Compubias Bias Tool

[selloutrr]

I used the Compubias for the first time yesterday on a 1966 Fender Bandmaster head. It was incredible how easy it was to use. The amp was switching to a pair of Svetlana 6L6GC with a butterfly logo in replace of some older winged C's that had aged rather hard after one had started to leak and glow a sexy blue hazz causing loss of volume mostly noticable in the low power chording / palm muting.. anyway.
I threw the new tubes on the At1000 and for a quick matching test they where with in .6GM of each other. I plugged them into the octal sockets of the compubias and flipped the on switch letting the tubes warm up for a couple minutes after seating them in the amplifier. before flipping on the standby with a flat head screwdriver already in the groove of the bias pot. The compubias has a handy wattage reading for the common tubes such as 6L6 8-13watts and the 6L6GC 15-21watts ( I could be off be a bit it's not infront of me ) The coolest feature is the display being able to monitor both tubes at the same time. I don't how many times i've used a biasrite in the field and set a tube only to flip the switch and find the other is running to hot or cold leaving the amp lithargic and harsh. finding the sweet spot with the compubias was so easy i felt like i was cheating. I started with a conservative 17.8 watts on the stongest tube. I plugged in a old les paul that i was familiar with the tone combination and tried out the amp it was very alive and responsive with the new tubes, huge volume increase and lots of head room but still not voiced for the clean definition in the mids... not a problem! i turned the volume down and let the tubes settle in and upped the wattage by .6W. Played and repeated several times, mostly to let the tubes burn down and settle my goal was to get the most out of the tubes but i didn't want to go for broke right away being my first time using the tool i didn't want to risk glowing the plates. The entire bias took maybe 10 minutes to set and another hour of rocking it out cause it sounded so good! with a recheck to see how it held I ended up stopping at 20.2 watts on the strongest tube which left the amp with a very open almost MXR Dist+ distortion when cranking the preamp and holding back the master volume ( mod'd ) very smooth top end, controlled feedback at a useable volume and the clarity and punch in the mids that a late 70's rock band would die for. I can't say enough great things about the compubias. As a first time user I had my doubts it was any better then all the other bias tools but I was hopeful! After reading the manul and trying it out I am nothing but pleased and proud to own them. I put the amp on a scope w/ 1K sine wave just to see how close it was voiced the notch was as smooth as if i had scoped it. the plates were nice and cold and the tone is stunning and alive ( nice tubes )! If you want the best the compubias is the one to use! anyone want to buy my Weber BiasRite I'll never use it again!

Thank you for turning me on to this great tool!

[Allynmey]

The CompuBias is the best Biasing tool I've ever used!

[km6xz]

Maybe I am missing something but why on earth would an expensive biasing tool be needed?

If you have a scope, DMM, low distortion oscillator and dummy load you can do better in less time. Better, add a spectrum analyzer and be sure. A computer sound card is all that is needed and free spectrum display software to have both time and frequency domain displays giving much more informative indication of the balance, distortion and harmonic content. Cathode current is only part of the story when biasing an amp for both operating point and sound.

[skyboltone]

Maybe I am missing something but why on earth would an expensive biasing tool be needed?

If you have a scope, DMM, low distortion oscillator and dummy load you can do better in less time. Better, add a spectrum analyzer and be sure. A computer sound card is all that is needed and free spectrum display software to have both time and frequency domain displays giving much more informative indication of the balance, distortion and harmonic content. Cathode current is only part of the story when biasing an amp for both operating point and sound.

Why not write the procedure up and post it? Sounds interesting.

[strato17]

Maybe I am missing something but why on earth would an expensive biasing tool be needed?

If you have a scope, DMM, low distortion oscillator and dummy load you can do better in less time. Better, add a spectrum analyzer and be sure. A computer sound card is all that is needed and free spectrum display software to have both time and frequency domain displays giving much more informative indication of the balance, distortion and harmonic content. Cathode current is only part of the story when biasing an amp for both operating point and sound.

This is considered a very innacurate(and possibly harmful) way to set your amp bias...

You can read more info here...
http://www.aikenamps.com/CrossoverNotchBiasing.html

[km6xz]

The article assumes only time domain waveform information is used, which it is not, by anyone who knows anything about amplifier operational characteristics. I am not sure what he is trying to sell but his analogy would be close to "avoid driving a car using a map because it doesn't tell you enough" when any sane person would combine the map with visual observations of the current conditions and traffic in addition to the map.

An amp is a simple device that as a vast array of interacting characteristics and determining the operational state of an amp requires many views into the dynamics and static conditions. The more elements that can be tracked at once, the better the understanding of its performance. Setting bias without regard to dynamic performance is like driving that car only looking at the idling tachometer, when the other gauges, the windshield and sounds tell a more complete story.
Achieving not only a safe operting point for power tubes, but a desirable sound is a balancing act that requires compromises that can only be best set by knowing as much about what the amp is doing as possible. I feel blind dealing with an amp with only limited info, say, from a DMM alone, despite knowing a hell of a lot about design and the resultant sounds.

My set up includes more than most hobbiests would want to be dealing with but the results give me confidence in having few doubts when finished.
I monitor a lot of different parameters at the same time, for not only steady state but also dynamics in real time for amplitude, time, current, distortion, time domain measurements, frequency domain measurement, AC mains current, True RMS and Peak reading signal values input and output, arbitrary complex signals( a set of programmed waveforms in an HP function generator that mimics diferent guitar notes with full harmonic content, and more....all at the same time.
To me, that is like putting on my reading glasses after struggling and squinting to read fine print without them.

Anyone asuming from reading that article that biasing using more than cathode current is harmful is hurting themselves by illogical acceptance of an article that only addresses a subset of an issue. Do you really want to drive your car by tachometer alone? I prefer to have a clean windshield, ears, and a full array of instruments. And I bet I get to the destination faster, safer and cheaper than you and your tachometer.

[skyboltone]

The article assumes <snip> I prefer to have a clean windshield, ears, and a full array of instruments.

It's a bit like Dumble's plate resistor balancing pot isn't it. You put the thing in, DC balance it, then say to heck with it and set the dang thing by ear.

Like I said, write it up. I'm interested. I don't have a signal generator that mimics a guitar note but I got a guitar that does My signal generator is the old HP 200C, a Tek 453B, bunches of DMMs and a HP VTVM. I suppose I could add the spectrum analyser to my second laptop.

[km6xz]

I suppose I should write about this topic. Reading the article gives the impression that the power disappation of a tube in a circuit is all that is needed to balance the compromises in different operating environments of tubes. He was using different plate voltages to show his point but he ignores that the operational environment of a properly designed amp with 550v on plates is a different animal than a 350 B+ and will not have the same transformer turns ratio, same drive etc. He was railing about phantoms. Lower the plate voltage of a Kelly amp and tell anyone that setting the idling disappation alone is going to result in similar transfer function as the same amp running 500+V B+ and they will laugh. He is arguing against straw men. The whole circuit design would need to be revisited to get a 6V6 amp to sound the same with different potentials.


Either he was selling something that relied on discrediting something not advocated by anyone in the way he describes, or he really does not understand the nature of nonlinean electronic circuits.
Either way I have little respect for his attempt to deceive his readers and possible customers.

Balancing: You make a valuable point. Why do guitar amps seldom have a balance control? Most of the bizzarre bias and asymetrical waveform issues are directly traceable to imbalance of drive/gain between the power tubes. The phase inverters used in most amps are fine with steady state balance, by adjusting the load resistor of the follower side....for one operating point., Stuff it with a wide dynamic range signal, near clipping and there is real imbalance in drive to the PA section which no doubt has its own dynamic balance problem. Using a full differential phase splitter would resolve the drive issue. The photos of my 3CX300 amp I posted on the Jim Kelly amp post shows a bit more complex but much more predicatable and balanced driver that can be used in almost any size amp.
A driver stage is not a good place to try to generate harmonics for tone, let the gain stages and the output do that.

[Ears]

The article assumes <snip> I prefer to have a clean windshield, ears, and a full array of instruments.

It's a bit like Dumble's plate resistor balancing pot isn't it. You put the thing in, DC balance it, then say to heck with it and set the dang thing by ear.

Like I said, write it up. I'm interested. I don't have a signal generator that mimics a guitar note but I got a guitar that does My signal generator is the old HP 200C, a Tek 453B, bunches of DMMs and a HP VTVM. I suppose I could add the spectrum analyser to my second laptop.

Dan speaks for me too!. From your description I gather your proceedure/s would pretty iterative, but I'm intetrested to know just how you balance the spectrum analyser for desired aural targets and just what you mean by monitoring mains current etc. It would be great if you might find the time to expand on it. Perhaps start a new thread on bias proceedures etc.

[Ears]

<snip>
A driver stage is not a good place to try to generate harmonics for tone, let the gain stages and the output do that.

Your posts are generating lots of interesting (to me )questions. Here is one, a little off topic. See this thread

https://tubeamparchive.com/viewtopic.ph ... ght=patent

here is a patent doing just that. I was considering using this idea to generate adjustable inbalance in a differential splitter. So why exactly don't you think this is a good place?

[selloutrr]

not sayings the only way to bias but if u do off site repair or want to check your bias at a show its a great tool and by far the best bias tool i've encountered.
I have the signal generator and dedicated counter the scope the simpson 260 the fluke DMM a distortion meter smarrt live multiple dummy loads and 12 more feet of lab test equipment. all calalibrated and perfect. so yes a bias tool is a luxery but as far as luxery goes the compubias is very useful easy to use and gets a professional result quickly. when i'm not on my phone i'll write up a bias procedure and take pics to step u threw the process

[CaseyJones]

Either he was selling something that relied on discrediting something not advocated by anyone in the way he describes, or he really does not understand the nature of nonlinean electronic circuits.
Either way I have little respect for his attempt to deceive his readers and possible customers.

Aiken posted a vast resource of information simply to make that information available on the 'net. His intent was to educate a new generation of amp techs. Much of the information available on Aiken's site is still beyond the grasp of many would-be techs.

Mr. Aiken deserves respect for the time and effort he has put into making information available to the public. Aiken's amps sell themselves, no hype and no deception required.

Driving by tachometer... wish I had a tachometer. I close my eyes and listen to the engine stutter against the rev limiter. I assume those dull thuds are pedestrians. Either way the ECM reads crank degrees (compared to air flow and temperature) and fires injectors and ignition right where the map tells it to. I'm pretty sure the map is correct, the oxygen sensor catches it and lights the "check engine" light if it isn't.

[km6xz]

Every library in the country is full of unread books that would be of great value to aspiring techs so there is no lack of availability of information about electronic theory and engineering. What apparently is desired is a quick answer even if it is just opinion, instead of an understanding, or else those books would have been read, and the math worked out. In the days of the internet, opinion is valued as much or more than scholarship.

As an attempt to short cut the need for understanding fundamentals, such an article would distort the facts for anyone leaning on the article as the last word. The poster who supplied the link obviously believes that it was, without questioning the statements made, nor apparently understanding them. Out of hand, he claimed biasing using instruments has been discredited and is harmful. No, ignorance is harmful, not instruments.

He might make wonderful amps but what he writes is bogus in the way he inaccurately frames the issue, and uses it to disparage "so-called gurus" by restating a case they never made. That is trying to sell something that the facts do not support, and we have seen enough of that in politics so the public is so used to it that they accept it as normal discourse. Apparently more than a few bought into it without questioning. That is salesmanship in itself and an objective view must conclude that was the intent, to sell something; build up a product image or establish a cult following, or something similar. You bought into it and probably others have as well.

It appears that amp discussions have become the next high-end hi-fi or New-Ageism, full of mystery, magic, spirits and cults rather than engineering and reproducible, predictive reduction of theory to practice.

[strato17]

The poster who supplied the link obviously believes that it was, without questioning the statements made, nor apparently understanding them. Out of hand, he claimed biasing using instruments has been discredited and is harmful. No, ignorance is harmful, not instruments.

Wow. A bit harsh. Well... if your going to brand me as an ignorant hack, atleast have the courtesy to quote something I actually stated.

I never said biasing with instruments was harmful. I said that the particular method you described is "possibly harmful," and I mean harmful to the amp and tubes. In my opinion, I believe that setting a bias to the point where the crossover notch dissapears can sometimes lead to a bias that is either too cold or too hot. I realize that we do not share the same opinion on this matter, but lets agree to disagree and move on.

I should have expressed my original opinion in a more objective manner. It was wrong for me to express my opinion in the manner I did. I apologize.

Maybe I am missing something but why on earth would an expensive biasing tool be needed?

If you have a scope, DMM, low distortion oscillator and dummy load you can do better in less time.

If one did not own all of these things, a "expensive" bias tool would indeed be cheaper than purchasing all of these things. Also, in my opinion, I cant imagine that the crossover notch bias method would be any quicker than using a bias tool.

My set up includes more than most hobbiests would want to be dealing with...

Exactly. So, is a compubias an acceptable tool, then?

I am not trying to belittle you by any means. It is very apparent that you have more knowledge, background, and experience than I will probably ever have. But please dont brand me as ignorant if we disagree on something.

Once again, I do apologize for the fowardness of my first post. I should have stated it as an opinion I hold, and not as proven fact.

[km6xz]

uot;]

The poster who supplied the link obviously believes that it was, without questioning the statements made, nor apparently understanding them. Out of hand, he claimed biasing using instruments has been discredited and is harmful. No, ignorance is harmful, not instruments.

Wow. A bit harsh. Well... if your going to brand me as an ignorant hack, atleast have the courtesy to quote something I actually stated.

I never said biasing with instruments was harmful. I said that the particular method you described is "possibly harmful," and I mean harmful to the amp and tubes. In my opinion, I believe that setting a bias to the point where the crossover notch dissapears can sometimes lead to a bias that is either too cold or too hot. I realize that we do not share the same opinion on this matter, but lets agree to disagree and move on.

I should have expressed my original opinion in a more objective manner. It was wrong for me to express my opinion in the manner I did. I apologize.

No where did I say you were an ignorant hack, yet in fact, if someone does not know something, it is an appropriate term, and says nothing about intelligence, linage, or character. It says someone does not know. I said ignorance is harmful and the rest of the comment supports that ignorance of actions that are taking place, and the methods proposed IS harmful. You have no idea of what method using a scope and other instruments was being proposed because it is not described in either the article or by me. your reading of the article left an impression, a wrong impression, of what other techs who use test instruments use them for. The article created a straw man arguement against something that no one advocates, and goes about showing irrelevent photos with extreme examples. After reading that you say

"This is considered a very innacurate(and possibly harmful) way to set your amp bias... "

Oh, really? What way do techs use test instruments, surely not the methods attributed to them by the author. Besides, even if it is not advocated, setting by cross over notch alone without any other measurements can be more effective in terms of sound, and performance than simply setting based on cathode current. Where did I describe the methods you assume that are being used? Not in this or any thread on forum.

Let me asume your conclussions that says cross-over notch observations/adjustment is harmful.

Setting cathode current is effective IF you know the:
Tube curves
tube type
heater current and voltage
load impedance
DC resistance of the transformer
dc degeneration level
screen current, supply regulation and impedance
plate voltage and supply impedance
And several other characteristics.

The "70% rule" is only for amps that you know these characteristics, besides it is not ever a very effective rule of thumb and is being stretched far beyond its original intended scope. It applies toratings that have nothing to due with high duty cycle wave forms.
First off, you have no idea of the curves for the tubes, because none of us know precisely what the tube was supposed to be. There are no 6L6s, 6V6s or EL34s made today, there is not enough volume to set up production lines for such tubes. Exsisting lines and similar tubes built for local markets in China, Russia and FSU countries are producing tubes but none those sold now were intended to be the common US tubes from the 40s on. Western marketers found similar performing tubes and ordered them with octal bases and have them marked in familar identification labels. That is fine, those countries had very interesting technology and some of those tubes are better than what was produced back when manufacturers could produce tubes in the west. As long as we know what the characteristics, we can build anything we want from them. But we can't rely on spec sheets for those old tubes. They do not sound, test or look like the old tubes, not necessarily worse but different. Most designers work with a limited number of sources so they can build up their own specs based on testing the tubes and stick with what they know. I do that, most who want to know what their amp is doing do it also.

But setting for a given cathode current based on published characteristic curves for tubes that are not around anymore, even NOS are not often like new production back when the tubes where produced for a number of reasons, is leaving way too much to random chance. A dart board choice of cathode current is just as likely or unlikely to represent "accurately" the optimum bias point. One reason that those tubes are even available is that they were "expired" and surplused out of the military supply systems. Tubes do drift in characteristics over time. They are probably still good, but not the same.

What is accuracy any way? With so many variables not known, it is a strange concept, not too differnt than believing in the Unicorns. "But if they did exist...they would be real" is the same as "set it at this point:24.245ma....as if the tubes were what I imagine them to be".

There are other concepts of accuracy like in degree of conformation to expectation, such as getting it to sound the way the artist and producer want for a particular track, and longevity be damned.
Is a Kelly amp accurate just because it sounds good, or inaccurate due to knowing the tubes are going to self distruct any moment? The answers, the correct answers are different for different players and owners.

Setting by current alone, as with the bias computers, ignores the interests of sound, the circuit topography, the operating environment of the tube, longevity, reliability, and the list goes on.

On a production line it makes sense to use such as device, but a simple DMM would do as well, because a setting fitting the average of a large lot of tubes, and circuits of the same components can be reasonably "accurate". But how was the bias spec deternined by the engineers before they turned it over the production workers? They, as any decent tech would, weigh the many variables, while monitoring all the characteristics they can, to determine what works best. They use all the tools they can to get a picture of the total operating environment of those tubes.
A hobbiest or owner of an amp that has had tubes changed or speakers reconed or whatever does not have that option unless they take it to the tech who does the same thing that the engineers did, get a picture of characteristics, and use experience, along with customer priorities to come up with a compromise in sound, longevity and reliability.

Setting to some specific value might be confidence inspiring, that it was done "accurately" but it IS ignorance beleiving it is based on anything other than dart board relevence.

Maybe I am missing something but why on earth would an expensive biasing tool be needed?

If you have a scope, DMM, low distortion oscillator and dummy load you can do better in less time.

If one did not own all of these things, a "expensive" bias tool would indeed be cheaper than purchasing all of these things. Also, in my opinion, I cant imagine that the crossover notch bias method would be any quicker than using a bias tool.

Having decent equipment is not expensive, and they are needed to design and prototype an amplifier with any degree of confidence that it will work as expected. If the equipment is set up, and the information is presented to the tech in ways he can grasp what the amp is doing it is rather fast and probably needs no tweeking for sound characteristics. A computer with built-in sound card can, with a home made interface, can perform an amazing number of sophisticated measurements include spectrum displays, amplitude, very low distortion wave generation, phase measurements, power output, noise figure, tone burst measurements etc, all except DC measurments which can be handled by a $14 DMM. All these displays are concurrent so it is particularly revealing.

My laptop can analysize acoustics, delay, act as a precision wave analysizer, spectrum analyizer, low distortion multi-channel signal generator and AC/DC current and voltage measurements with the help of a home made interface connected to one of the USB ports. Total cost; <$50

But from comfort and tradition, I still prefer a bench full of lab type gear for that, that is my comfort zone and results in quick effective diagnostics. Just like when recording, I am best with a large format analog control consule and tape while others are more effective with ProTools or similar DAW.

My set up includes more than most hobbiests would want to be dealing with...

Exactly. So, is a compubias an acceptable tool, then?

I am not trying to belittle you by any means. It is very apparent that you have more knowledge, background, and experience than I will probably ever have. But please dont brand me as ignorant if we disagree on something.

Once again, I do apologize for the fowardness of my first post. I should have stated it as an opinion I hold, and not as proven fact.

The post mentions scope, generator DMM, and loads but I went on to describe a different system that could also characterize a circuit under test, that is a big difference and a rereading would reveal that. A hobbyist needs some basic equpment to have any hope of troubleshooting or tweaking a new project without resorting to shotgunning or blindly fiddling around. Sort of like to be a hobbyist mechanic, the bare minimum is a good set of handtools and basic diagnostic tools. Anything less and he is an "appliance operator" not a hobbyist.
It is a useful tool? It would be if all the variables were known that would indicate a predicted conduction rate for those particular tubes and operating environment. If not, no, it is a waste of money, and setting by sound alone would at least result in a pleasing result.