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-   -   F5 Bias Problem (http://www.diyaudio.com/forums/pass-labs/184289-f5-bias-problem.html)

maartentje 2nd March 2011 11:48 PM

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

maartentje 2nd March 2011 11:50 PM

Its on both channels. So something has to be wrong :confused:

6L6 2nd March 2011 11:55 PM

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?

Andersonix 3rd March 2011 12:03 AM

Is your PS large enough? Bottleneck somewhere?

maartentje 3rd March 2011 12:16 AM

Voltage across R11-R12 is 0.3v. Devices are IRFP9240 IRFP240.

Rail voltage 25volt+ and 25volt-

maartentje 3rd March 2011 12:17 AM

Nothing toasted ;) not yet

Nelson Pass 3rd March 2011 12:27 AM

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

:cool:

maartentje 3rd March 2011 12:36 AM

Hi Nelson,

Yeah i was thinking the same thing. But the circuit should work like this right with the original value's?

cjkpkg 3rd March 2011 12:56 AM

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.

nigelwright7557 3rd March 2011 01:08 AM

Quote:

Originally Posted by Nelson Pass (Post 2489349)
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

:cool:

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


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