F5 Bias Problem

maartentje · 2nd March 2011, 11:48 PM

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

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

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 · 12:58 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

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.

2nd March 2011, 11:48 PM	#1
maartentje diyAudio Member Join Date: Jun 2002 Location: USA, FL	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

2nd March 2011, 11:55 PM	#3
6L6 diyAudio Member Join Date: Oct 2010 Location: Denver, Colorado	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? __________________ Pass Labs Pearl Phono -- Pass B1 Preamp -- Pass F5 amplifier -- Pass Labs Pearl 2

3rd March 2011, 12:56 AM	#9
cjkpkg diyAudio Member Join Date: Aug 2008 Location: Roswell GA	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 12:58 AM.

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2nd March 2011, 11:50 PM	#2
maartentje diyAudio Member Join Date: Jun 2002 Location: USA, FL	Its on both channels. So something has to be wrong

3rd March 2011, 12:03 AM	#4
Andersonix diyAudio Member Join Date: Jan 2004 Location: Silicon Valley	Is your PS large enough? Bottleneck somewhere?

3rd March 2011, 12:16 AM	#5
maartentje diyAudio Member Join Date: Jun 2002 Location: USA, FL	Voltage across R11-R12 is 0.3v. Devices are IRFP9240 IRFP240. Rail voltage 25volt+ and 25volt-

3rd March 2011, 12:17 AM	#6
maartentje diyAudio Member Join Date: Jun 2002 Location: USA, FL	Nothing toasted not yet

3rd March 2011, 12:27 AM	#7
Nelson Pass The one and only Join Date: Mar 2001	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

3rd March 2011, 12:36 AM	#8
maartentje diyAudio Member Join Date: Jun 2002 Location: USA, FL	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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