mesa's cold bias...

[carloc80]

I know that mesa's run at cold bias.
Do you know the bias value in Mv ?
for instance I have a Cornford mk 50 with adjustable bias and the manufacturer says that when biasing the amp "the reading should be 35Mv on both valve bases". Is this value(35 Mv) cold or hot?
And what is the Mesa's value? I have a Mark4.
thanks

 

[Whoopysnorp]

There's not really an answer for that question...certain tubes will pull more current than other tubes will. Mesa-branded tubes tend to pull more current than many tubes, which counteracts the cold bias.

 

[stokes]

There's not really an answer for that question...certain tubes will pull more current than other tubes will. Mesa-branded tubes tend to pull more current than many tubes, which counteracts the cold bias.

That answer contradicts itself.If a tube "pulls more current" it is hotter than the tube being described as cold.Mesa branded tubes dont "pull more current".Even the "hottest" Mesa tube is still cold.
Whether your Cornford is hot or cold depends on the plate volts.I would guess that 35mv's is on the cold side,or towards the middle.The MKIV draws about 25ma's,which is cold for the 450v's on the plate (approx.,I'm not sure offhand the exact plate volts,which will vary depending on what your tubes are actually drawing,but it is in the area of 450).With 450v's on the plate a tube wouldnt be considered "hot" until about 45ma's.

 

[carloc80]

maybe on the cornford forum where I red how to bias the amp there's a mistake,I think that with Mv they intended ma...

 

[carloc80]

now if I put 2 6l6 mesa tubes in the cornford(with bias at 35 ma) without rebiasing it,these 2 mesa tubes will work "hot","too hot",or cold or etc...???

 

[stokes]

Probably no mistake.A common way to measure a tubes current draw is to use a 1ohm resistor between the cathode and ground.You then use a voltmeter to measure the voltage drop across the 1ohm resistor which will convert directly to ma's.So if you read 35mv's it is 35ma's.

 

[stokes]

now if I put 2 6l6 mesa tubes in the cornford(with bias at 35 ma) without rebiasing it,these 2 mesa tubes will work "hot","too hot",or cold or etc...???

Just because the tubes you had in the Cornford that read 35mv's or ma's,doesnt mean the next pair you put in will draw the same.Each pair of tubes will have their own current draw.You have to measure and adjust the bias every time you change tubes.With Mesa tubes they are tested and then sorted so when they sell you a pair of the same rating they will draw the same as the last pair you used.If you were to buy a batch of 20 tubes from Sovtek for instance and put them in a tester they will likely not all draw the same current.You would have to sort them and match the ones that are close in pairs or quads.This is why amps have adjustable bias pots.Contrary to what Mesa would like you to think,their amps are adjustable.Fixed bias does not mean non-adjustable,the term refers to a "fixed" amount of negative voltage applied to the control grid.Mesa just sets this in their amps and matches tubes that draw a safe,but cold current,to match their amps.Leading people to believe they have to use their tubes,and unless you know how to measure and adjust the bias,you should use them.A lot of tube suppliers have become aware of this marketing scheme and now sell you tubes that match the Mesa specs.

 

[Whoopysnorp]

That answer contradicts itself.If a tube "pulls more current" it is hotter than the tube being described as cold.Mesa branded tubes dont "pull more current".Even the "hottest" Mesa tube is still cold.

That's the same thing that I am saying. I was trying to say that knowing what a particular set of tubes draws in a particular amp tells you nothing about the amp.

 

[carloc80]

now if I put 2 6l6 mesa tubes in the cornford(with bias at 35 ma) without rebiasing it,these 2 mesa tubes will work "hot","too hot",or cold or etc...???

Just because the tubes you had in the Cornford that read 35mv's or ma's,doesnt mean the next pair you put in will draw the same.Each pair of tubes will have their own current draw.You have to measure and adjust the bias every time you change tubes.With Mesa tubes they are tested and then sorted so when they sell you a pair of the same rating they will draw the same as the last pair you used.If you were to buy a batch of 20 tubes from Sovtek for instance and put them in a tester they will likely not all draw the same current.You would have to sort them and match the ones that are close in pairs or quads.This is why amps have adjustable bias pots.Contrary to what Mesa would like you to think,their amps are adjustable.Fixed bias does not mean non-adjustable,the term refers to a "fixed" amount of negative voltage applied to the control grid.Mesa just sets this in their amps and matches tubes that draw a safe,but cold current,to match their amps.Leading people to believe they have to use their tubes,and unless you know how to measure and adjust the bias,you should use them.A lot of tube suppliers have become aware of this marketing scheme and now sell you tubes that match the Mesa specs.

i know that mesa's are fixed bias,but even cornford and marshall are fixed bias,but instead a resistor(in mesa's) they have an adjustable bias pot. now if i put a pair of matched and "in specs" tubes in my cornford without rebias i'll have the same results as putting a pair of tubes in my mesa. or i'm wrong?
i know that biasing the amp when changing tubes is the best thing to do.(and in mesa's i can't do this unless i change the resistor or install a bias pot).

 

[stokes]

You will only have the same results in your Cornford if the bias pot is set to deliver the same negative voltage and your plate volts are the same as they are in the Mesa.You have to take things a step further also.What you are actually interestred in is the idle watts.If your Mesa has 450 plate volts and draws 25ma you are idling at 11.25 watts.If your Cornford has 400 plate volts and you draw 25ma's you are idling at 10 wattts.Either case is extremely cold.A 6L6GC can idle at a max 30watts,class AB1,which is what most push/pull amps are is about 70% of max plate dissipation,21 watts in this case.So putting those Mesa tubes in your Cornford wont necessarily get the same results.In order to get the same results your plate volts and bias (negative voltage) would have to be the same in both amps.

 

[stokes]

That answer contradicts itself.If a tube "pulls more current" it is hotter than the tube being described as cold.Mesa branded tubes dont "pull more current".Even the "hottest" Mesa tube is still cold.

That's the same thing that I am saying. I was trying to say that knowing what a particular set of tubes draws in a particular amp tells you nothing about the amp.

If a tube draws more current,it doesnt "counteract the cold bias",it is not in fact biased cold.Mesa tubes dont draw more current than other tubes,they in fact draw less,that is what makes them cold to begin with.

 

[metal190]

But, different colors of tubes draw different amounts, correct? Thus, you can run a hotter tube, which can slightly compensate for a colder bias (granted the tube could run even hotter if you adjusted the bias).

Correct me if I'm wrong.

 

[stokes]

You could look at it like that,I suppose.But you really aren "compensating".The hottest tube Mesa sells is still cold when compared to what is considered optimum performance,which in a push/pull output is about 21 idle watts,as opposed to the 10-15 idle watts you will get from a Mesa tube.

 

[metal190]

Fair enough.

 

[ANIMATED SUSPENSION]

If people hadn't read on the internet about the cold bias 'issue' would anyone really have noticed ? To me Boogies sound great, miles better than any Marshall biased hot or cold. And that's all that matters at the end of the day !! If you have an amp biased correctly with known spec tubes you can replace them with new ones of the same type/spec without rebiasing of course.

 

[carloc80]

If you have an amp biased correctly with known spec tubes you can replace them with new ones of the same type/spec without rebiasing of course.

this is what i'm interested to.. i want to change tubes in my cornford without rebiasing it cause i don't know any tech in my town that is able to do it..
i want to do in my cornford what i usually do on my mesa(that sounds great!).

 

[metal190]

Yeah, there's definitely a reason Mesa does it the way they do.

I actually didn't read about the cold bias on the internet, rather I heard about it from our local mesa tech who offers to add adjustable bias to any mesa amp. He basically mentioned that it was an easy way to "hot rod" any mesa amp. He acknowledged that Mesas were already smokin amps, but that after doing a couple mods everyone he had done them for were blown away by how much better it could sound.

I haven't ever gone for it cause I'm not stoked about voiding my warranty, which is definitely one of mesa's strong suits. And besides, I've never found myself cursing my road king for not having enough gain.

 

[carloc80]

is it safe to pull out the 2 inner tubes and run mark4 in classA?

 

[mr_fender]

I love the way my Boogie sounds, so I've never really given it much thought. That said, I wouldn't mind trying out an adjustable bias moded amp just to see how much difference it really makes. I doubt it would be a huge difference. Preamp gain is what 99% people are listening to when they talk about needing more gain. If you need more gain (distortion) than a Mark IV or Recto amp can offer you've got other problems! Keep in mind too that a hotter bias might make your power tubes wear out faster.