SVT3pro, sounds fuzzy,

elementx · 18th November 2006, 12:06 AM

I have a bass amp I repaired, an Ampeg SVT3 PRO. I put new transistors and resistors amongst a few other things. I replaced the transistors with what farichild cross referenced:

IRFP9240/IRFP240 (original, obsolete)

FQA12P20/FQAF12P20 (new, cross referenced)

Now, I get sound/power. However, the sound is very compressed, almost fuzzy sounding. Something else is wrong. Do you think the transistors would have any dramatic effect like this?

The tubes are supposedly new, I have a 2nd set I put on a tube checker (theyre good) ill try them next.

What if two tubes were switched, would that "fuzz" the sound. i heard that was a problem between the 12ax7 and 12au7.

anyone have a SCHEMATIC they would so kindly send me???

Gold_xyz · 18th November 2006, 08:48 PM

Hi

IRFP240 & IRFP9240 these are in current production !

to see website of International Rectifier :
https://ec.irf.com/v6/en/US/adirect/...uctID=IRFP9240
https://ec.irf.com/v6/en/US/adirect/...uctID=IRFP9240

Also IRFP240 is a N-Channel type
and IRFP9240 is a P-Channel type
but
FQA12P20 is a P-channel type
FQAF12P20 is a P-channel type (F=isolated package)

Both of P-Type ???

elementx · 21st November 2006, 10:14 PM

I'm sorry, the exact chips in there now are:

19N20C (N)
12P20 (P)

I've spoke to a repair tech and he said often times the following are often switched:

J12 - 12AU7/ECC82
J11 - 12AX7/ECC83

Mine were switched, I corrected it. Still sounds the same, like a light distortion/fuzz on the output. Like I have a "grunge" pedal on it.

Is it worth spending about $25usd to get the right fets in there? or is that not my problem??

elementx · 4th December 2006, 09:12 PM

Well the problem ended up being the output bias, since I used different model transistors. We'll see what happens when I plug it into my stack...

18th November 2006, 12:06 AM	#1
elementx diyAudio Member Join Date: Jul 2005 Location: cleveland, ohio	SVT3pro, sounds fuzzy, I have a bass amp I repaired, an Ampeg SVT3 PRO. I put new transistors and resistors amongst a few other things. I replaced the transistors with what farichild cross referenced: IRFP9240/IRFP240 (original, obsolete) FQA12P20/FQAF12P20 (new, cross referenced) Now, I get sound/power. However, the sound is very compressed, almost fuzzy sounding. Something else is wrong. Do you think the transistors would have any dramatic effect like this? The tubes are supposedly new, I have a 2nd set I put on a tube checker (theyre good) ill try them next. What if two tubes were switched, would that "fuzz" the sound. i heard that was a problem between the 12ax7 and 12au7. anyone have a SCHEMATIC they would so kindly send me???

18th November 2006, 08:48 PM	#2
Gold_xyz diyAudio Member Join Date: Dec 2005 Location: Italy	Hi IRFP240 & IRFP9240 these are in current production ! to see website of International Rectifier : https://ec.irf.com/v6/en/US/adirect/...uctID=IRFP9240 https://ec.irf.com/v6/en/US/adirect/...uctID=IRFP9240 Also IRFP240 is a N-Channel type and IRFP9240 is a P-Channel type but FQA12P20 is a P-channel type FQAF12P20 is a P-channel type (F=isolated package) Both of P-Type ??? __________________ My English originates from here

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21st November 2006, 10:14 PM	#3
elementx diyAudio Member Join Date: Jul 2005 Location: cleveland, ohio	I'm sorry, the exact chips in there now are: 19N20C (N) 12P20 (P) I've spoke to a repair tech and he said often times the following are often switched: J12 - 12AU7/ECC82 J11 - 12AX7/ECC83 Mine were switched, I corrected it. Still sounds the same, like a light distortion/fuzz on the output. Like I have a "grunge" pedal on it. Is it worth spending about $25usd to get the right fets in there? or is that not my problem??

4th December 2006, 09:12 PM	#4
elementx diyAudio Member Join Date: Jul 2005 Location: cleveland, ohio	Well the problem ended up being the output bias, since I used different model transistors. We'll see what happens when I plug it into my stack...

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