Compubias Bias Tool

[CaseyJones]

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..

Maybe in Saint frickin' Petersburg but not in the good ol' U.S. of A. Not anymore. Libraries here are spotty at best especially when it comes to what could be considered obsolete technical manuals. It's pretty simple: Librarians here are aging girlie-girls. Our culture assigns balls to machinery and tits to home decor so when a librarian sees a book from McGraw-Hill or Western Electric printed in 1949 they assume it's obsolete at shitcan it without a second thought. Can't relate to it at all. Long gone. Sad but true.

Next trend, the dumbing down of Western society. We don't want our kids to sweat, we don't want them to struggle. Math skills should arrive as an epiphany, just an effortless miracle. Like athletic prowess only without the sweat. Little wonder that our kids walk around wif a head full o' dope and their noses in some video game.

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.

Ignorance is bliss. For those who choose the misguided path of knowledge there's always booze and drugs to return us to our natural state. We are born naked, ignorant and red-faced bawlin' our fool heads off. Lot to be said for it.

A simple answer like "bias for dissipation" is technical enough for the vast majority of would-be techs. Like anything else maybe one percent rise above the wannabes. Refer back to "ignorance is bliss". For every action there is an equal and opposite reaction, the reaction here is "knowledge is a curse". We can be freakin' rocket scientists but keep it to yerself. The Dee-dee-dees feel very threatened by intelligence. Hide yer light under a bushel lest ye be crucified, lest ye be burned at the stake.

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.

Yeah. So what? I like to bone hippy chicks. I'm not gonna get there in a white lab coat wif a pocket protector!

[UR12]

I think that this debate may go on for a while but I see two camps here. I think the compubias is a great “tool” for what it was intended for. It works great and it gives you information that can be used to set your bias in a range that won’t harm you tubes if you know how to use it. (If you are truly ignorant no amount of test equipment or even the compubias will help) When I see some pedals in the same price range as the Compubias I wouldn’t call it expensive. I guess that is a relative term. It is great that, unlike a laptop and a bench full of equipment, you can throw it in a small “Tool” box and have something to use to get info on an amp at say a venue or somewhere you can’t have all of that test equipment available. The second camp seems to prefer using a truckload of equipment or a laptop with homemade interface and a lot of scope probes (good probes aren’t cheap either) to emulate that bench full of equipment. Why not include a good tube characteristic curve tracer in that arsenal of test gear? That way you could plot the curve for the exact tube you are using instead of having to go with those inaccurate published values. Please forgive my ignorance, but I haven’t seen one of those for a laptop. I actually know one guy, who shall remain nameless, that tests his amps in a test chamber that simulates a 30,000ft environment. I guess some people want to believe that this is all rocket science.

My 2 cents………I have been using all of that test equipment mentioned for too many years to count to set the bias on amps using the crossover distortion method. This will get you a baseline, for at least the minimum bias needed for no crossover distortion. It assures that if you have good tubes you shouldn’t red plate those expensive jewels. After finding the point where the crossing distortion just disappears, you now have a “range” between the current drawn at your baseline bias voltage and the max current that the tube is capable of handling with out red plating (may be 70% / may not). This range I speak of will end up being a point on a linear portion of the tube’s characteristic curve for a given plate voltage. Setting the bias anywhere in that range will make the amp work but may change the way the amp sounds and feels to the player. This is shifting the operating point of the tube on it’s particular curve. If you know the bias voltage where the crossover distortion just disappears and the bias voltage that cause your tubes to red plate you can safely bias your amp with your ears at any bias voltage in between. (This will occur on a linear portion of the curve of most of the common type tubes we use today. The compubias will be able to give you the info you need to set your bias in this manner once you know your limits, either published or measured.

Unless you are designing an amp from scratch you are stuck with what ever plate voltage you have in the amp. The limit is set by the B+ winding on your power tranny and not something that you will be changing unless you’re into swapping trannys in between sets to get a different sound out of your amp. My point is that once an amp is built you are stuck, so to speak, with a set of parameters like plate voltage, OT impedance, etc, etc. These things don’t normally change and for a person to go out and buy a bunch of equipment just so they can set the bias on their JCM800 that they have owned for years and has had the same OT and plate voltage for years seems a little over kill IMHO.

If you are designing amps from scratch then you can have all of the test equipment known to man and all of the graphs at your disposal, find the perfect operating point for the exact tube you are using and still have an amp that sounds like crap. (Might make a nice sounding HiFi amp though) I think a lot of the guys here on this forum are guys who build a few amps and are not into designing amps from scratch. I think the ignorance level of this forum and our forum members would go down if posts like this included a lot more “how too” and a lot less look how much I know that you don’t.

[selloutrr]

thank you
I didn't mean to start a pissing match just wanted to give a product review.

the amplitrex at1000 has a software based curve tracing program if your interested

[skyboltone]

I think that this debate may go on for a while but <snip> I think the ignorance level of this forum and our forum members would go down if posts like this included a lot more “how too” and a lot less look how much I know that you don’t.

Thanks Dana, great post Anyway, I'm one of the poor ignorant bastards, but unfortunately I can't seem to build a direct clone of anything. Not so far anyway. I guess it's just the rebel in me. But I got some ideas for real departures from the two amps most often discussed here.

The point I tried to make was could somebody just explain what it the heck we're talking about here? I think you just did. I bias by the 70% rule as a starter then just move back and forth around that point until it sounds right. Pretty ignorant I know.

[UR12]

thank you
I didn't mean to start a pissing match just wanted to give a product review.

the amplitrex at1000 has a software based curve tracing program if your interested

Wow , Just checked out the at1000 and it's $2550. Not something I would think most hobbiests would have in their arsenal. My brother has a compubias and I have used it a couple of times. It is a great tool!

[Richie]

yep, i have one and use it quite often. Right or wrong, i like using it,and never had a problem when using it. Might also say, you need to read up on it, as it has a max plate voltage,along with a couple other things not to do,when your checking the amp.

[Ears]

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..

Maybe in Saint frickin' Petersburg but not in the good ol' U.S. of A. Not anymore. Libraries here are spotty at best especially when it comes to what could be considered obsolete technical manuals. It's pretty simple: Librarians here are aging girlie-girls. Our culture assigns balls to machinery and tits to home decor so when a librarian sees a book from McGraw-Hill or Western Electric printed in 1949 they assume it's obsolete at shitcan it without a second thought. Can't relate to it at all. Long gone. Sad but true.

Yet the books are still very easily available through online auction houses (local ebay equivs) 2nd hand bookshops etc. I've quickly built up a library of pre 1970's books over the past two years. Several titles by Terman, Radiotron D H , MIT, Happell Hesselberth, Reich, and several more.
Granted these aren't technical manuals but are engineering books, which I assume km6xz is referring to providing understanding. Coupled with modern undergrad university texts Sedra&Smith, Nilsson, Papoulis, Smith&Dorf and host of others the theory is mostly covered. Augment with Kuehnel, O'Connor etc. IMO the hardest information to come by is general design proceedures using real-life componentry as opposed to the theoretical.
Many of the classics are also available on the net in pdf.

[Mr "HiWatt" advocates notch method for biasing (see Duncan's Amp pages)]

[bnwitt]

Boy I hope there aren't any girlie girl librarians reading this thread.

[CaseyJones]

Boy I hope there aren't any girlie girl librarians reading this thread.

My favorite librarian is an aging New Waver but she'd be the first to admit... the quiet of the Public Library is stark contrast to the heavy bass and crisp drums of a modern mix.

[ryanburton]

I recently purchased one of these and i love it. it's fast, easy, and small.

[Jana]

Entertaining thread. I can see it coming to a theater soon, "Bias Tap," a spin-off of the "Spinal Tap" movies but focusing on amp builders.


But seriously, or not, simply measuring the voltage drop across the OT primary works for me. If I had an amp with an external bias adjustment, the bias tool would be a handy thing to have. But, if I'm going to open the amp up to get to the bias adjustment all I need is a DVM. The amp doesn't even need the 1 ohm resistors on the cathodes. Pretty simple for a simpleton like me.

[Andy Le Blanc]

I built a socket adapter with a couple old tube bases and a couple PCB mount
sockets. I internally wired the thing and ran the wiring to box where I could
plug in my DMM's. A 1ohm resistor in the box competed the circuit. The goal
was to measure plate voltage and cathode current relatively safely and at the
same time. The point is to get the data you need to make the math work in
a way that is consistent. I don't trust "products", you really do need to do
your homework, if you don't these products can be misleading. My next
goal for my test socket is to create a test criteria to measure zero signal
conditions and then max signal conditions. The data from such a test will give
you more than you would ever want to know about the amp.

[Tubetwang]

I had purchased a bias tool made by one of Aspen Pittman's partner way back when.

Sold it when i lost interest...

When Mr Meyer's recommended this one, i jumped and am a happy camper.

[Mostaks68]

I think that this debate may go on for a while but I see two camps here. I think the compubias is a great “tool” for what it was intended for. It works great and it gives you information that can be used to set your bias in a range that won’t harm you tubes if you know how to use it. (If you are truly ignorant no amount of test equipment or even the compubias will help) When I see some pedals in the same price range as the Compubias I wouldn’t call it expensive. I guess that is a relative term. It is great that, unlike a laptop and a bench full of equipment, you can throw it in a small “Tool” box and have something to use to get info on an amp at say a venue or somewhere you can’t have all of that test equipment available. The second camp seems to prefer using a truckload of equipment or a laptop with homemade interface and a lot of scope probes (good probes aren’t cheap either) to emulate that bench full of equipment. Why not include a good tube characteristic curve tracer in that arsenal of test gear? That way you could plot the curve for the exact tube you are using instead of having to go with those inaccurate published values. Please forgive my ignorance, but I haven’t seen one of those for a laptop. I actually know one guy, who shall remain nameless, that tests his amps in a test chamber that simulates a 30,000ft environment. I guess some people want to believe that this is all rocket science.

My 2 cents………I have been using all of that test equipment mentioned for too many years to count to set the bias on amps using the crossover distortion method. This will get you a baseline, for at least the minimum bias needed for no crossover distortion. It assures that if you have good tubes you shouldn’t red plate those expensive jewels. After finding the point where the crossing distortion just disappears, you now have a “range” between the current drawn at your baseline bias voltage and the max current that the tube is capable of handling with out red plating (may be 70% / may not). This range I speak of will end up being a point on a linear portion of the tube’s characteristic curve for a given plate voltage. Setting the bias anywhere in that range will make the amp work but may change the way the amp sounds and feels to the player. This is shifting the operating point of the tube on it’s particular curve. If you know the bias voltage where the crossover distortion just disappears and the bias voltage that cause your tubes to red plate you can safely bias your amp with your ears at any bias voltage in between. (This will occur on a linear portion of the curve of most of the common type tubes we use today. The compubias will be able to give you the info you need to set your bias in this manner once you know your limits, either published or measured. That said, for those who prefer more lightweight or AI-assisted tools, I’ve seen people use solutions like https://overchat.ai/chat/best-free-ai-chat to double-check theory or lookup reference curves while in the workshop or studio. It's definitely not a hardware replacement, but for quick info or clarification on tube data or distortion theory, it's surprisingly handy.

Unless you are designing an amp from scratch you are stuck with what ever plate voltage you have in the amp. The limit is set by the B+ winding on your power tranny and not something that you will be changing unless you’re into swapping trannys in between sets to get a different sound out of your amp. My point is that once an amp is built you are stuck, so to speak, with a set of parameters like plate voltage, OT impedance, etc, etc. These things don’t normally change and for a person to go out and buy a bunch of equipment just so they can set the bias on their JCM800 that they have owned for years and has had the same OT and plate voltage for years seems a little over kill IMHO.

If you are designing amps from scratch then you can have all of the test equipment known to man and all of the graphs at your disposal, find the perfect operating point for the exact tube you are using and still have an amp that sounds like crap. (Might make a nice sounding HiFi amp though) I think a lot of the guys here on this forum are guys who build a few amps and are not into designing amps from scratch. I think the ignorance level of this forum and our forum members would go down if posts like this included a lot more “how too” and a lot less look how much I know that you don’t.

That’s a well-balanced take, and I think you nailed the practical reality a lot of us live in. There really are two camps — those who want absolute precision with lab-style setups, and those who are looking for a reliable, efficient tool that fits in a gig bag and gets the job done in the field.

CompuBias (and tools like it) seem to serve the second group very well. You’re right that once you know your limits — red plating threshold, crossover distortion point, plate voltage — you’re working within a safe and functional range. Having a quick-reference device that can give you that info without hauling a bench setup is incredibly valuable for working musicians and techs alike.

Also completely agree with your point about diminishing returns on gear when you’re not designing amps from scratch. If your amp has had the same plate voltage and OT for a decade, you probably don’t need a scope array just to bias it safely. It’s great to geek out and push the envelope, but that doesn’t mean simpler methods are any less valid.

And yes — posts that focus more on how to actually do things, with clear explanations like yours, are way more helpful than flexing technical jargon. Thanks for keeping it grounded.