Converting Smoky to a more useful 'Little Gem'

[javacody]

I've got some newbie questions. Here is my background in effects and electronics thusfar:

Modified a TS-5 to TS808 specs with additional bass response and clipping diode mod for sweeter distortion.
Built an Orman Mini-booster with Radio Shack parts.
Modified several guitars with different pickups, pots, tone caps, etc. etc.

So I think I have a halfway decent start, but have gotten away so far without really understanding what does what, other than simple stuff (capacitors and resistors).

Anyway, long story short, I want to convert my smoky to a Little Gem (as it has 4 of the components already and I've already overpaid for it by about $25) and was trying to figure out the smoky schem, but the LM386 pins are wired differently. What are the differences?

Is there any good/easy (you choose ) way to use the existing pcb of the smoky and mod it to a little gem?

Also, can anyone recommend a good, cheap, small speaker readily available locally in the midwest?

Last but not least, how is the reostat wired for the Little Gem?  Can I simply use a regular potentiometer here?

[Peter Snowberg]

Welcome Javacody,

I don't have the schematic of the smokey so I'm not much help there, but all the 386 circuits are pretty similar.

Perhaps the best place to start is by looking at the National datasheet for the chip:

http://cache.national.com/ds/LM/LM386.pdf

That gives you a good jumping off point at least. I'm sure it will be pretty easy to reuse the PCB with only minor changes at most.

When it comes to the speaker, the most important thing for great tone is to use a real guitar speaker which is fairly efficient. The smaller units are OK for a pinch, but if you want good sound, I would recommend going with an 8 or 10 inch unit.

Weber VST makes some really nice speakers and they have a clearance special on Signature Series C10S units at $20. I don't think you can go wrong there. The smaller the speaker, the more brittle the tone. I played through a 386 amp with a 3 inch speaker for a long time before trying out a full sized, real-deal unit. The difference was night and day and then some. I know that's not the small speaker you requested, but I think it's beyond worth it and they are in Kokomo, Indiana.

http://www.webervst.com/bargain.html
http://www.webervst.com/

You can also use just about any car speaker. The best way to pick one is to take your amp along with a guitar and audition the possible units at the local car stereo place. They may even have a single speaker from a damaged pair sitting around... you never know. The variety of sounds is quite large and a surprising numbr of expensive speakers will not sound very good in comparison to a speaker made for guitar amp use. Let your ears guide you if you can in this case.

For an attenuator, I've used a regular 500 ohm 1/4 watt linear pot in line with one of the speaker leads without any problems. The level is almost whisper quiet at max resistance. Works GREAT!

Take care,
-Peter

[javacody]

Thanks for the tips Pete. That one about the Weber speaker on sale for $20 is almost worth a finders fee.       Also, thanks for the tip about the pot.

[javacody]

Ok, I hit the shack today and got all of the parts except for the 100 uF cap. I only had a 47 uF cap lying around so I used it. Can anyone tell me the effect this will have on the little gem?

Anyway,  I slapped it together and it worked great the first time around. I ended up going with a Jensen 8" Ceramic reissue for $18. Right now the amp is in a radio shack tin project box and the speaker is in the carboard box it came in (I cut a hole in one side). Does anyone have any recomendations for a cheap speaker box?

This amp sounds great! Even better than the smoky. Honestly, it took me longer to drill the holes in the box and tighten down all of the nuts than it took me to solder the components on the board.

One last question, I want to add an LED to this little amp, how would I do that? I'm assuming it wouldn't be as complicated as the millenium bypass, as I simply need a power indicator. The power is switched on via a stereo 1/4" jack.

[Peter Snowberg]

Cool!  

The smaller cap (I'm assuming that's the one in-line with the speaker) will just reduce the bass a little. When you get a 100uF, try adding it in parallel with the 47uF and see how you like the response. Play that by ear. A very common value in that spot is 220uF, but a wide range will work fine.

Since you said you had the speaker mounted in a box, is it open back or closed? If the box is closed up, that will increase your bass response. You might want to try out some other boxes for experimenting. Speaker enclosure design is quite an art. I say play with it and find what you like most.

Adding a power indicator is very easy.... just connect an LED in series with a good sized resistor like 4.7K to 10K and put that across pins 6 and 4 of the LM386. Presto, you're done.

I'm glad your upgrade went well.

Take care,
-Peter

[javacody]

I used the schem at RunoffGroove (http://www.runoffgroove.com/littlegem.htm, the first one with a single lm386) and the cap I'm talking about is the one that is connected to +9v and goes to ground. What is this cap for?

Thanks for the tip about the LED, my little amp is about to get a cool little upgrade!  

Rather than upgrade my smoky, I started over. I figured I could still leave the smokey in my gig bag as a backup.[/url]

[Ansil]

power supply filter.. is what that cap is for.

also i don't know where you are located but if you are in the us you can usually find a grocery store that has a bakery they will give you one of there cake icing buckets for nothing just so they don't have to throw it away.  they have these short squatty buckets that will hold a 12" speaker oh so nice and if you put in a little insulation it makes a great little speaker cabinet.. also if you use a taller bucket it makes a great resonant bass sound.  you can also use these as a deadbox or an isobox.  has a great reverb sound that is really fun.

[javacody]

Ok, so will having a 47 uF cap there instead of a 100 uF cap cause any problems?

Also, just finished my LED upgrade. Man I love blue LEDs! Very easy. Again, it took longer for me to drill the stinkin hole than it did to solder the components. Looks sharp! Thanks Peter.  

Ansil, that plastic bucket sounds like its worth looking into. You kind of just inspired me to head down to my local hardware store to see what kind of other stuff I could rig up. Maybe a round bucket with some pvc pipe legs?   :twisted:

[Peter Snowberg]

Yep, that's power supply cap. In this circuit, that cap acts like a reservoir that lets the 386 draw more power easily when it needs it for producing bass. The value is very non-critical. If you run the circuit from an AC adapter, that cap will filter potential hum from the supply too.

That's cool that this is a new build. That makes it all the sweeter!

Take care,
-Peter

[javacody]

Ok, the LED dims and brigthens to my playing, is there a way to fix that?

Also, I used a 10K resistor in series with the led, what affect does lowering the resistance to the LED have? Will it cause the led to brighten?

Last but not least, what effect will raising the voltage have on this circuit? Will 12 volts get me any more volume or headroom? I've looked at the voltage doubler article at geofex, is there an easy way to get 12 volts from a 9 volt source?

[Peter Snowberg]

If you lower the 10K, you will have a brighter LED. Try 4.7K, but remember it's all a tradeoff... More power for the LED means less power available for the amp and lower battery life. The effect of the smaller power supply cap you used is directly visible as you play harder. That 9V doesn't have the ability to source very much instantaneous current, which is the reason for the reservoir cap in. Bass takes a LOT of current to produce relative to treble. If you increase the cap size, the LED will be more constant. Try something like 470uF or 1000uF or even higher. One thing to note is that the LED is showing you the amount of "sag" in the power supply.... just like tube amps have.

When it comes to higher voltages, you will get more headroom and cleaner, clearer tone, but your LM386 may not be able to handle it. The 386 has been produced in several different versions over the years, mainly because of refinements in the semiconductor process and the lower requirements of the bulk of the 386 produced. This has lead to the LM386-1, which was originally the 6 volt version of the chip. Now the NS datasheet says it's good to 12v and there is no longer (for a very long time now) an LM386-2, which was the 12 volt version. Translation: the original -1 die is gone and the -2 die was used to replace it. The -3 die has larger output transistors for more wattage. The -4 die has the larger transistors and the ability to run from voltages up to 18. The LM386-4 is the only National Semi chip I would use with more than 12V. New Japan Radio makes a chip called the JRC386BD, which is the -4 equivalent.

When it comes to voltage converters, remember that they take power themselves and they are not too efficient. You’re already having trouble running the amp and LED with a huge series resistor so adding a double will only make matters worse… much worse. For higher headroom, use two batteries in series and always change them in a pair for maximum life.

Take care,
-Peter

[javacody]

I'm using a handy little 9-volt adapter that plugs right into the battery connector, so I don't have to worry about battery life.

I suppose I could just leave well enough alone. Nah.   :lol:

[Peter Snowberg]

I'm using a handy little 9-volt adapter that plugs right into the battery connector, so I don't have to worry about battery life.

I suppose I could just leave well enough alone. Nah.   :lol:

If it 'aint broke, improve it!

Two options there... doing both is best... (1) try a larger cap, and (2) try a larger adapter with higher current. Both of these will improve the sound most likely. Better is such a subjective term, so I'll say that it will be louder and the tone will be more even.

Two more things you might want to try at some point would be to add a high impedance JFET buffer to the input, and with that in place you also have the option of adding a tone stack. Options abound!

Take care,
-Peter

[javacody]

Peter, I want to add a buffer, but it looks like the AMZ site has exceeded its bandwidth for the month and I want to build this puppy NOW!    Can you point me to a JFET Buffer schem?

Also, I've downloaded Duncan's tone stack calculator.  Which Fender version should I use? He says that the E series approximates the older Fender tone stack, but I'm not sure what exactly this means? Older as in tweed era? Also, what is z source, the input?

[javacody]

Would this work?

http://www.till.com/articles/GuitarPreamp/index.html

He says that this sounds much like the first stage of a Fender preamp, which is a sound I wouldn't mind emulating.

[Peter Snowberg]

Luckily JFET buffers are fairly generic circuits so you look around a little, you can find something to adapt. Almost any high impedance audio stage will work. You don't really need any gain, just buffering. A google search for "JFET buffer" turn up.... wow.... imagine that.... Runoffgroove.
http://www.runoffgroove.com/grace.html

I've never used the E series Fender tone controls so I can't tell you much about them. I think they're from REALLY old amps. I just use the "standard" stack (but I like different cap choices). This is dangerously close to a design I've been looking at producing for the least three years so I'm going to get vague from this point on. Sorry. It's funny how DIY minds think alike.

The Zsrc figure is the output impedance of the buffer stage. The JFET buffer is considerably lower impedance than a 12AX7 so if you're running simulations, try a value like 5 to 10K there, but don't worry about it too much. The cap choices are more important. Also important is the value of the slope resistor. Whenever I've made this a pot (33K in series with 100K pot) on an amp upgrade, the owner has been very happy with the versatility it gives. I would suggest trying a thumbwheel trimpot there and set it to the place your ears like the best.

Good luck!  
-Peter

[Peter Snowberg]

I just saw your next post.... and yes, that would be an excellent choice.

Take care,
-Peter

[javacody]

Are you talking about the cap choices you mentioned here http://www.diystompboxes.com/sboxforum/viewtopic.php?t=1030?

Would the schem look something like this?



Also, would the slope resistor be R1 in the above schem? What does it do?

Thanks for all of your help on this Peter. I've learned a ton already. Now if I could just get a tiny reverb of some kind on this, I'd be in heaven!

Almost forgot, in the post linked above, you mention a fixed resistor for the mid, and that Fender usually sets their mid at 7 in amps with only treble and bass tone controls. Am I correct in thinking that this would be approximately a 7k resistor instead of a 10k pot? Doh! Just read you suggested 6.8k, sorry.  

I have to admit, I'm very excited about putting all of this together. The lil gem is already a smokin amp, but if I can make it more Fenderesque, I will be very, very pleased.  It's almost more fun tweaking the thing that it  is playing it. Almost.  :wink:

[Peter Snowberg]

Are you talking about the cap choices you mentioned here http://www.diystompboxes.com/sboxforum/viewtopic.php?t=1030?

Would the schem look something like this?

Good sleuthing. That's one of my favorite combos given my very limited range of guitars and pickups. That's also a VERY common hotrod choice. It's more Marshally with a little less "bite".

Let your ears guide you there.... You might also want to consider a toggle switch in there to select a couple different choices. There isn't one best value for all occasions. A DPDT to switch between 33K/.022uF/.022uF and  100K/.1uF/.047uF is a pretty powerful feature.

Also, would the slope resistor be R1 in the above schem? What does it do?

Yes R1 is slope. The slope resistor will vary the amount of signal that gets bled off by the low-cut part of the stack. This is a balance with the treble cap and the ultimate effect is to vary the frequency of the mid scoop. Try the simulation with 33K in there and with controls set to 5/0/5, hit the snapshot button, and then switch R1 to 133K. You'll see the range of frequencies available. Note what it does to increase bass too.

Another mod to the stack is the use of a different bass pot. Try 500K and 1M also. So many options. Only your speaker and ears will tell you the right value.

Thanks for all of your help on this Peter. I've learned a ton already. Now if I could just get a tiny reverb of some kind on this, I'd be in heaven!

Glad to be of service. I believe that passing knowledge along is a very important thing. The more we all do it, the smarter we all become.

Now on the reverb.... Take a look at the 8AB2A1B tank under "amp parts-->reverb tanks-->Accutronics" at this place: http://www.tubesandmore.com/

If you build a second 386 amp with lower gain (no fuzz) and feed it with the wiper from a 100K pot (dwell) attached to the buffer output through a cap like.... oh say... .022uF, and then use that to drive the reverb tank, you can use another JFET to amplify the tank output and recombine the signal at the input to your main 386. Sorry that's such a general description.

Check out the "Stage Center Reverb" at GGG for another way to do it. You'll need a different tank with that circuit, but you could always combine the 386 driver and tank I spoke of above with the recovery and mixing side of the Stage Center. That will do you great.

Happy building!
-Peter

[javacody]

Wow Peter. This just turned into a step by step guide on building a tiny, full featured amp! I stayed up until the wee hours of the morning researching all the stuff you've been talking about. I've got enough information now to keep me busy for at least a month (if I totally ignore my family   ), probably longer.  

I think I'm going to build in the order that we discussed things. I've got the basic amp done, next comes the buffer.

After I have that working, I'm going to bread board the tone stack so I can try out all of the things you talked about, and then last (probably sometime in January!) I'm going to figure out how to make that reverb work. Any idea what the dimensions are on the particular tank you pointed me to?

When I have it all working, I'm going to sit down and draw up my first ever layout and etch my first board.