Biasing (yes, again!)

[Keven]

So I finally got that pair of winged c el34 installed. measuring with the bias rite i got a plate voltage of 277v (i run my amp in low power mode, so the plate voltage runs low and the current is high)

I upped the current from 44ma to around 55ma. as far as overdrive goes, it kinda made it fuller and more responsive to agressive grinding, but where it shined was the clean tone. big and full and alive. 

what does biasing cold and hot do to your tone? at 277v and 55mA i'm at around 15w of plate dissipation per tube, i read that for class AB you don't go over 70% of plate dissipation of your tube (el34's have a rating of 25W right?) so i shouldn't go over 17-18 Watts if i want to keep my tubes for a while (we play around 3 hours every saturday and that's it, so they should last a while anyway...), but what would happen to my tone if i notched up the current just a bit more? does it get louder or does it saturate better? I have a feeling my amp got louder with more current, but actually, that was after a comment from my drummer that he felt my guitar and the bass was loud tonight, overpowering the other guitarist (who runs a solid state peavey, though this thing packs quite a punch for a tranny...) so maybe it was my imagination. does hotter necessarily mean louder?

 Biasing is such a black art (at least the webpages i read try to make it seem harder than it is for my amp, gotta love the trimpot access without removing the chassis) but if i know what's safe and not safe, i can get my amp running up to its full potential.

thank you in advance again for the educated answers, i'm starting to see the light at the end of the tunnel

BTW... I'm quite disappointed in my quad of TAD 12AX7... they aren't better or worst than the EHX i had in there. it's time to start looking for more preamp tubes!

[jpfamps]

The "70% of maximum" is a good rule of thumb for most amps running in class AB at around 450 VDC for EL34s.

For an excellent explanation as to how this number was calculated have a look at Randall Aiken's web-site:

www.aikenamps.com/Why70percent.html

However, there are circumstances where biasing hotter is acceptable:

1) If the amp is can be biased to operate in class A then you would normally bias at 100% of plate dissipation. To run an amp in class A without exceeding the recommended plate dissipation requires a lower plate voltage, so in low power mode with 277 VDC on the plate you can safely bias the static plate dissipation above 70%.

2) Amps with valve rectifiers you can also safely be biased hotter as the additional current draw when the amp is played hard causes significant voltage drop across the rectifier and hence reduces peak plate dissipation.

Conversely in amps using very high plate voltages, eg Ampeg SVT, I shoot for a lower static dissipation of around 50%.

Ultimately, if it doesn't compromise the reliability of the amp and you like the sound then it is biased correctly.

[Keven]

but even if the plate voltage got really low, wouldn't it still be class AB push pull? because it runs with two tubes? that's the part i'd like to understand with my amp. the low power mode is simply a switch that drops plate voltage by half, that doesn't automatically make it class A right?

reading the article, my amp definitely falls into class AB. because i left the bias rite connected, made someone play my guitar while i took measurements... and the cathode current went up depending on the ''playage''.

from what i gathered in the article, class A 100% biasing is safe because when played the cathode current actually goes down. or did i get it all mixed up?

[jpfamps]

One common misconception is that class A means cathode bias (and single ended); in fact many guitar amp companies help perpertrate this myth as "class A" sounds better from a marketting point of view.

You certainly can have push pull fixed bias class amps, although this is unusal for guitar amps it is more common in hi-fi amps.

To run an amp in class A you need to have lower plate voltage than that found in most guitar amps, and also the reflected impedance needs to be correct.

Again have a look at Randall Aikens tech info for more on this.