# Yamaha G-5 Practice amp... mods??



## Studiocat (Apr 24, 2007)

Found this little gem on the side of the road today.

It's got a really nice cabinet with a natural finish and finger joints, spring reverb, two band EQ, and two gain stages. Headphone jack on the back, and the speaker is 6.5 inches, 10W, 4 Ohm. 

I opened it up to look at the components, and there is an opamp that says "JRC 1312," and some transistors that are either "C2240" or "A970."

The amp sounds pretty good (for a tiny little thing), but I'm wondering if anyone might know what components I might want to swap out to improve the overdrive circuit. I don't have high expectations or anything; I'm just curious.

I could start by looking for a better speaker, too. Suggestions welcome.

Thanks
Adam


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## Studiocat (Apr 24, 2007)

nuthin? :bow: please?


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## mhammer (Nov 30, 2007)

It's hard to make any recommendations without knowing what's in there. And as nice as the pictures are, they don't really provide enough critical information to understand the circuit well enough to suggest anything.

Incidentally, the chip *is* a JRC, but its a 4558 from what I can see, NOT a "1312".

Chances are, that somewhere in the vicinity of that 4558, there are two diodes close together. They would be identifiable as orange and glasslike with a black stripe near one end. Anything like that?

Edit: BTW, nice score! I'm impressed that something that small has spring reverb.


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## Studiocat (Apr 24, 2007)

Hey!

It's totally a great score. I realize that this is going to be difficult, if not impossible without a schematic. I can't seem to find one. It's pretty obscure.

Anyway, there are a pair of orange diodes with white stripes (not near the op-amp), and another bunch of diodes covered in plastic near the large caps in the top right. Those ones are black, with a white stripe I think. There's another lone orange one in that group too.
Looks like there's a red zener diode in the middle.

I'll keep looking for a schematic...

Cheers
Adam


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## mhammer (Nov 30, 2007)

The black ones near the big caps are for the power supply. Don't monkey with those.

The pair of orange ones with the white stripe are the clipping diodes, commonly found in amps of a design like this. For example, here's a schematic taken from an old Japanese projects book for a 2W practice amp. I built it and it's pretty much what I test things out on these days. Like your G5, it has a solitary JRC4558 used to handle all the preamp chores. More than likely the two orange diodes will be sitting in a location very similar to the one shown for the pair of 1N914s in the schematic. Typically, in many overdrive pedals, a small-value capacitor will be placed in parallel with those diodes to shave off the fizzy part of the distortion when it kicks in. Unfortunately, if these is no distortion present, the addition of such a cap shaves off the treble and crispness, so you really only want it to be in effect when aiming for overdrive by cranking the gain control. Here is where it pays to have a schematic of your amp, so that it would be possible to find out if that treble cutting effect might be eased in, in some manner, as the gain is turned up.

At the very least, however, you can soften up the clipping a bit if you lift one end of that diode pair (they should be connected together at each end) and solder in a small-value (say 470R-2.2k) resistor in series with them.

Not sure if that's what you're aiming for when you say "improve the distortion".


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## Studiocat (Apr 24, 2007)

I'll give that a try!
Did you mean to attach a schematic of your amp?

Cheers,
adam


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## mhammer (Nov 30, 2007)

Studiocat said:


> Did you mean to attach a schematic of your amp?


Oh I meant to, all right. And now, I'm actually gonna *do* it. To see the entire thing at once without having to scroll around, right click on the image and select "view image":


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## UberKalimero (Oct 3, 2010)

Please help me guys : the host of : http://hammer.ampage.org/files/Miniamp.png refuses to give me access to this schematic ...
Can one of you paste the picture inside this thread or send it to me by email, please ?


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## GoreDoh (5 mo ago)

Any chance yall still on here? i got one of those Yamaha g-5 mini amps with USA reverb it ..

power cord was stretched at the retainer on the back and severed so i fixed that. got power no sound then a slight hum... no sound from headphones or speaker... then something popped after getting rather hot at the Mosfets.. the pop id say happen near the lagre cap and covered diods near the fuse... 

did anyone find schematics for this?

Thanks! 
Nick,


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## Latole (Aug 11, 2020)

We like pictures, link do not work long time ;

Schematic ; answer #8 ?


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## mhammer (Nov 30, 2007)

Here is the actual schematic of the G-5. That's a sweet little amp, from the circuit design and pics.


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