6L6GC Bias

[chunkitup]

I know how to bias but I do have one question. I put in new JJ's that are rated for 30W each. This is a 60W amp. It is a Peavey so there is no bias pot and the current is 28mA. I have a chart for biasing which uses 70% of 25w, which seems to be the norm. So I have 474v and that gives me 13w. Clearly I need to bump this up to around 35mA. Here is my question:
Why shouldn't I use 70% of 30W? If I use 30w then I need current in the range of 44mA and that is well above the high range on the chart. 9-10mA more would definitely change the sound for the better or worse but I want to get input before I go that high and greatly shorten the life of the tube. And of course I take into account when i do raise the current the voltage changes so these numbers are round-about.
Thanks

[Bruce / Mission Amps]

I know how to bias but I do have one question. I put in new JJ's that are rated for 30W each. This is a 60W amp. It is a Peavey so there is no bias pot and the current is 28mA. I have a chart for biasing which uses 70% of 25w, which seems to be the norm. So I have 474v and that gives me 13w. Clearly I need to bump this up to around 35mA. Here is my question:
Why shouldn't I use 70% of 30W? If I use 30w then I need current in the range of 44mA and that is well above the high range on the chart. 9-10mA more would definitely change the sound for the better or worse but I want to get input before I go that high and greatly shorten the life of the tube. And of course I take into account when i do raise the current the voltage changes so these numbers are round-about.
Thanks

Personally I think running your 6L6s at 21 watts each is unnecessary.
I know there folks out there that swear up and down that this 70% thing is the magic number for all power tubes but it simply is not true. It is a very subject topic and I for one think it is NET driven hype.
With everything else being equal, I don't know very many players at all...who can actually identify which amp is running it's 6L6s at 21 watts each vs 15 to 17 watts each.
You probably will not hurt anything by increasing the idle current but the tubes will last longer at under 17 watts and still sound great.
I agree 13 watts each is a little cool for a saturated power tube sound but it is still within the normal "satisfaction curve" of "bias" vs "good tone".
I'd try about 15-17 watts each and let it go at that... unless you can afford to wear out your power tubes faster and cost performance doesn't matter.
If that's the case and your amp's B+ rail (including the power transformer) can stand the 75% increase in current demand with no damage, then really sock it to them and run the tube real hot at 21-25 watts each.
That will sound different then 13 watts each and you might really enjoy it.

[defaced]

Go by JJ's specs. If that's 30 watts, then use it. Manufacturer data generally superseded nominal data. My guess is that your chart came from a book, and while that can get you "close enough" if you've got manufacturer specific data, use it.

[Enzo]

Clearly I need to bump this up to around 35mA.

Why?

I am with Bruce here. People chant bias so much it becomes a mantra. Is there something WRONG with the amp? Does it sound bad? I know that 70% has become some sort of standard, but when I see someone deciding he absolutaley MUST make his 5150 hotter, I have to wonder. It is already a gain monster. (No, I don't know which PV you have)

Yes, we generally consider the 6L6 as a 30w tube and the EL34 as a 25w tube. Older vintage 6L6 types may have some lower ratings, but not modern tubes. But there is nothihng wrong with running less than 70% of max.