Little Gem MK II

[christobean]

i am going to try to make a Little Gem MKII but i cant find the pcb layout designs or anything or the mkII, just the little gem

does anyone have this?
also does anyone have any other hints that would help me in this project?

all response appreciated
thanks

[TheBigMan]

I've done a Veroboard layout for it, it's in my Gallery.  It's not verified (I haven't built it) but it has been node checked.

[christobean]

thank you!!
it looks pretty much like the little gem original before the transister splits into the 386's, then afterwards is a bit different, am i correct?
also i didnt see any volume or gain lug specs. is this because there arent any?

thanks again, i was going crazy looking for information online, but with no luck

[TheBigMan]

The Mark I doesn't have a buffered input, so that part is different as well.  I think you could add a gain pot if you wanted to, but it would have to be dual-ganged so it works on both chips.  I'm sure there was a thread a while back about how to add a volume pot to the Mark II as well.

[Peter Snowberg]

There are at least three ways to easily add a gain pot to the Mark II.

The first method is almost mentioned on the schematic and it requires a dual pot. I say almost because it mentions using a switch to disconnect the two 10uF caps and that's like having a gain control you can set to either 20 or 200. I would use a dual 5K with linear taper instead of the DPST switch in order to get gains between 20 and 200. Connect the + end of each of the 10uF caps to the two wipers. Now connect the clockwise ends of each pot to pin 1 of the corresponding 386. The Little Gem schematic shows this arrangement, but without the cap in series. The cap should be there.

Method 2 would be to replace the input buffer with an input booster. Compare the buffer circuit with any of the JFET boosters out there. The two differences between these basic building blocks are that the drain of the JFET will have a resistor in series with it and there is usually a capacitor in parallel with the source resistor, but the cap isn't mandatory. The resistor values will depend on the JFET used and how much current you want to run through it. The source resistor is often replaced with a pot and that becomes your gain control. The wiper gets connected to the + end of the cap and the other side of the cap is grounded along with the CCW end of the pot. The CW end of the pot connects to the source lead of the transistor. When using a booster in front, you many not want or need the 10uF caps on the 386. Check out this article for more on boosters: http://www.geofex.com/Article_Folders/foolwfets/foolwfets.htm

Method 3 is to put a pot on the output of the buffer, as the Ruby does. Simply break the connection between the input cap and the inputs of the 386s, and install a 10K pot in there with the wiper connecting to the 386s and the CW end connecting to the cap. The CCW end gets grounded.

All three methods work, but method 2 will overdrive the 386 input stage much more while methods 1 an 3 will cause the later stages of the 386 to make more distortion. I've never compared the two tones side by side. Something else for the "to do" list.  Methods 1&2 will give you a gain control, while method 3 is really more like a master volume control.

[TheBigMan]

I've just built an LGII using my layout, so it's now verified.  The two 10uF caps for the gain are the wrong way round though, I'll correct that just now.

[christobean]

Methods 1&2 will give you a gain control, while method 3 is really more like a master volume control.

so if i use method 3 and it it acts as a volume control, will i be able to use before the amp to control od/distortion?

thank you for such detailed directions, theyre very helpful and answered my questions straight off