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Ampeg svt 3 pro Power Amp
Ampeg svt 3 pro Power Amp
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Old 18th September 2013, 10:17 AM   #81
served is offline served
diyAudio Member
Join Date: Aug 2011
Yes. That pot makes you want to have a scredriver that has a 1m diameter handle to get it right.
Multiturn pot would make a huge difference. Though I got it pretty close.
And I think you should be fine with thouse transistors. They are also pretty close.
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Old 14th August 2014, 06:35 PM   #82
diy didi is offline diy didi  South Africa
diyAudio Member
Join Date: Sep 2009
Location: Pretoria
Originally Posted by fuzzman72 View Post
Hello Served

I think I've solved the problem.
I feel stupid now, but I was frightened about the description of that sensible trimmer and I was not turning it far enough in the CW direction.
Now I've been able to set all the mosfet at right bias.

I will post another schematic with all voltages and mosfet at bias for future reference.

In the meantime here are all the voltage across 0,47 ballast resistors:

IRFP240 IRFP9240

24,3mV 25,2mV

26,3mV 25,1mV

24,3mV 25,4mV

23,0mV 23,6mV

What do you think about this? I guess they're in the ballpark, aren't they?

However that trimpot is a nightmare, I'm thinking to replace it with a 20 turns pot for more precise biasing.
Those are some nice figures man.
How did you match the Fets? Or did you just use same batch numbers?
Keep it up...
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Old 15th August 2014, 07:08 AM   #83
served is offline served
diyAudio Member
Join Date: Aug 2011

I matced the Fets.
I happened to get two bags of FETS from a friend.
And I found suitable ones from there.

Otherwise this would have been a really tough and expensive repair.
I had about 200 fets to choose from. Some were really way off.
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Old 18th August 2014, 08:44 AM   #84
fuzzman72 is offline fuzzman72  Italy
diyAudio Member
Join Date: Sep 2010

I've matched the fets using an ATLAS PEAK DCA 75 semiconductor analyzer (Peak Electronic Design Limited - Atlas DCA Pro - Advanced Semiconductor Analyser - Model DCA75).

I've bought 30 pieces for each mosfet and matched them by their Vgs(on) parameter.
Ampeg suggest a tolerance of +/- 0,3v, but you can do better than this and that's what I did.
I would suggest you to change the original trimpot with a 20 turns one, otherwise setting a precise bias point it's really a nightmare
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Old 17th August 2017, 10:50 PM   #85
LD Repair is offline LD Repair
diyAudio Member
Join Date: Apr 2017
Did you ever complete that schematic? It'd be GREAT to see a copy! Thanks-
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