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American Bass VFL-1100.1
American Bass VFL-1100.1
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Old 17th October 2020, 06:15 AM   #11
caraudioguy2100 is offline caraudioguy2100
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There are 6 banks of fets in this amp . Along with 6 transformers and 6 banks of driver transistors .

Can I put 2 in each bank to test or would that be a lot of load on only 2 fets per bank for testing
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Old 17th October 2020, 06:31 AM   #12
Perry Babin is offline Perry Babin  United States
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There are two banks per transformer. One bank drives the primary to ground then it switches off while the other bank does the same.

You can install only one FET per bank (2 FETs per transformer).
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Old 17th October 2020, 06:59 AM   #13
caraudioguy2100 is offline caraudioguy2100
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Power supply is working correctly now with 1 fet per bank .

Not sure why when I replace the drivers I always have issues the first time around
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Old 17th October 2020, 08:19 AM   #14
caraudioguy2100 is offline caraudioguy2100
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With all of the fets installed and the power supply board disconnected from the output board .

The fets still get warm and continue to heat out of the heatsink at idle . Not sure for his is normal or if I still have an issue .

Here is what I get on all of the banks of fets .

Scope set to 10us 5 volts/div
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Old 17th October 2020, 03:23 PM   #15
Perry Babin is offline Perry Babin  United States
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What's the drive look like on the other side of the gate resistor?

What value gate resistors are you using?

What FET?
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Old 17th October 2020, 04:45 PM   #16
caraudioguy2100 is offline caraudioguy2100
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Here is what the drive looks like on the other side of the gate resistor .


The power supply gets are IRFP1405

Gate resistors are 47 ohm
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Old 17th October 2020, 07:43 PM   #17
Perry Babin is offline Perry Babin  United States
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I'm assuming that the trace wasn't aligned to the reference line.

Try changing the gate resistors to 22 ohms for the 2 FETs on one transformer. Do the FETs run cool?

How long does it take the FETs with the 47 ohm resistors take to heat up too hot to touch?
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Old 17th October 2020, 08:30 PM   #18
caraudioguy2100 is offline caraudioguy2100
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It takes around 3 mins to get to hot to touch .

I replaced 2 gate resistors for 1 transformer to 22 ohm gate resistors those 2 remain cold the other 4 in that bank with 47 ohm resistors heat up

Last edited by caraudioguy2100; 17th October 2020 at 08:52 PM.
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Old 17th October 2020, 10:05 PM   #19
caraudioguy2100 is offline caraudioguy2100
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So I should switch them out to 22 ohms
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Old 18th October 2020, 01:56 AM   #20
Perry Babin is offline Perry Babin  United States
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3 minutes to heat up isn't awful and could be normal.

That said, for all of the diagrams I have that use the 1405, all but one uses the 22 ohm gate resistors.

Replace all of the gate resistors for that transformer with 22 ohm, install all of the FETs for that transformer and confirm that they still run cool. If they heat up slightly more, install of the FETs for a transformer with 47 ohm resistors and see how much quicker they get hot.
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