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razorrick1293 20th November 2011 12:14 PM

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.


DF96 20th November 2011 03:14 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 04:21 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 04:33 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.

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