Ampeg svt 3 pro Power Amp

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#81
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.
 
#82
fuzzman72 said:
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.
Click to expand...
Those are some nice figures man.
How did you match the Fets? Or did you just use same batch numbers?
Keep it up...
 
#83
Hi.

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.
 
#84
Hello

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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