this?

5. I see where the 9v+ goes. Does 9v- go to ground? What is common ground? (can you tell this is my first project? )

I'll be eternally grateful for the answers to these questions!!

Dezz

[Primal]

1. If you look at the capacitor, it should have a blue stripe down the side with a - symbol. That is the negative side.

2. The middle terminal, i.e. the sweeper, is the on that has the arrow pointing to the resistor symbol in the potentiometer symbol (make sense?).

3. The base is the left side, the collector is the upper-right, and the emitter is the lower-left. With that chip, the base and collector, I believe, can be interchanged (see your packaging to make sure).

4. Adjusting the trim pot adjusts the bias. Take your multimeter and put one terminal (or whatever you call those things on the multimeter, haha) on the B+ and the other after the pot. Adjust it so that when reading voltage, the voltage is 4.5v.

5. Yes, 9v- is generally always ground. Common ground just means that every part of the circuit that goes to ground goes to the SAME ground.

[Tony Enamel]

Thanks a lot, Primal.

Just to clarify:

1. What does the sweeper connect to?

2. You say base, collector and emmiter, My data sheet says drain source and gate. Which is which?

3. Do you mean to say the 9v- IS common ground (every thing that goes to ground goes to 9v-)?

Thanks again.

[xtjdx]

I'll try to answer a couple of these (even though I have less than 1% of an hour of background in electronics).

2. I'd assume that the base is gate, collector is drain, and emitter is source.

3. 9v- goes to the ground point on the PCB and all commons go to that point.

Grain of salt, all that stuff, I'm probably wrong, but this whole thing is helping me learn too, so guesses are fun!

Edited July 11, 2006 by xtjdx

[Primal]

My fault, I was looking at a reference page for a bipolar transistor, not a JFET. But yes, the base is the gate, collector is drain, and emitter is source.

For further reference: http://encyclobeamia.solarbotics.net/articles/symbols.html

As for the potentiometer, the sweeper for the 100k trim pot is directly connected to one of the other terminals, depending on which way the pot is oriented. In the case of the volume control, the sweeper is connected to pin 2 of the opamp.

[Tony Enamel]

Thanks guys. I'm almost there....

9v- goes to the ground point on the PCB and all commons go to that point.

...and what does the ground point connect to? (on a guitar, all the grounds end up at the output jack.)

[xtjdx]

If I'm not mistaken, the negative on the 9V IS the ground. Otherwise you'd ground to a chassis I assume. Although that doesn't make much sense so someone else should clarify.

[Primal]

The guitar and the circuit share the same ground. Otherwise, you wouldn't be able to get the signal from the pickups to the driver amp. The best way to do it, I believe, is to have a ground on the circuit bored, then run a wire from the circuit ground to guitar ground.

[Tony Enamel]

OK, I rebuilt my amp on the breadboard with a new FET (I fried the first one) and attached a small speaker to the output and my guitar to the input. If I adjust the vol and gain pots to the EXACT right position, I get sound. If the pots move even a little I get this loud humming that changes pitch as the pots are adjusted. What could I doing wrong? I had my poles mixed up on the MPF102 the first time so I replaced it. Do you think I fried more than the FET? I'm going to re-build the entire curcuit with replacement parts and see.

Also, I still can't get a proper reading on the drain bias. The trim pot adjusts from 0v up to 3.5ish. But I'm supposed to bias the drain to 4.5v. Any thoughts?

Thanks,

Dezz

[Primal]

Make sure you are using a brand new 9v. As batteries get old, their voltage drops. Something is obviously wrong, either with your battery or your trim pot. If the pot is wide open (i.e. set to "zero"), then you should be getting the full 9v (in actuality its more like 10-11v) to the transistor, which shouldn't hurt it. My experience with the circuit leads me to believe that a 25k trim pot is all that is necessary for biasing the drain. The 100k doesn't give enough accuracy. I ended up settling for a 4.65v or so bias.

Make sure you have the polarities on the electrolytic caps correct. Other than those, the transistor, and the IC can be fried. Are you soldering things yet? When you do start, please purchase a few IC sockets. Heat alone can fry the ICs.

Remember, go one step at a time. First get the preamp working (the section with the transistor). Once you get that working, then move on to the poweramp (LM386 chip).

[Tony Enamel]

Right... I rebuilt the circuit AGAIN and I have the same bias problem. The battery is new. The trim pot is new (100k as per schematic). 9v+ goes to pole 1 on pot. Sweeper and pole 3 are together and out to the drain pole of JFET. Is this right? When I measure from 9v+ to just after the pot I get a reading of 0.683 or something no matter where I set the pot.

Edited July 16, 2006 by Tony Enamel

[Primal]

You don't have 9v- hooked up. Or if you do, the trim pot isn't working. If you end up getting a new trim pot, see if you can find a 25k or so pot. Also, make sure you know how your breadboard works. I had some trouble myself when I first started breadboarding.

Out of curiosity, is there any way you could take a picture of your breadboarded circuit so we could take a look?

[Tony Enamel]

Thanks Primal,

I'll try to get a pic up here. If a trim pot has no value written on it, how do I measure it? Is it Ohm meter on the two outside poles?

[Primal]

An ohm meter will work. It should have some numbers on it, though. All of mine have a number code. For instance, one might say 223. The first number is the first digit, the second number the second digit. The third number is how many zeros are at the end. So, one that says 223 would be a 22,000 or 22k ohm pot.

[Tony Enamel]

I'm obviously missing some basic bit of knowledge about circuits. I took everything out of the circuit except 9v+ to 100k trimpot to 9v-. I measured from every point in the circuit but the pot does nothing. I thought that the trimpot was supposed to add resistance to the current as it is adjusted but the voltmeter reads full voltage no matter what the trimpot is set at. The drain bias is the only part of my circuit that I can't figure out. HELP!!!

I'm gonna slap myself a few times, crack a frosty brew and try this again.

[Tony Enamel]

No luck. Anyone?

[Tony Enamel]

Thanks for all the help. I'm happy to report that my fetzer-ruby is fully functional. I still don't know why I was getting the wrong readings before but today everything worked out great. I'm using this preamp for a sustainer driver and it works! I got sustain on all 6 strings! The circuit is still on the breadboard so I got some soldering to do.

Anyone know anything about printed circuit boards? I plan on mounting this on a PCB using the toner transfer method to make the board. I'm having a little trouble making the transfer template. What is the best (easiest) software for designing PCB's?