EL84 fixed fixed bias

razorrick1293 · 20th November 2011, 11:14 AM

Hi there,

Just wondering, would it be ok to run -12/13v to an EL84 and then never touch the bias again? I know some amplifiers do this with EL34s/6L6's (eg mesa boogie) but would it do for EL84s? I have a 15w PP amp on a PCB that has no easy way of setting any kind of bias, but it wouldnt be hard to modify the bias supply with a 12v zener instead of a trim pot.

Ricky

DF96 · 20th November 2011, 02:14 PM

OK if you are happy with one or both of: short valve life, poor sound.

Alternatively, use a bypassed cathode resistor. This is the established engineering solution for reasonably accurate, emission compensating, zero-adjustment biasing.

razorrick1293 · 20th November 2011, 03:21 PM

ok, why would colder biasing make valve life shorter? if never found the sound of biasing slightly colder (a few mA) that bad.

DF96 · 20th November 2011, 03:33 PM

Fixed bias means the valve is likely to be either too hot or too cold. Either can cause poor sound. Too hot also means shorter life.

20th November 2011, 11:14 AM	#1
razorrick1293 diyAudio Member Join Date: Nov 2010 Location: England Hertfordshire	EL84 fixed fixed bias Hi there, Just wondering, would it be ok to run -12/13v to an EL84 and then never touch the bias again? I know some amplifiers do this with EL34s/6L6's (eg mesa boogie) but would it do for EL84s? I have a 15w PP amp on a PCB that has no easy way of setting any kind of bias, but it wouldnt be hard to modify the bias supply with a 12v zener instead of a trim pot. Ricky

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20th November 2011, 02:14 PM	#2
DF96 diyAudio Member Join Date: May 2007	OK if you are happy with one or both of: short valve life, poor sound. Alternatively, use a bypassed cathode resistor. This is the established engineering solution for reasonably accurate, emission compensating, zero-adjustment biasing.

20th November 2011, 03:21 PM	#3
razorrick1293 diyAudio Member Join Date: Nov 2010 Location: England Hertfordshire	ok, why would colder biasing make valve life shorter? if never found the sound of biasing slightly colder (a few mA) that bad.

20th November 2011, 03:33 PM	#4
DF96 diyAudio Member Join Date: May 2007	Fixed bias means the valve is likely to be either too hot or too cold. Either can cause poor sound. Too hot also means shorter life.

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