To Bias or Not to Bias...That is the Question.

[paulcampo]

Thanks for the info Joe...this in response to your reply from another post that I pasted below..
I DO know that changing tubes from 6550 to EL34 needs to be biased...I also know adjusting the bias is not a function of the tube, but I thought it DOES affect the tone by the amount of (exact current?) going to the tubes from the amp...my old tech had me play while he adjusted the bias and I told him to stop when I liked it...which apparently was considered a 'cold' bias for 6550 tubes in a Marshall, but I like the cleaner tones you can get from the JCM 800...I think it was either 16wts per tube or 22wts if that sounds right...so I always wanted to 'demystify' the issue of biasing or not and its affect on tone and tube life...my old tech told me once you bias for a certain type & brand of tube, its not that neccasary...in my case, its always 1965-1975 Tung Sol 6550. He said quality control was pretty tight then, so its ok with those old Tung Sols.
I wanted to learn how to bias my amp myself with those  'bias sockets' you see on ebay, but dont want to give myself a new hairdo either since I dont really know too much...should I bother? Does biasing have ANY effect (+ / -)on tube life and tone? Does all this talk apply to  removing the solid state rectifier plug in my Solarus and just putting in an old 5AR4 rectifer?
Thanks Joe



In the case of fixed biased amps, changing output tubes is not going to change the bias voltage applied to the grid of the output tubes. The bias voltage is a function of the bias power supply, adjustable or otherwise, not a function of the tubes. The bias voltage is not going to change if you simply take out old tubes and put in new ones. It doesn't "wear out" with the tubes.

The bias voltage is specified by the manufacturer of the amp to work with a particular output tube type and handle the range of tolerances within that type and to provide an acceptable balance between tone and tube life. The only real reason for amps having an adjustable bias in the first place (those that have them) is so that the manufacturer can easily set it during production without having to hand select individual components.

As long as you stick with the manufacturers tube recommendations there is no real reason to bias a normal production tube amp during tube replacement. People changed their tubes without a bias adjustment requirement for roughly three decades without any problems.

If you change output tube types (e.g. 6550 to EL34) then the amp does need to be biased to work correctly with the different parameters of that tube. That also goes for rectifier replacements, but only if you change tube types or use a solid state replacement, as this may affect the B+ voltage which has a relationship to the bias voltage. However, when you start talking about changing tube types, then you enter the realm of "modification" and none of this applies.

[djc]

My understanding of this is limited but I am learning and like you would like to bias my amps myself if they require or if I want to change the tone.  Bias does affect tone.  hotter the tubes (higher, I think, bias supply) the warmer the tone is but shorter tube life.  colder, more brittle sound, clean crisp tones you talk of, longer tube life.  there is a point were it begins to sound very badly, likewise there is a point on the hot side your tubes will blow within seconds. 
I can't remember who it was but some famous player years ago had a tech with oven mitts on replace his tubes multiple times during a concert because they would burn out.  great tone, but spent a lot of money to play with that sound.
those bias sockets are a great way to go, you're not opening up the amp and they give you the voltage reading by connecting a multimeter to two leads.  use a screw driver to turn the pot and voila, you've changed the bias.  pretty simple and you are not a grounding rod.
there are specs that you will need to follow ie the recommended voltage for a specific tube type, readily available on the web and I've seen it posted here.  that is what is considered a safe operating level or a good balance between tone and tube life.
some of the Sr members could confirm this or please, correct me if I am wrong or misunderstand something here.  Biasing is not rocket science, it really is very simple and if techs get away charging $75 to do it, I'm gonna learn and make a mint.

[JoeArthur]

I have two rules that have never been proven wrong:

Rule 1) Tone is always a personal preference.

This is similar to that old saying that one man's garbage is another man's treasure.

Rule 2) There is no electronic equation or formula that yields TONE as a result.

I can give you any electronic formula you want - capacitive reactance at a particular frequency... etc. But there is no guarantee of TONE - good or bad. This should be obvious because of Rule 1.

---

No, biasing a tube amp is not rocket science. You measure the grid voltage and adjust it until it meets specifications. In many amps, like the model T, you don't need a "special bias adapter" - it is already provided. Just stick your meter in and adjust.

But what is claimed to be rocket surgery - is biasing an amp with total disregard for the manufacturer's specifications... based on the current flow through the tubes, not the grid voltage. This provides the illusion of matching the tubes to your amp (or vice versa), to achieve some "magic number" of tube current. To do this, you need a real "tube tone tech" that doesn't believe rule 2 (and by default, rule 1 - he has already made up his mind what good tone is and you simply default to his opinion).

Does this statement make sense: "Tube tone is a warm tone and if you run a tube hotter the tone will be warmer still." If so, waste no time in finding one of those tube tone tech gurus to take all the money you are more than willing to give up. (rule 2). The only reason for running tubes "hotter" is that they will wear out quicker making you a more frequent customer for tubes. The tube amp guru tech will keep your hopes up until your money runs out or you give up hope.

If you're not convinced, then consider this. The amp (more than likely) has a single bias adjustment. Do you really believe it is possible to adjust the current through each tube with a single adjustment affecting all two (or four) output tubes?

The bottom line is simple, although you may not want to hear it: If you are not getting the tone you want from your amp, set to the manufacturers specifications for bias voltage, you won't find any magic in tweaking the bias. You simply have the wrong amp for the tone you want.

But hey, if you are tweaking on your own, you have nothing to lose but your amp/tubes... and patience. I say go for it!! You will soon realize the "bottom line" I mentioned above without spending much extra money.

[djc]

The amp (more than likely) has a single bias adjustment. Do you really believe it is possible to adjust the current through each tube with a single adjustment affecting all two (or four) output tubes?

Good point, mine has only one adjustment.  I currently run to the specs set out and love the tone of my amp.  no desire to change it.

is there any truth to the "hotter the warmer"? if not, why did (do) so many run according to that myth?  please debunk it, I've always been a fan of reading instructions and actually following manufacturers specifications, and, thankfully so does the tech who worked on my amp and called it and treated it like 'a work of art'.

[EdBass]

is there any truth to the "hotter the warmer"? if not, why did (do) so many run according to that myth? 
please debunk it,

What is "warmer", and is it a good or bad thing? If by "warmer" you mean more distorted, I would say yes, the hotter you bias the "warmer" the sound. The hotter you run, the lower levels it takes to get saturation, and as the distortion builds the tone will get thicker with harmonics (warmer?) until the tube gets so saturated it can almost become difficult to distinguish the actual note being amplified. Keep in mind, with the Sunn amps we are all so fond of, the design was originally based on Dynaco hi-fi amplifiers. Harmonic distortion was the enemy, not part of the design. Also, IMO 6550 types lean towards nasty, more abrupt breakup when pushed hard, than their smaller octal siblings. Don't get me wrong, I often intentionally push 6550's/KT88's into saturation, but the big tubes seem to snarl and grow instead of getting warm and fuzzy.

I don't think there is any thing to debunk; it's a matter of personal preference;

I have two rules that have never been proven wrong:

Rule 1) Tone is always a personal preference.

This is similar to that old saying that one man's garbage is another man's treasure.

Rule 2) There is no electronic equation or formula that yields TONE as a result.

Maybe there is a formula for "warmer" though?

I mostly agree that if the amp of choice isn’t getting the job done set at specification, you probably are using the wrong tool for the job.
But do what you want; I don’t think a player can find their personal tone niche without experimentation. Most long time players with the wherewithal to indulge themselves and probably all professionals have evolved their rigs/instruments/accessories/setting into their own tone over years of experimentation, usually it’s an unending quest.
Of course, be aware of  the financial aspect, you will go through expensive output tubes at an alarming rate if you bias for dissipation rates exceeding tube specification, and of course the potential collateral damage a flame out can cause.
Other than that possible nastiness; if you want breakup, crank them up, if clean is your bag, cool them off.

[JoeArthur]

Well, I'm not sure if the myth can be debunked. Rule 1 claims the myth is valid if the user prefers the sound. There is nothing to debunk with an opinion based solely on personal preference.

The rest of this is very general and an extreme case - it is for example only and all numbers are totally made up so I don't have to contend with math. It's a thought experiment.

There are other things happening in the amp when you shift the bias besides the tube running "hotter or colder". If the myth of "better tone" is only based on the "heatness" of the tube, it is incorrect because it doesn't consider these other effects.

You know about operating classes - class "A" means the tube is conducting current 100% of the time. Class "B" means the tube is conducting current 50% of the time. The majority of guitar power amps are considered class "AB" - meaning the tube conducts current somewhere between 100% and 50% of the time. None of these classes of operation can be considered more "toneful" than the other although there are plenty of people that claim otherwise.

Say we have our imaginary amp that can operate with everything perfect and linear. The output tubes can pull 100ma of current maximum. Assume the design bias point is so that the output tubes idle at 25ma of current.

Let's also assume the phase inverter is capable of cleanly driving the output tubes the full 75ma of capability between the 25ma idle and 100ma maximum for the positive side of the signal. With that assumption, we can also assume the negative side of the signal can drive the tube from idle down to zero current - cutting off the tube. At maximum clean power output, each tube is operating 75% of the time.

This would be class "AB", but until the negative signal is strong enough to actually cut off the tube, it is by definition operating class "A". So the amp actually transitions from class "A" to class "AB" at some point depending on the strength of the signal and the bias point.

This is baseline. Now let's reduce the grid bias voltage so the tubes are now idling at a higher current. Let's assume 35ma. Regardless if we consider it "warmer", what is happening inside the amp that might cause any change in sound?

The most obvious thing that happens is that the tubes operate warmer because they have to dissipate more power sitting around doing nothing. This change also shifts our operation point closer to class "A", taking more negative signal to cutoff our tube and shift it into class "AB", which means it is operating a higher percentage of time keeping the tube even hotter since it has less time to cool off. All of this reduces the effective life of the tube.

Remember our phase inverter. It will still try to drive the output stage to produce 75ma of positive side current, but now the output tube only has a reserve of 65ma to work with. The result is earlier clipping of the output stage and reduced output power. Why reduced output power - because the output stage is now only capable of swinging 65ma instead of 75ma through the output transformer.

What might not be so obvious is that the tubes aren't the only thing that has to deal with the increase in idle current. It also affects the output transformer since tube current flows through it as well. At idle it will also have to dissipate the extra heat from the increased current.

Also (very simple) a transformer works by generating a magnetic field. How much of a magnetic field it can ultimately generate depends on various factors - the amount of "iron" being one. At some point it will be unable to generate more magnetic field with an increase in current. This is called "saturation".

With saturation, the result is a decrease in bandwidth, basically less highs and lows make it through the output transformer creating a midrange, nasal type sound. To be honest, some amps intentionally use too small a transformer to create this sound - like a Bandmaster head for example. With the increase in idle current through the transformer - it will now operate in saturation longer, reducing the bandwidth as you crank up the volume - since it doesn't have as far to go to saturate.

Now back to the myth. If you define "warmth" as a midrangy sound lacking in highs and lows with more distortion, then it really wouldn't be a myth.

[djc]

So much to learn.  it amazes me that man has the capacities not only to design but understand how electrical currents work.  most people just turn the toaster on.

As for 'warmer' what I meant was 'the tone will get thicker with harmonics'.  and as I am realizing it is more of a buzz word than a descriptive word.  what didn't even dawn on me is the number of other variables that are affected as values are changed.  that in itself will change the way I play the instrument and the amp.  you'd think I'd know this stuff teaching 'gain structure' to audio techs.

Thank you JoeArthur and EdBass for your patience and willingness to share your knowledge.
keep it up

[paulcampo]

So...I get the concept of tone being subective and theres no 'right' tone...I have never kept an amp very long that I didnt like...so true that an amp has a 'basic' voice...if you're looking for 'Fender' clean....use a Fender or better yet a Sunn!...no point in trying to make your Marshall do something it doesnt want to do...but what about biasing the amp in regards to tube life? That was my main question...If the amp has been biased recently, how often do you really need to check to see if its in the factory parameters? I'm totally happy with the tones I get, but was actually more curious of tube life in regards to hot or cold biasing...what kind of life difference with the tube are we talking about, or is this very difficult to determine with a generalized statement?...these damn tung sols can be expensive on a bad day!

[JoeArthur]

Don't know about tube life - it would be a generalization. There are so many different variables that enter into determining the life of a tube, and it would be difficult to even guess. The difficulty is only changing one variable and assuming everything else to remain exactly the same - doesn't happen too often in real life.

As for frequency of checking bias, I can tell you what I do.

o When I get it (as in new or new to me), I check it. If it's really new, it's normally dead on. If it's new to me, it usually needs adjustment.

o If I make any repairs or modifications, I check it. Normally it's still dead on, unless of course it was a repair to the bias circuit - like replacing the electrolytics or resistors.

o For periodic checks, maybe an average of every five years - give or take. No real set schedule. I remember one amp I hadn't checked for a little over 10 years. When I did it was dead on.

o Finally, if anything seems weird with the amp, or it developes some kind of problem, it's one of the first things I check as part of normal troubleshooting.

[EdBass]

So...I get the concept of tone being subective and theres no 'right' tone...I have never kept an amp very long that I didnt like...so true that an amp has a 'basic' voice...if you're looking for 'Fender' clean....use a Fender or better yet a Sunn!...no point in trying to make your Marshall do something it doesnt want to do...but what about biasing the amp in regards to tube life? That was my main question...If the amp has been biased recently, how often do you really need to check to see if its in the factory parameters? I'm totally happy with the tones I get, but was actually more curious of tube life in regards to hot or cold biasing...what kind of life difference with the tube are we talking about, or is this very difficult to determine with a generalized statement?...these damn tung sols can be expensive on a bad day!

I assume you are referring to a fixed bias amps with adjustable bias. The higher the - bias the longer the tubes will last under the same circumstances (same amp, same settings, identical* tubes, same usage). the lower the bias, the greater the current through the tube and theoretically shorter tube life. Is that what you are asking?
As far as how often; I set them once and only check it if something goes sour.

*The term "identical tubes" may be close to an oxymoron. I have found tubes, more specifically modern tubes to be very inconsistent as far as durability. I've had single octals in matched pairs that fail randomly while the other one still tests strong as new, even after a good burn in and retest, sometimes after many hours of playing time. Sometimes they both are junk in 2 weeks, sometimes they both last for years of hard use/abuse.
Kind of a shame about the new tubes because I hate to tap into my stash of NOS Gold Lions. That gross I bought back in '71 is down to 64 pairs...
The old power tubes are much more durable, but even more importantly more consistent. However, they can still fail indiscriminately.

[JoeArthur]

One thing I think I need to point out.

In terms of tubes... bias is defined as the voltage difference between the cathode and the grid of a tube. Bias is not the current flow through the tube - current flow through the tube is the RESULT of the bias voltage applied to the grid.

The voltage between the cathode and the grid is always NEGATIVE in polarity. It is measured with respect to the cathode - the cathode is normally grounded at ZERO volts, so the grid will be negative.

A higher bias means going from a lower negative voltage to a higher negative bias voltage - e.g. from -55 volts to -60 volts. It's more of an absolute value direction than a direction on a number line.

The current through the tube has an inverse relationship to the bias voltage applied to the grid.

To increase current flow through the tube you have to reduce the bias  and to reduce current flow through the tube you have to increase the bias.

REMEMBER - bias is the voltage difference between the cathode and the grid of the tube. It is not the current flow through the tube.

However, you will read where people claim to have their tubes biased at 30ma (or whatever the number happens to be) of current. THIS IS NOT BIAS - this is the RESULT of bias.

HOTTER or COLDER usually refers to the current flow through the tube - and not the bias simply because it refers to current. So HOTTER means decreasing the bias, and COLDER means increasing the bias.

Does that make sense? If not, thank your local tube tone guru for making it more confusing than it really is. 

[EdBass]

OK, now I'll see if I can REALLY muddy the water...

One thing I think I need to point out.

In terms of tubes... bias is defined as the voltage difference between the cathode and the grid of a tube. Bias is not the current flow through the tube - current flow through the tube is the RESULT of the bias voltage applied to the grid.

Very key point. Not just in terms of tubes, the literal definition of the word "bias".

The voltage between the cathode and the grid is always NEGATIVE in polarity. It is measured with respect to the cathode - the cathode is normally grounded at ZERO volts, so the grid will be negative.

We'll just leave cathode biased amps for another thread, but for this discussion, why does a tube need to be biased? It has to do with the term used for tubes across the pond, "valves", named for Fleming's original oscillation valve. It's called a valve because current can only flow one way through it. This is why they make dandy rectifiers. But... sound signals are sine waves, like AC, which phase or oscillate through zero (reverse) themselves and flow backwards, which is something tubes won't do. Yet, we all know they do indeed pass, even amplify a sound signal. It's all about where this "zero" value is in relation to the phase peaks of the signal, if you bias the relative zero point above actual zero volts, the sine wave can phase (ebb and flow) while still maintaining constant positive current flow through the one way "oscillation valve" that a tube actually is.
Oh yeah, also try to forget that we all know that electrons flow from negative to positive, it will just make this entire discussion that much harder to comprehend if this is new to you. (see below)

A higher bias means going from a lower negative voltage to a higher negative bias voltage - e.g. from -55 volts to -60 volts. It's more of an absolute value direction than a direction on a number line.

The current through the tube has an inverse relationship to the bias voltage applied to the grid.

To increase current flow through the tube you have to reduce the bias  and to reduce current flow through the tube you have to increase the bias.

REMEMBER - bias is the voltage difference between the cathode and the grid of the tube. It is not the current flow through the tube.

However, you will read where people claim to have their tubes biased at 30ma (or whatever the number happens to be) of current. THIS IS NOT BIAS - this is the RESULT of bias.

Well, you can set tube bias by measuring tube current, or even by measuring plate dissipation, but it's a fact that neither of these values are actually the bias of the tube. I personally am a devotee of using a multimeter in the test hole and trusting the tube manufacturers specs.

HOTTER or COLDER usually refers to the current flow through the tube - and not the bias simply because it refers to current. So HOTTER means decreasing the bias, and COLDER means increasing the bias.

If that isn't what I said in my last post, that's what I meant!

Does that make sense? If not, thank your local tube tone guru for making it more confusing than it really is. 

Clear as mud, Joe!

[JoeArthur]

Ok Ed, I'll avoid cathode bias (aka self-bias). After all, it involves OHMS 

Figure me this one. While everyone "knows" they need to take their tube amp into an ex-spurt to have the bias adjusted periodically...

Why does no one recommend this for solid state amps?

Seriously -> solid state amps on the average have more adjustments in their power amp stage than the average tube amp. If something "adjustable" was necessary to be adjusted on a periodic basis wouldn't you think this same recommendation for solid state amps would be a prime source of income for amp techs?

Perhaps there is not enough garlic necklaces and crosses for all of the tube amp ONLY tech's out there to approach a solid state amp within 50 feet.

Ok, that was sarcasm. I admit it.

But seriously ask yourself - why does an adjustment on a tube amp have to be adjusted more... Yet a similar adjustment (or any adjustment) on a solid state amp is never even recommended to be checked much less adjusted?

"follow the money". 

[EdBass]

People do take solid state amps to techs, Joe.
When they break!
Seriously though, I think the reason is that solid state stuff is based in actual science and anyone with a solid education in electrical engineering can work on them, while everyboby knows that tube amps work by magic and require a Shaman to work on them properly.