High bias FTW?

[Nigel]

Am I the only guy that thinks this 70% max rule is BS?

Typically, I like JJ E34L's and 6L6's at 80-85% of 30 watts, or 22.5-25 watts at idle.

KT77? 25-28 Watts at idle. It's said they're a 32 watt tube, but rated for 25...

Personally, I'll buy a new set every month or two if I get tone. These things just don't BREATHE unless biased that hot. Then then get that squishy yet defined bliss clean/OD that makes hair stand up...

Anyone else feel this way?

 

[hunter]

yes and no

 

[Sixtonoize]

It's not an absolute.

70% bias is a guideline. If you bias lower, your tubes will last longer but won't sound as good.
Hotter, and they might sound better, but you'll lose headroom and lifespan.

Of course, it all depends on the amp. Some amps sound great with hotter bias, some amps sound mushy and undefined.
Some amps sound better with colder bias, but some sound cold and sterile.

It all depends on the amp you have and the tones you're trying to get.

 

[glpg80]

It's not an absolute.

70% bias is a guideline. If you bias lower, your tubes will last longer but won't sound as good.
Hotter, and they might sound better, but you'll lose headroom and lifespan.

Of course, it all depends on the amp. Some amps sound great with hotter bias, some amps sound mushy and undefined.
Some amps sound better with colder bias, but some sound cold and sterile.

It all depends on the amp you have and the tones you're trying to get.

and even more to do with the power supply design, the transformer specifications, tube variances/tolerances, class of bias, and also the volumes you are playing at.

 

[hunter]

Larry posted in a German forum once his recipe about SED =C=s and that he always puts them at 80-85%

 

[Nigel]

Seems to me the KT-77's are the vox dei when biased up to 28 watts at idle.

I killed 'em in my Mofo at 570v of B+.

They seem to do okay in the Rivera M60 at 445 with the tranny mismatched 2:1; i.e.: 16 ohm load/8 ohm tap. 1:1, I'd see 130ma per tube under drive. Mismatch keeps it under 80.

With this high bias, I can totally have hammond style feedback, definition, and bloom with ease. It's hard to stop playing it. Guess I'll need a fan in the back. =P I can crank the treble, and all it does is sound better. Not a hint of harshness anywhere...that "fill the room with inviting waves of tone" experience. Perhaps this is what players in the glory years of Mullard abundance enjoyed?

 

[Greazygeo]

I would never bias that high....they sound terrible to me even a 70%. But I play cranked.

 

[Capulin Overdrive]

I bias to what sounds good. There is no number...although I check to make sure I am within safe operating voltages. I usually end up between 55 and 75%. Most likely 60-65%

yep, same here!


i usually end up around the 65-70% range.

usually a little compromise, between sound and how they feel.

 

[Nigel]

I heard Brian May would run his tubes super hot, as well as the guy from Thin Lizzy.

Seems like most players "play it safe" these days. That's not rock 'n roll, man.

 

[diagrammatiks]

i use my ear to bias the tubes where they sound the best, use my eyes to make sure they aren't blowing. Last time I stuck a multimeter in one of those things I shocked one of my nuts loose. Don't do that anymore.

 

[Nigel]

Lol. I put the meter in just to know where they are...but yeah, I agree, sound > numbers.

I want a specific result, and that depends on the form that provides it. A STABLE form is what's needed...Ice-pick is the enemy, and not a friend of me...

 

[novosibir]

Larry posted in a German forum once his recipe about SED =C=s and that he always puts them at 80-85%

This is only half of the truth and the word "always" is definitely misleading.

First of - I've noticed that =(C)='s are the most detailled sounding EL34's of current production, but they also sound a bit 'slender' in the lows, when compared with RFT's, JJ's or those chinese big bulbs, when biased to the same percentage of max. plate dissipation - i.e. these ominous 70%

And the fact, that =(C)='s do have carbon plating inside the plates and gold plated screen grids lead me to the conviction, that these tubes maybe can handle some more abuse, than all the others current produced stuff, which don't have the mentioned 'features'.

I've experimented and found out, that some =(C)='s you even can bias to almost 100% w/o red plating. But keep in mind, that no red plating at idle doesn't mean, that the tube doesn't red plating in regular operation. Also at full blast the danger of red plating is not much higher than at idle - the most danger of red plating is at about 40...60% of max. power. This area you have to check with a sine/square generator, a load and a scope

But while experimenting I've found out, that =(C)='s are getting the before missing lows beyond these ominous 70%, what's de facto only a number, nothing else and never the point, where you can say, that you've biased 'correct'

Hence my method with biasing (not only) =(C)='s is, first to bias it to 65-70%, then 5 minutes I'm driving the tubes into saturation with a square wave and a load, then I re-check the bias and adjust bias, until the missing lows occur, where I sometimes end up at a max. plate dissipation of 80-85% - but not always!

Finally I re-check for possible red plating especially in the 40-60% of max. power area - and sometimes I have to go a small step backwards (colder) with my bias adjustment.

This all requires a lot of experience and 'empathy' for tube amps, so if you're lacking these qualities, then better stay 'on the safe side' and don't bias beyond 70%, no matter, what brand of output tube

Larry

 

[Anonymous]

how do i find out what range to bias my ruby 6L6's or groove tube 5881's in my tourmaster?

or, i guess, how do i find out what those tubes are capable of rating wise (wattage, etc).

 

[hunter]

Ok Larry, duly noted. I'll quote more carefully in the future.

By the way your past advise did make me more courageous and made me use my ears more, using the measurements more as a supportive information rather than the truth. Also I did end up biasing the =C=s hotter in both, my Marshall and the Steavens with great results.