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Old 3rd March 2011, 12:48 AM   #1
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Default F5 Bias Problem

Hi there,

Finished building the F5 amp. Everything looks fine but the bias does not want to go higher then .6amp. P2 is set to max and output set to 0vdc. Used Peter Daniels board and components from Tech-Diy.

Martinus
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Old 3rd March 2011, 12:50 AM   #2
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Its on both channels. So something has to be wrong
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Old 3rd March 2011, 12:55 AM   #3
6L6 is offline 6L6  United States
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If it's on both, that's actually a good thing.

What is the voltage across R12 and R11?

Is it all warmed up and toasty?

You said Tech-DIY, but to be sure, these are not Toshiba outputs?
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Old 3rd March 2011, 01:03 AM   #4
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Is your PS large enough? Bottleneck somewhere?
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Old 3rd March 2011, 01:16 AM   #5
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Voltage across R11-R12 is 0.3v. Devices are IRFP9240 IRFP240.

Rail voltage 25volt+ and 25volt-
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Old 3rd March 2011, 01:17 AM   #6
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Nothing toasted not yet
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Old 3rd March 2011, 01:27 AM   #7
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You can increase the value of the 2.2K resistor which is
in parallel with the pot to get some more bias. Try 2.7K
or 3.3K

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Old 3rd March 2011, 01:36 AM   #8
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Hi Nelson,

Yeah i was thinking the same thing. But the circuit should work like this right with the original value's?
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Old 3rd March 2011, 01:56 AM   #9
cjkpkg is offline cjkpkg  United States
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silly question but have you validated the ohm value of R11 and 12?

You should be able to easily get 800-1000mV(not that you would run there unless you had some MONSTER sinks) across with the stock values and 25V rails.

Also maybe doublecheck the values and orientation of the trimpots themselves - the adjustment screws should be to the outside of the boards.

Last edited by cjkpkg; 3rd March 2011 at 01:58 AM.
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Old 3rd March 2011, 02:08 AM   #10
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Quote:
Originally Posted by Nelson Pass View Post
You can increase the value of the 2.2K resistor which is
in parallel with the pot to get some more bias. Try 2.7K
or 3.3K

The IRFP240/9240 each need 3v5 to switch on.
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