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Old 28th November 2006, 02:01 AM   #1
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Default substituting FETs

I have an amp (bass guitar actually) that had some blown transistors. I punched them into fairchildsemi.com and they cross referenced to a new part. I installed this part but I still have a distorted sound (but at least i have sound now...). I'm just wondering if it could have anything to do with the FETs or is it simply a deeper problem? I'll check it on a scope tonight...

original:
IRFP9240/IRFP240
referenced to:
19N20C/12P20

Now, I had those so they were free. Is it worth the $20 or so to get an original FET set? Will it have any impact on sound quality?
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Old 28th November 2006, 05:47 PM   #2
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The replacement devices are probably close enough to get you by. But, there's a very good chance something else is still wrong with your amp. So, either your old output devices failed and took out other parts, or other parts failed and took out the outputs. Either way something's still wrong and needs further troubleshooting. I doubt your original outputs simply failed for no reason. I'd continue troubleshooting paying attention to the gate-drive resistors, bias circuitry, class 'A' drivers, etc. to locate additional problems.
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Old 28th November 2006, 10:54 PM   #3
Thack is offline Thack  United States
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I hope you know you have your "replacements" listed backwards. The 12p20 is the P channel and the other is the N channel but you have the IR mosfets listed reverse. So the 12p20=IRFP9240 and 19N20C=IRFP240. Did you install them reversely?
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Old 29th November 2006, 01:50 AM   #4
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no, i just didn't pay any attention to how i wrote them here, sorry.

I went ahead and bought a set of original transistors that were a good deal. I'll continue to troubleshoot it. It's tough because I can't find the schematic anywhere. its an Ampeg SVT-3 Pro.
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Old 29th November 2006, 03:27 AM   #5
Thack is offline Thack  United States
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You will have to pay for the schematic but here it is.
http://www.musicparts.com/products.asp?Company=Ampeg

Just go down the list to find your amp. I think you have something else wrong that is effecting your FETs.
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Old 5th December 2006, 01:16 AM   #6
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the distortion turned out to be a bias issue, maybe due to the different FETs being used. When put on a scope and sine wave, there was distortion between the neg/pos wave.

hopefully this is it....
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Old 5th December 2006, 03:40 PM   #7
Thack is offline Thack  United States
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So did you source the distortion? Was your bias too hot?
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