One for the techies out there....

[dodger916]

Hi:

I am in the middle of a discussion with a guy selling some Groove Tubes 6CA7s on ebay. In my response to his response to my initial inquiry, I explained that Mesa's bias is fixed (non-adjustable), and that I needed Groove Tubes in the 4-6 range in order to comply with Mesa's operating and warranty specifications, to which he responded with the following:


I'm, sorry but your mistaken. First of all Mesa amps are cathode biased not fixed biased. Fixed bias refers to amps which have an adjustable bias/ a bias pot. Secondly no tube amp has parameters that are that narrow if they use el34. In tube guitar amps components will work if they are within 20% of one another but often they will work if the ranges are even wider. The claim to fame of groove tubes is that tubes don't need to match electrically anywhere near as well as the average end user believes and that when tubes burn in they drift in value and still work fine anyway. This is mostly true. Groove tubes claims that they use a distortion meter to determine the point at which tubes break up and they match them accordingly. They say that their tubes are distortion rated and the hardness rating refers to the point at which tubes begin to break-up. To see what I think about this read my previous message. Mesa amps tend to try to squeeze the maximum wattage out of tubes and so it make sense to use Sylvania Fat boys because they can handle the highest plate voltages and offer slightly higher output. I don;t have any more time to spend on this.
best


Any techie with an opinion on this?

Thanks.

Frank

 

[msi]

Well, you are correct and he is wrong about how mesa amps are biased. They are fixed biased, not cathode biased. And some amps can have a variable voltage. Most parts allow for a certain degree of drift, usually between 1-5%. Finally the exact voltage coming from your wall may not be the same from place to place. So you can have a decent amount of variance from amp to amp. But to sum it all up, that guy sounds like a douche.

 

[Monsta-Tone]

But to sum it all up, that guy sounds like a douche.

:lol: :lol: :lol: :lol:

I wouldn't buy anything from him, no matter how good the price is.

Fixed Bias refers to a biasing method. Most Mesa amps are fixed bias and do require tubes in the general range that you stated.

Some of the Mesa lineup are Cathode Biased and do not require tubes in that range, and do not require the tubes to be closely matched.

Any Mesa amp that says, "Class A," is a Cathode Biased amp. The rest are not.


This guy has no idea what he's talking about and do not trust him. I would email him a link to Mesa's website and tell him to shove it.

 

[stokes]

This guy,like a lot of peaople,has read things and doesnt fully understand what they are talking about.Most Mesa amps are fixed bias,and can be adjusted.Class A doesnt have to be cathode biased,Monsta,and cathode biased power tubes should be as closely matched as fixed bias.Just because Mesa says "Class A" doesnt mean it is cathode biased,or that it is a true class A amp.In fact the MKIV has a class A switch and is fixed bias,and if it is "true Class A" is debatable.This guy is speaking a lot of "half truths",I wouldnt buy from him.GT's are readilly available and cheap enough,just order some from a reliable supplier.

 

[phyrexia]

Is it Uptown Audio?

 

[dodger916]

No. It's International Audio Tube. He's been fine to deal with and seems knowledgable. He just might be misinformed with respect to Mesas.

 

[stokes]

He's misinformed about more than Mesa.Although he uses the name "International Audio tubes",looking at his ebay "store" and reading this tells me he is just some guy with tubes for sale who has half the story on tubes.Power tubes that are mismatched by 20% will cause a loss of power and an increase in hum in the output.Those tubes may very well be good,but if they fall below the 4-7 recommended rating,they will be way too cold in your amp and will sound like ****.If he cant tell you what the GT ratings are,theres something wrong.I would also be leary about paying $90 for a used set of tubes,especially from someone making the staements he made.

 

[Monsta-Tone]

Class A doesnt have to be cathode biased,Monsta,and cathode biased power tubes should be as closely matched as fixed bias.

Every Mesa amp that I have drawings for that says Class A is actually Cathode Biased, with the exception of the Lonestars that actually go to 5 or 10 watts. These supposedly are true Class A, single ended when in the 5 or 10 watt setting. But, from the drawings, I cannot tell. I'll have to take a meter to my amp and see.

If you read about the BiValve, or most Class A amps with Cathode Bias, and what most manufacturers say about their Cathode Biased amps, they specifically tell you that you do not need for the tubes to be matched that closely. 20% is not that tight of a tollerance.

 

[stokes]

While it is common for a class A amp to be cathode biased,it isnt an absolute.A fixed biased amp can be biased so the tubes draw max (although true classA draws about 90%) current for the full cycle.I have an amp I built, with a pair of 7027A power tubes,fixed bias class A,it needed a much bigger OT since it is drawing about 150ma's.It is also more common to use single ended output for class A,but again isnt an absolute.In the case of single ended,your tubes dont have to be matched (in a parallel config.) but they will perform better if they are matched.Push-pull amps should be "matched",cathode biased or not,not necassarilly an exact match,but close enough not to cause a loss of power or excessive hum.There are many different opinions as to what constitutes "true" class A,so we could on for days as to what amps are and arent class A.Mesa has a class A switch on their MKIV,but it is fixed bias.Vox calls some of their amps class A just because it is cathode biased,but in fact they arent,many manufacturers have claimed class A over the years,but arent actually class A,it just sounds more attractive.I have my doubts as to Mesa's MKIV actually being "true" class A because if you bias power tubes into class A with 400 volts on the plate your tubes will burn out quite fast,if you look at tube data sheets,most will recommend no more than 250 volts for a 6L6 type tube because of the heat class A will develop.The amp I described above gets so hot,you can feel the heat across the room after an hour or so.

 

[Monsta-Tone]

The amp I described above gets so hot,you can feel the heat across the room after an hour or so.

:lol: :lol: I bet it sounds good though!


At the risk of hijacking the thread, this is all I was trying to say....Don't believe any of the hype, and don't buy tubes from that guy. I wasn't looking for a pissing contest. :lol: It's too windy today! :lol:

 

[stokes]

No,I didnt mean it to sound like a pissing contest either,sorry if I came off that way,and I totally agree with your original post about staying away from this guy,just trying to dispel some myths,like the fact that an amp HAS to be cathode biased to be class A,and pointing out that not all amps that claim class A,actually are.Class A has become more of a marketing ploy of late,and more amps that claim it are more "in class A territory" than actually class A.Sorry for the hijack,dodger,but i think you got your answer anyway.I find it amusing that in his response to you,he says that matching isnt all that important,yet his ad touts how closely and carefully he matches them.

 

[Monsta-Tone]

I find it amusing that in his response to you,he says that matching isnt all that important,yet his ad touts how closely and carefully he matches them.

:lol: :lol: That's the cool thing about the internet. Any mental midget can easily become Andre the Giant. Still not that smart, but much much bigger! :lol:



Stokes, I think we are arguing the same side of the same point. My posts were just pre-coffee and incomprehensible. :lol:

 

[dodger916]

No,I didnt mean it to sound like a pissing contest either,sorry if I came off that way,and I totally agree with your original post about staying away from this guy,just trying to dispel some myths,like the fact that an amp HAS to be cathode biased to be class A,and pointing out that not all amps that claim class A,actually are.Class A has become more of a marketing ploy of late,and more amps that claim it are more "in class A territory" than actually class A.Sorry for the hijack,dodger,but i think you got your answer anyway.I find it amusing that in his response to you,he says that matching isnt all that important,yet his ad touts how closely and carefully he matches them.

This is a great discussion, but I'm confused about one thing stokes. In your first post you say "Most Mesa amps are fixed bias and can be adjusted." How are they adjusted?

Thanks again, guys.

Frank

 

[stokes]

Monsta,didnt think it was an argument at all,just an exchange of opinions on what constitutes class A.If you look around,there are many opinions on what makes an amp class A,all deviations of the "textbook" definition,all "somewhat" correct.I think it is always good to have these kind of exchanges,that is a big part of what these forums are about.

 

[stokes]

This is a great discussion, but I'm confused about one thing stokes. In your first post you say "Most Mesa amps are fixed bias and can be adjusted." How are they adjusted?

Thanks again, guys.

Frank[/quote]
Frank,first of all you have to look at what "fixed bias" really means.It actually refers to the "fixed" amount of negative voltage applied to the control grid,or the bias voltage.This negative voltage is what determines the operating parameters of the tube,or how much current the tube draws.This voltage is "fixed" by means of a resistor or a pot in the bias supply.In the old days,most amps used a resistor.When you changed tubes you would replace this resistor to set the bias voltage to get the tube in the right range.This is the way Mesa builds their bias supply.What Mesa then does is buy huge lots of tubes and test them in a control amp to see how much current they draw,all tubes will draw a different amount of current,even from the same production run,just the nature of the beast.They then select the ones that will operate safely in their amps so you dont have to re-set the bias when you use their tubes.There is a fairly wide range that will be considered safe,a 6L6GC in a Mesa amp can safely operate at about 25 to 40 ma's.In my experience Mesa's tubes are biased very cold,I have never found any tubes drawing more than 20 to 30ma's which doesnt allow the power tubes to get too hot,so generally you are dealing with a very clean output section coupled with a preamp that can be overdriven very easily,so for the most part you are getting all preamp distortion.Many people,myself included,like some power tube saturation,I prefer more power tube saturation and little preamp distortion.To acheive this I have to re-set the bias to draw more current to get the power tubes to break up.This is done easily by replacing the resistor with an adjustable pot,which is still "fixed bias",but more easily adjusted.Cathode biasing doesnt use negative voltage.It uses a cathode resistor to raise the voltage of the cathode,so the control grid seems to be negative in comparison to the cathode,so the end result is the same.It is not necessary to adjust a cathode biased amp because the cathode resistor is self balancing to some degree,it keeps the tube from drawing too much current.But these amps can be adjusted by tweaking the value of the cathode resistor,in fact on all the cathode biased amps I have,I use an adjustable wire wound resistor,I am kind of anal about biasing and like to find the "sweet spot" in the tubes.So in reality any bias scheme can be adjusted.Fixed bias should be checked and adjusted,if necessary,any time you change power tubes,cathode bias can be adjusted if you are so inclined,but doesnt have to be and most people dont and amp builders will tell you to just "plug them in and go".

 

[dodger916]

Frank,first of all you have to look at what "fixed bias" really means.It actually refers to the "fixed" amount of negative voltage applied to the control grid,or the bias voltage.This negative voltage is what determines the operating parameters of the tube,or how much current the tube draws.This voltage is "fixed" by means of a resistor or a pot in the bias supply.In the old days,most amps used a resistor.When you changed tubes you would replace this resistor to set the bias voltage to get the tube in the right range.This is the way Mesa builds their bias supply.What Mesa then does is buy huge lots of tubes and test them in a control amp to see how much current they draw,all tubes will draw a different amount of current,even from the same production run,just the nature of the beast.They then select the ones that will operate safely in their amps so you dont have to re-set the bias when you use their tubes.There is a fairly wide range that will be considered safe,a 6L6GC in a Mesa amp can safely operate at about 25 to 40 ma's.In my experience Mesa's tubes are biased very cold,I have never found any tubes drawing more than 20 to 30ma's which doesnt allow the power tubes to get too hot,so generally you are dealing with a very clean output section coupled with a preamp that can be overdriven very easily,so for the most part you are getting all preamp distortion.Many people,myself included,like some power tube saturation,I prefer more power tube saturation and little preamp distortion.To acheive this I have to re-set the bias to draw more current to get the power tubes to break up.This is done easily by replacing the resistor with an adjustable pot,which is still "fixed bias",but more easily adjusted.Cathode biasing doesnt use negative voltage.It uses a cathode resistor to raise the voltage of the cathode,so the control grid seems to be negative in comparison to the cathode,so the end result is the same.It is not necessary to adjust a cathode biased amp because the cathode resistor is self balancing to some degree,it keeps the tube from drawing too much current.But these amps can be adjusted by tweaking the value of the cathode resistor,in fact on all the cathode biased amps I have,I use an adjustable wire wound resistor,I am kind of anal about biasing and like to find the "sweet spot" in the tubes.So in reality any bias scheme can be adjusted.Fixed bias should be checked and adjusted,if necessary,any time you change power tubes,cathode bias can be adjusted if you are so inclined,but doesnt have to be and most people dont and amp builders will tell you to just "plug them in and go".

Excellent description, stokes. I've read several items about this over the past few months, but this really pulled it all together for me. Thanks!

I'm on the threshold of constructing this mentally, so If I may impose further. So if I'm understanding this correctly, current flows into the positively charged cathode and wants to jump to the negatively charged plates. The negatively charged grid stands between the two as a gatekeeper, and as the charge or "bias" of the grid is changed, the amount of current flowing from cathode to plate is changed.

So I have two more questions:

1) Increasing the negative bias of the grid will increase or decrease the current flow to the plates?

2) When you say:
"There is a fairly wide range that will be considered safe,a 6L6GC in a Mesa amp can safely operate at about 25 to 40 ma's.In my experience Mesa's tubes are biased very cold,I have never found any tubes drawing more than 20 to 30ma's"

are you referring to the inherent current draw of the tube or the bias?

Thanks again.

Frank

 

[stokes]

Lets look at a fixed bias situation first.Think of a tube as a valve (as they are called in europe).The cathode in a fixed biased tube is at ground potential.The electrons flow towards the positively charged anode or plate from the cathode.Since you have a flow of positive current flowing to the plate,the control grid,when negatively charged,will restrict the flow much like a valve.So if you have more negative voltage,lets say -52v,you will restrict the flow of current more than if you had less negative voltage,say -25v's.So to answer your first question,more negative voltage will mean less current.So a cathode biased tube works like this.Instead of the grid being negative,the cathode resistor puts the cathode at some voltage above ground,lets say +25v's.Since the control grid is at zero volts it fools the plate into seeing the grid as being negatively charged as compared to the cathode.As for your second question,the 20 to 30ma's is what Mesa sets the bias at when using their tubes.This is fairly cold,my belief is that it is mainly a safety issue for Mesa.If they were to set the bias at or near max dissipation,they would have to be very precise in selecting tubes with a narrow margin.If they set the bias to draw 40ma's and a set of tubes that draw more current were to get to the end user you could end up blowing a tube and causing them a liability issue.Keeping the bias cold gives them more margin for error and makes more tubes usable.Mesa's tone is based on the preamp being able to distort easily,so they dont need to bias hot enough for power tube saturation.In fact most people I have heard that dont like Mesa amps is for just that reason,no power tube saturation and they tend to be somewhat buzzy when pushed.

 

[dodger916]

Again..thanks very much, stokes.