Do tubes affect bias?

[frehley]

I'd like to ask some of you amp techs here if power tubes affect the bias on
an amp? I ask because I own a Mesa 2:100 power amp and like all Mesa amps,
it's a fixed bias amp. The people at Mesa state in their manual that tubes DON'T affect bias but the folks at Eurotubes who sell JJ tubes seem to say they do. If I'm not mistaking, Mesa says that if the bias on an amp is set correctly in the first place, the current to each tube will be the correct amount if using in-spec tubes.

The JJ people seem to say that if a fixed bias amp is set too cold depending on how cold it is, using hotter tubes will bring the bias up to where it belongs.

What the real deal here. Am I misunderstanding something?

Please set me straight.

 

[krankguitarist]

Re: Do tubes affect bias?

Different tubes will draw different amounts of current. This is why ya see "matched" sets of tubes; In a matched set all the tubes will draw the same (or close to the same) amount of current.

Using a bias probe, this is the formula I follow:

(plate dissapation of tube) / (plate voltage of amp)= (max bias setting)

(max bias setting) x .75= (min bias setting)

So, to put that into pratice:

I use JJ/tesla 6l6GC's in my Fender showman. Measuring the plate voltage by measuring the voltage between pin #3 and #8 on my power tube sockets, I get about 440 volts. So 440=(plate voltage of amp).

Now, the jj/tesla 6l6gc is supposed to dissapate up to 30 watts. So 30=(plate dissapation of tubes).

30/440= .068

.068 x .75=.051

So...when I use the bias probe to set the bias on my amp, I should set it between 51mA and 68mA. Right now I've got it around 60mA or so.

If I was to swap the power tubes in my Fender for the power tubes in my KRANK, that reading would likely change a bit. Same model tubes, same manufacturer. But...even in tubes of like model and make, there's still some difference in how much current they pull.

I'm a little wary of Mesa's fixed-bias system to be honest...It definitely makes for an amp that's easier to take care of. But I prefer to be able to adjust the bias on my amps to my liking.

(P.S. I'm still an electonics n00b, if any other forum members notice that my language on this is grossly incompetent, please PM me so I can learn to better my ways )

 

[Lake Placid Blues]

Re: Do tubes affect bias?

I believe Mesa don't provide a bias adjustment (Mesa's are not cathode biased in most cases) so that people can't screw them up, and so they don't require a tech every time you replace tubes.

With adjustable bias you can fine tune the bias, but you can also set the bias up wrong, if you don't know what your doing. An amp with the bias set way off, will sound worse, and wear tubes faster, than an amp with the bias close, but not spot on. For the average player, adjusting the bias is also a technically duanting, and potentially dangerous task.

As long as the new set of tubes draw close (within a tolerance) to the same current as the tubes the amp was designed for, the bias will probably be close enough.

 

[LesStrat]

Re: Do tubes affect bias?

I'm biased on this issue.

 

[wixedmords]

Re: Do tubes affect bias?

I believe LPB is correct here, Mesa's are fixed bias - non adjustable as opposed to fixed bias - adjustable, which most fixed bias amps are. You will have to get tubes that will bias close to the originals.

Pretty close krankguitarist. Max dissapation is usually around .70 or 70% rather than .85. You are really looking for how it sound between 50-70% or .50-.70. That would give you this:

.068 x .70 =.047

rather than this:

.068 x .85=.051

Then take the .068 x .70=.047 and multiply it by 1000 to get 47mA

EDIT: I also run 6L6's at 23 watts, rather that 30. As per the Duncan Amps bias sheet. That would change your equation to:

23/440= .052
.052 x .70 = .036
.036 x 1000 = 36mA

Edited to explain fixed bias - adjustable.

 

[cream123]

Re: Do tubes affect bias?

I'm biased on this issue.

:laugh2:

 

[marvar]

Re: Do tubes affect bias?

This is why Mesa insists that you use their tubes (Mesa) for replacement, they will be within the bias set for your amp.

 

[cream123]

Re: Do tubes affect bias?

This is why Mesa insists that you use their tubes (Mesa) for replacement, they will be within the bias set for your amp.

The only problem is that their tubes are freakin expensive and they still aren't very good sounding.

90 dollars for a quad!?!?!?! :eek13:

 

[LesPaulRules]

Re: Do tubes affect bias?

All good info. I'll just add that if you play your amp full up, or at least cranked up half way or more, you would probably be better off biasing your tubes a little colder (about 60%). I have a Marshall that if I bias it any where near 70% max dissapation, then turn it all the way up... the tubes will redplated in less then a minute.

 

[Grandor]

Re: Do tubes affect bias?

The only problem is that their tubes are freakin expensive and they still aren't very good sounding.

90 dollars for a quad!?!?!?! :eek13:

So u just talk to bob over at eurotubes and get tell him what u got and he'll hook u up.

 

[cream123]

Re: Do tubes affect bias?

So u just talk to bob over at eurotubes and get tell him what u got and he'll hook u up.

yeah, I just bought a set of JJ 6l6GCs but I have yet to install them. This weekend they'll be put in as well as a check up. My Nomad has been out of comission for close to a month now. I'm pumped to get it back!

 

[krankguitarist]

Re: Do tubes affect bias?

Pretty close krankguitarist. Max dissapation is usually around .70 or 70% rather than .85. You are really looking for how it sound between 50-70% or .50-.70. That would give you this:

.068 x .70 =.047

rather than this:

.068 x .85=.051

Just fixed my original post...Meant to type .75 rather than .85.

 

[marvar]

Re: Do tubes affect bias?

FYI=Here is a scale from Miles Rose's Amplifier Blueprinting Website, that shows the GT replacement vs Mesa tubes
http://www.guitaramplifierblueprinting.com/pastinfo.html
"Mesa Scale crossed to Groove Tubes scale
Red 4
Yellow 4
Green 5
Gray 5
Blue 6
White 6

The reason for Mesa's narrow range, is that their amplifiers do not have an adjustable bias. This way if you
have one of their amps, you can use any of their tubes safely.

For amplifiers without an adjustable bias, if you want more headroom and/or power as an example ... and you
amp had a #5 or Mesa "Green" tube in it .... and had an idle dissapation of about 50%, you would just go to a
"White", "Blue" or a Groove Tubes #6. You will see your idle dissapation go up to maybe 55%. This is a way to
fine tune your amp."