Ampeg svt 3 pro Power Amp

[served]

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.

 

[diy didi]

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...

 

[served]

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.

 

[fuzzman72]

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

 

[LD Repair]

Did you ever complete that schematic? It'd be GREAT to see a copy! Thanks-