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Old 15th February 2008, 08:14 PM   #1
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Default Need help cloning a Gibson g-20 amp

This is the 3rd forum that I've posted this question is, but maybe you guys can help me. I have a Gibson g20 sitting at my house that I want to make a replica of. It is not too too loud (possibly because the speaker is partially blown.) I'm gonna fix the speaker for my friend who actually owns the amp, but I was to copy it while it is still in my house.

I don't know what kind of transformer I need. The back of the amp says that it is 30 watts. That doesn't tell me a whole lot. I have found some evidence that Gibson was not consistent in producing this amp, because the transistors are of different types that those that are listed in this schematic. http://www.schematicheaven.com/gibso...g30-guitar.pdf

I think this is the correct schematic though. Please tell me what kind of transformer I need. Thanks
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Old 15th February 2008, 08:40 PM   #2
gl is offline gl  United States
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For a G20 the schematic says that the rectified DC at the output of the diodes is 37 volts. So the transformer secondary needs to be around 27 or 28 volts AC. 28 is a standard value so go with that. If the output of the amp is 20 watts then look for a transformer thats rated at 50VA or higher. Also easy to find. Try Avel-Lindberg or the Mouser catalog.

The transformer here is not the hard part to source. It's the transistors. There are lots of better circuits out there for guitar. It would be much easier to build a Fender clone or a solid state amp using op-amps in the pre-amp/eq section and a chip amp or class D amp for the output.

Good luck with the project.

GL
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Old 15th February 2008, 08:54 PM   #3
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Hey thanks a lot. Finally a straight answer. I have a 15 watt Fender front man and that works for me, but the G20 has an intense vintage sound. I was thinking about getting a blown speaker somewhere and reconing it with an aluminum voice coil to help it sound more vintage. So what I'm trying to say is, I like that amp's tone a lot. The transistors have not proved to be too difficult to track down. 3 or some FET type ones have been. I might have to test them

Thanks again. I'll post more questions later
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Old 15th February 2008, 11:35 PM   #4
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Ok well I'm looking on mouser. Under 56VA 28VCT there are several transformers all at either 2 amps or 4 amps. Which one should I go with? I don't know if I'm reading the schematic right, but I think it has to be 1 amp.
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Old 15th February 2008, 11:50 PM   #5
gl is offline gl  United States
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Either will do. Don't use the center tap.

GL
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Old 16th February 2008, 02:54 AM   #6
PB2 is offline PB2  United States
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The G20A looks like a better design, however there is an error in the schematic where E and C for Q8 are reversed:
http://www.gibson.com/Files/schematics/G20A,%20G30A.pdf

Pete B.
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Old 16th February 2008, 03:08 AM   #7
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The schematic doesn't show it, but the amp that I have to look at has the center tap running to the switch and from there running under the board to an unknown location.

This schematic is concerning me. In the reverb section it shows a thing that I assume controls the reverb effect. I can't seem to find it in the amp. Do you know what it looks like and what it is called? There is also a push button thing on the back of the amp that says "push to reset" and it is connected to the switch via to large wires. What is this?
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Old 16th February 2008, 04:15 AM   #8
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Sorry for the double post. I took a look at the schematic for the g-20a and was surprised. That looks like my amp! I went through the circuit and matched it up with the schematic. They seem to match. I'm pretty sure the resistors matched (anyone else here that's color blind?) and the caps matched.

Since a lot of things matched up, it looks like this is the actual schematic for the g20 that I'm looking at right now. The schem states that it is the diagram for G20A and G30A amps. How is that possible? Is is just up to what kind of transformer I put in that makes it louder?
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Old 16th February 2008, 03:54 PM   #9
PB2 is offline PB2  United States
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Yes probably a lower supply voltage to the power amp in the G20, however there doesn't seem to be any notation about this in the schematic.
+/-35 (70V total) is significantly more than the +48 in the other schematic. I suggest that you measure the supply voltage across the caps in the amp that you have there and let us know.

Pete B.
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Old 16th February 2008, 04:36 PM   #10
PB2 is offline PB2  United States
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Actually, it's possible that they just standardized on the amp for both models and if one has a 10" and the other a 12" that would provide enough of a difference. 10" guitar speakers are generally less efficient than 12s all else being equal, more or less.

Pete B.
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