Biasing a Power Valve

[Jay]

Biasing the power valves almost seems a mystical procedure which supposedly brings out the magic tones of the valves...

Some bias them hotter than plate dissipation allows for and others much lower to allow for valve longevity.

I have biased my EL84 SE amp such that I can switch between 73% and 89% of max allowable plate dissipation which is 12W for this valve.

I do not hear any difference between either bias selection and someone who's ears I trust couldn't either.

So is bias equals tone a thing or not? Can you hear it at any volume or only when dimed?

Please enlighten me.

[robthemac]

From memory, Jean, you run your amps pretty quiet. Have you been testing them at volumes when the power section is likely to be running at maximum wattage?

[Jay]

Despite it being a single EL84, I thought it was quite loud in my little room so the power valve must have been working a fair bit.

[AiRdAd]

I had a Carr Slant 6V amp, and I could adjust the bias on it. I could definitely hear a difference when I changed the bias.

[MikeC]

Will this amp push the EL84 into distortion? Perhaps that is where you may hear some differences?
For tech types like me I bias as per the recommendations for a combination of tube longevity & best performance (i.e.tone & power). So no more than 70% MPD for a grid (fixed) bias push-pull amp such as a DR and at 100% or near enough for a cathode biased tube (or tubes). There is much "mojo" generated from ignorance IMHO and I have believed my share!

[Jay]

Will this amp push the EL84 into distortion? Perhaps that is where you may hear some differences?
For tech types like me I bias as per the recommendations for a combination of tube longevity & best performance (i.e.tone & power). So no more than 70% MPD for a grid (fixed) bias push-pull amp such as a DR and at 100% or near enough for a cathode biased tube (or tubes). There is much "mojo" generated from ignorance IMHO and I have believed my share!

Yes, I think that is a sensible approach. 100% for cathode bias probably gives a bit more headroom. If I ll do another PP amp, I ll go for separate cathode resistors to avoid the expensive matched pairs me thinks.

[Slowy]

Despite it being a single EL84, I thought it was quite loud in my little room so the power valve must have been working a fair bit.

No, that's not loud; it's just coming good.

[Jay]

Despite it being a single EL84, I thought it was quite loud in my little room so the power valve must have been working a fair bit.

No, that's not loud; it's just coming good.

Ah, but I have hearing aids and you don't

[Slowy]

Despite it being a single EL84, I thought it was quite loud in my little room so the power valve must have been working a fair bit.

No, that's not loud; it's just coming good.

Ah, but I have hearing aids and you don't

Switch 'em off and turn up the amp.

[Bg]

No, that's not loud; it's just coming good.

Ah, but I have hearing aids and you don't

Switch 'em off and turn up the amp.

Is the correct answer! You win a $5 book token!

[Jay]

From memory, Jean, you run your amps pretty quiet. Have you been testing them at volumes when the power section is likely to be running at maximum wattage?

Right, no one home so put it in the lounge and ran at full bore. Volume of lounge is way more than music room so much easier on the ears.

I can hear a slight improvement, like a bit more headroom, when I flick it into a hotter bias but no big deal really.

What annoys me is that there is still a little bit of grid blocking or clipping. It is like a bit of noise rustling on top of the main note. Not sure if it is caused by the second triode or the EL84. Bit more tinkering to do.

Other than that it sounds glorious on full bore as the rustling ain't noticeable then...