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Old 18th November 2006, 12:06 AM   #1
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Join Date: Jul 2005
Location: Cleveland OH
Default 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???
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Old 18th November 2006, 08:48 PM   #2
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Join Date: Dec 2005
Location: Italy

IRFP240 & IRFP9240 these are in current production !
to see website of International Rectifier :

Also IRFP240 is a N-Channel type
and IRFP9240 is a P-Channel type
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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Old 21st November 2006, 10:14 PM   #3
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Location: Cleveland OH
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??
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Old 4th December 2006, 09:12 PM   #4
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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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