Need help cloning a Gibson g-20 amp

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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/gibsonamps/g10_g20_g30-guitar.pdf

I think this is the correct schematic though. Please tell me what kind of transformer I need. Thanks
 
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
 
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
 
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?
 
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?
 
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.
 
Wouldn't a 12 inch speaker be less loud than a 10 inch? Well I guess the voice coil and magnet would be the same... 12 might give me more of a less compressed sound...

So how should I measure the supply voltage? Do you mean to place a multimeter in between the load and one of the transformer's secondary leads?
 
I still have a few questions.

Would a 10" or 12" speaker sound better/ louder?

Now that we know what schematic we are using, what transformer power rating and secondary rating should I use?

What is and where can I find a "spring type H"?

The transistor types are too blurred to be read. Are they ending in B's or 8's?

Thanks for all the help
 
DeadSpeaker said:
Wouldn't a 12 inch speaker be less loud than a 10 inch? Well I guess the voice coil and magnet would be the same... 12 might give me more of a less compressed sound...

So how should I measure the supply voltage? Do you mean to place a multimeter in between the load and one of the transformer's secondary leads?

Generally, within a family of drivers with similar motors it is true that the cone for a 12" is heavier than that for a 10" which does reduce efficiency, however the increase in surface area usually more than offsets the difference.

Your question about the meter shows that you need some basic instruction in electronics. What you describe is how you would measure current, not voltage and you can blow out your meter and more using it incorrectly. You should also learn about the safety hazards. Here's some basics on how to use a meter:

http://mechatronics.mech.northwestern.edu/design_ref/tools/multimeter.html

Pete B.
 
DeadSpeaker said:
I still have a few questions.

Would a 10" or 12" speaker sound better/ louder?

Now that we know what schematic we are using, what transformer power rating and secondary rating should I use?

What is and where can I find a "spring type H"?

The transistor types are too blurred to be read. Are they ending in B's or 8's?

Thanks for all the help

Use a good 12" guitar speaker.
This is what we used:
http://baselaudiolabs.googlepages.com/G_AMP_FRT_CRP.JPG

More: http://www.diyaudio.com/forums/showthread.php?s=&threadid=81449&perpage=25&highlight=&pagenumber=1

The PNP pre-amp transistor that's used in many places is a 2N4058, a good sub is the 2N5087. The 2N5087 is lower noise and higher beta, if you want closer beta use a 2N5086. A PNP type was used because they're generally lower noise.

The outputs are only 10A devices and there is no protection, I would sub modern bigger devices. There are no driver transistors and you might want to use the 50W Gibson solid state design which includes drivers. 2N3773 would be a nice rugged output device. It would be smart to add fuses in the supply lines to the output stage.

I think the TIS98 is a current product from Fairchild, 2N5210 should be fine also.

TIPs are TIP30B, TIP33B, TIP34B

Reverb driver is a 2N1132, a 2N2905A should also work fine:
http://www.jameco.com/webapp/wcs/st...toreId=10001&catalogId=10001&productId=194378

Note pinout differences for all substitutions.

This is a fairly simple design and I think that makes it an interesting project. I prefer not to use OP amps since I think you'll get a better understanding of the basics.

Pete B.
 
Wow that helps a lot. Thanks for taking the time to explain all that. I guess I have a lot to learn about this stuff. I read up on measuring voltage so I'll measure the voltage across the caps in a bit.

Building a G50 instead of a g20/30 is tempting(very), but I think I'd be really confused without having a physical model for what I'm building. The layout on the board would look exactly like the schematic.

Earlier today I traced through the circuit trying to find the elusive "spring type H" and it appears that the 2N1132's collector is connected to a 2.2mfd 50v cap that go to a phono plug. That is really weird because the schematic says that it should go to the spring thing. What amp I missing here?


A little later:
I plugged the amp in, set my multimeter to "20" under volts dc. I put the black on the negative and the red on the positive and all the meter put out was 0.00. I don't under stand why this was.
 
Ummm this is weird. The two phono plugs are just chillin there. I don't see any box or anything of that size in the amp. I have it gutted, unless it is small enough to be in the half inch gap between the board and the chassis, I don't think it is in there. Could it be that it uses a reverb emulation circuit when there is no spring connected?

Oh I checked some of the caps btw. The two 1000 MFD's are getting 33.8 and 34.8 volts. The 500 MFD's are getting 30.4 and 30 volts. I also checked the 47u and got 26.3, 10.5, and 32.5. I can check specific ones if you tell me which ones to check.

This is an edit to what I had before. What I thought was reverb is actually tremolo. The reverb pot does nothing.
 
DeadSpeaker said:
Ummm this is weird. The two phono plugs are just chillin there. I don't see any box or anything of that size in the amp. I have it gutted, unless it is small enough to be in the half inch gap between the board and the chassis, I don't think it is in there. Could it be that it uses a reverb emulation circuit when there is no spring connected?

Oh I checked some of the caps btw. The two 1000 MFD's are getting 33.8 and 34.8 volts. The 500 MFD's are getting 30.4 and 30 volts. I also checked the 47u and got 26.3, 10.5, and 32.5. I can check specific ones if you tell me which ones to check.

This is an edit to what I had before. What I thought was reverb is actually tremolo. The reverb pot does nothing.

You're off to a good start. Please be careful in there, transistors are not forgiving about accidental shorts while probing.

I see you've solved the reverb mystery, no tank, no reverb.

Check the physical size of the transformer, and people here can probably help you find a substitute. I expect that the one in there sags more than most to provide high peak power but lower continuous RMS.

I'm off for the night, enjoy!

Pete B.
 
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