Can you bias an amp yourself and how?

[kars0747]

What do you need to bias your amp. Can you do it yourself or is it better to send it to the amp doctor?

[bclaire]

The better part of wisdom is, if you have to ask you need to send it to an amp tech.

Biasing an amp is not for a DIY-er. You really need electrical training to not kill yourself. The filter caps in amps store electricity CHEMICALLY. This means that even if you discharge them, the charge will return over time. If you really want to learn, ask your amp tech where he/she learned - and maybe take an electronics class or offer your tech some $$$ to teach you. You'll need some specialty equipment to do it right but the bottom line is, it's worth the money to have it done properly and safely.

[pedecamp]

What do you need to bias your amp. Can you do it yourself or is it better to send it to the amp doctor?

Its really easy to bias your amp, with that said it can be dangerous if you dont know what youre doing. Even when an amp is powered off and unplugged it holds a lethal amount of electricity in the circuit that can kill you if you touch something you shouldnt. There's nothing wrong with wanting to learn how to bias your own amp but they dont allow that conversation at the Orange forum. Theres lots of videos on electrical safety and biasing on youtube if you really want to learn.

Start with this video right here: viewtopic.php?f=3&t=53016" onclick="window.open(this.href);return false;

[fiveightandten]

I wrote this a long time ago (15 years or so), but it may help:

Biasing

DISCLAIMER: YOU'VE READ IT A HUNDRED TIMES, READ IT AGAIN...TUBE AMPS CONTAIN LETHAL VOLTAGES, EVEN AFTER THEY'RE UNPLUGGED. THIS ARTICLE IS MEANT TO EDUCATE YOU ABOUT THE PROCESS OF BIASING, SO THAT YOU (A COMPETANT INDIVIDUAL) CAN LEARN ENOUGH TO PERFORM IT. IF YOU ARE UNSURE ABOUT ANY PART OF THE PROCESS, IF YOU DON'T KNOW EXACTLY WHAT YOU'RE DOING, WHY, HOW, AND HOW TO DO IT SAFELY, YOU HAVE NO BUISINESS INSIDE AN AMP. IT'S NOT WORTH THE $30 TO GET KILLED AND/OR RUIN YOUR AMP. THAT BEING SAID...READ ON...

What is biasing?

"Biasing" an amp is the act of setting the idle current that flows through the power tubes. The tubes need to be running in a certain part of their operating range in order to amplify audio signal, they also need to be set in a more specific part of their operating range to perform this task well and do it reliably.

Do I need to bias my amp?

Maybe, maybe not. There are 2 main types of amps, as far as bias is concerned. One of those types can be broken into 2 subdivisions...

Type 1- Cathode biased amps: These amps are also called 'self-biasing' amps. They use a resistor connecting the cathode of the power tube to ground, which controls the idle current through the tube. Many EL-84 amps like the Vox AC-30 or Orange AD-30 are cathode biased...not to be confused with Class A, that's a whole other story! So how do you tell if your amp is cathode biased? The cathode of any octal based amp (EL-34's, 6L6's, 6550's, 6V6's, KT-88's, 7027's...) is on PIN 8. So if you see a big resistor between pin 8 and ground, it's most likely the cathode resistor, meaning the amp is cathode biased. Though not all amps with cathode resistors are cathode biased. It's rare to find an exception to that rule though. But for more info, read on. If you've been able to determine your amp is cathode biased, you don't need this info...go read a book, listen to some music, or play your guitar.

Type 2- Fixed biased amps: Any amp that does not have a cathode resistor on the power tube(s) is a fixed biased amp. These are amps that set the bias of the tube by injecting negative voltage into the control grid(s) of the power tube(s). They can be subdivided into 2 catagories; Adjustable, and Non-adjustable. Adjustable fixed biased amps have a small potentiometer (aka 'trim pot') in the amp that allows you to adjust the negative voltage flowing to the control grids. This, in turn, sets the bias...more on that later. Examples would be Marshall JCM800, Orange OR-120, Fender Twin, ect. NON-Adjustable fixed biased amps still use negative voltage to set the bias of the tubes. The difference though is that you can't adjust it (easily). Non adjustable fixed biased amps usually have a high amount of neg voltage coming into the grids, so they rarely run too hot. Examples would be Mesa Dual Rectifier, Peavey 5150, Hiwatt DR103 ect...2 points about these types of amps. First, you CAN adjust the neg voltage. This is done by changing the value of the resistor in the bias circuit that is responsible for the amount of neg voltage. Not something I recommend unless you know what you're doing. Secondly, you can modify any non-adjustable amp to be adjustable...just put a trim pot in the amp. Again, not something i'd recommend unless you know what you're doing. But these things can be done. If you have a non-adjustable fixed biased amp you can still read on, as this info will give you an understanding of how to check the bias to see if you need to go through measures to adjust it.

So how exactly is biasing done?

There are a few methods of biasing. The one we will be covering here is the cathode resistor method. There are other methods such as the output transformer shunt method or using an O-scope. But this is the most user friendly, and is not terribly difficult to understand.

Read on...but don't do anything inside the amp yet! This is the part where you understand what you are going to do and why. Don't skip it, trust me. It's more important to understand what you're doing and how not to get killed than to get the job done quicker. You'll waste more time later on reading through this again because you didn't the first time

What you will do is adjust the negative voltage flowing into the power tubes. Understand that the bias is a consequence of the neg voltage, and that the neg voltage itself is not the bias. The neg voltage means very little without other readings that you will be taking. Neg voltage is fed into PIN 5 of any octal based amp (EL-34's, 6L6's, 6550's, 6V6's, KT-88's, 7027's...). There are usually control grid resistors on these pins (common values may be 2.2K, 5.6K...) Any neg voltage measurement you take will be directly on the pin, not on the other side of the resistor. The MORE NEGATIVE the voltage (meaning farther away from 0), the colder the bias will be running.

You will also measure the plate voltage (aka B+) coming into the power tubes. The plate is PIN 3 (the PLATE) on any octal based amp (EL-34's, 6L6's, 6550's, 6V6's, KT-88's, 7027's...). And the last thing you will be measuring is the actual current draw through the cathode(s) of the power tube(s). The cathode is PIN 8 on any octal based amp (EL-34's, 6L6's, 6550's, 6V6's, KT-88's, 7027's...). PIN 1 will be tied to PIN 8 very often. PIN 1 is the supressor grid and is connected to the cathode for reasons that are beyond the scope of this article. NOTE: If you have an amp that runs 6L6's like a Fender or some Peaveys, PIN 1 will NOT be tied to PIN 8. Leave it that way, or you'll have a big (and expensive) surprise with lots of smoke.

So we know that negative voltage doesn't mean a whole lot by itself. The plate voltage also doesn't mean a whole lot by itself either. The current draw means more than the other 2, but again...it's not what we're looking for. The best way to set bias is by calculating something called "Static plate dissipation wattage". This is a number that takes both the plate voltage and the current draw into account, and is really the best way of getting an accurate understanding as to how your tubes are running.

Static Plate dissipation wattage is nothing more than the PLATE VOLTAGE multiplied by the CURRENT DRAW (in amps).

Again... STATIC PLATE DISSIPATION WATTAGE = PLATE VOLTAGE x CURRENT DRAW

So how do we get the plate voltage? Easy, we measure it directly from PIN 3 on any octal based amp (EL-34's, 6L6's, 6550's, 6V6's, KT-88's, 7027's...)...more on that in the section on the actual procedure.

How do we get the current draw? That's a little more complicated...but it is the MOST IMPORTANT part of all this information. If you read anything thouroughly, make it be this section, as this is the whole basis of what you're doing...

So we need to measure the current draw. How do we do that? Well, it turns out we have a clever way of making it as easy as possible. A widely used law of electricity called 'Ohm's law' shows us that CURRENT, VOLTAGE, and RESISTANCE are all intertwined and proportional (or inversly proportional, depending on what you're doing) with one another.

Ohm's Law:
V = I x R

V= VOLTAGE (measured in volts)
I = CURRENT (measured in amps)
R= RESISTANCE (measured in ohms)

VOLTAGE = CURRENT x RESISTANCE

So, what's the easiest way to make this silly math equation go away so we don't have to deal with it? Easy...remember your simple math from elementry school? Anything times 1 is what? Itself. Anything multiplied by the number 1, is itself. So...let's get crazy...what if we made the RESISTANCE in the equation equal to 1? Then that would make the CURRENT equal to the VOLTAGE? Cool...we don't even need Ohm or his stupid law anymore. So why did I even tell you about it? Well, now you know why we use 1 ohm resistors to bias an amp. If you want to understand things a little better, here's something to think about...this trick only works if the resistor is 1 ohm, right? Well, is it going to be EXACTLY 1.000000 ohms? No, it may be 0.99 ohms or 1.1 ohms. So there's an inherent inaccuracy with this method. Nothing to loose sleep over though.

So we measure the CURRENT draw across a 1 ohm resistor to make the math easy and make ohm's law go away. More on this in the procedure section...but right now you're wondering, but what about if I have a bias probe? Why am I learning all this math crap? Well, there's a drawback with using 1 ohm resistors. You have to actually solder them in the amp, physically. You have to buy 1 ohm resistors, disconnect the cathode connections, solder the 1 ohms in the amp, take your readings, and remove them when you're done. What a pain in the ***. Enter...bias probes.

What if we could still have a 1 ohm resistor off the cathode to read the current draw without soldering it in the amp. Can you do that? Sure...that's what a bias probe does. It's just a tube base and a tube socket connected together. Your tube plugs into the base, and the socket plugs into the amp. All the connections are made inside the bias probe. PIN 1 goes to PIN 1, PIN 2 goes to PIN 2, PIN 3 goes to PIN 3...and so on. But NOT PIN 8. PIN 8 has a 1 ohm resistor on it. The resistor is between PIN 8 of the socket (and the tube, consequently) and PIN 8 of the base (and the amp consequently). So what you have done is put a 1 ohm in there, without even touching the amp. Then all we do is just take 2 wires, connect one on each side of the little resistor in there, and make it all nice so you can just plug it into your multimeter and take your readings without even knowing the magic going on behind the scenes, in that little bias probe.

Something to note about using a bias probe vs. soldering 1 ohms in the amp. Using the probe is ALWAYS easier. Get a probe so you don't have to bother with soldering in 1-ohms. Also...A lot of new amps have the power tube sockets mounted directly on the board...you can't solder in 1 ohms even if you wanted to. Again, unless you REALLY want to be old fashioned, get a bias probe, or 2 (makes the job even easier)...it'll make things much more simple.

Let's get down to business...

So what do I need to measure/adjust the bias?
-Your new tubes
-A multimeter
-A bias probe OR some 1-ohm resistors
-A small screwdriver with an INSULATED HANDLE to adjust the trim pot
-A calculator to do the math easily and quickly
-An oven mitt or winter glove for handling hot tubes

The procedure

Again...DISCLAIMER, IF YOU AREN'T 100% SURE OF WHAT YOU'RE DOING, AND COMFORTABLE AROUND HIGH VOLTAGES DON'T ATTEMPT ANY OF THIS.

First thing's first. Get the amp in a good working position. You'll mostly likely have to remove it from the headsleeve...which i'm not telling you how to do. If you can't figure that out, you have no buiness trying any of this. Prop the chassis up (upside down, unless you have external bias points and adjustments) on books or something, high enough for you to be able to plug the bias probe in the tube(s) without it hitting the table. Before you even plug it in, make sure it's not going anywhere if you rest your hand on the chassis! You'll be pressing a lot harder than you realize on that thing when you get nervous taking readings on the pins, trust me. Just make sure it's not moving anywhere. Plug the amp into a speaker load, preferably a cab so you can play through it and make sure you like the sound as you're making adjustments.

Let's get a few basics down...


You CAN touch the metal chassis. You CAN rest your hand on it. You CAN NOT get careless with where you're putting your hand. Some switch contacts and other parts come dangerously close to the sides of the chassis. Be VERY mindful of where your hands are at, at ALL times. Most people will tell you to work with one hand behind your back. The idea here is if you catch a jolt, it won't go through your heart on the way to the floor. Sometimes that's not always feasible with the equipment you have. Just be VERY careful in there. Don't touch ANYTHING other than the chassis with your bare hands.

ALL measurements are in DC volts. Not AC, not current, DC volts. You will take readings by putting the RED probe on the PIN you are reading from, and the BLACK probe on a good ground point, preferebly the chassis. But use common sense here...if you have an amp with a nice enamelled chassis (like an Orange), the chassis is NOT a ground point. There's no bare metal to touch. A transformer mounting bolt or something similar will be best. Make sure it's something clean that you can make good contact with. It's ok to use an (insulated!) alligator clip to clip the black probe to ground so you don't have to worry about it.To check that you have a good ground connection, use the continuity tester on your meter (or just the resistance setting if you don't have one). Check for contunity (no resistance, or very low resistance) between 2 different ground points not near each other...maybe the chassis and a transformer mounting bolt. If it looks good, it is.

When making contact with tube socket pins, it's not uncommon to hear a scratching noise or a little squeal through the speakers. Don't let it startle you. Also, if you get the probe very close to the pin without touching it just yet, you may see a little electricity arc (jump) over to the pin. Again, don't let it startle you, it's perfectly ok.

That being said...here's what you'll do. This is the in depth version, there will be a step by step summary after this for you to keep in front of you as a quick reference so you know you're not forgetting anything without reading through all this junk again in the middle of taking readings...

If you are going to solder 1 ohm resistors in the amp, this is the time to do it. They go between PIN 8 and ground. If you are unsure hot to disconnect the ground connection and put a resistor in series with it, don't do this. It's beyond the scope of this article to teach you how to solder, how to make good connections, and plain old common sense (not shorting leads to other pins, ect.). If you're not 100% sure, don't do it. If you are, or you're using a bias probe...continue.

Your amp is plugged in...THE OLD TUBES ARE STILL IN IT. It's plugged into a speaker load, the chassis is secure, you can access the bias pot, and get at the tubes easily. You want to pull one of the tubes out and install the bias probe in that socket. Plug the tube back in. What you're doing is measuring the bias of the amp as it is now. You can practice on the old tubes without fear of hurting the new ones, and it's good info to know. What if you don't like the sound when you put the new tubes in? At least you'll have a reference point, you'll know where it was in the first place (generally speaking...you're not going to test every one, unless you want to). IMPORTANT: Be mindful of where the plug for that bias probe is at all times. You don't want it touching metal things, falling and touching you, or anything like that. It's not very high voltage, but be safe. If you only have one meter you'll be switching between that and the probes to take readings. Secure it with some tape if you have to.

Turn the power switch on. Leave the amp in standby and let it heat up like you normally would.

You can now check the negative voltage if you want. You won't adjust it yet, but it's good to know where it was. Put the BLACK probe on a ground point (like we talked about above), and put the RED probe on PIN 5. You'll see a reading anywhere from -34V to -55V or so, give or take. Write that number down, just so you have it. By this time your amp should be heated up and ready to flip off standby. Before you do, look at the bias probe plug again...is it safe?

Flip the amp off standby. Now you're good to go. The first thing you'll do is measure the PLATE voltage. Put the BLACK probe on ground, put the RED probe on PIN 3. Again, don't be startled by a scratching or squealing noise. You should see a number anywhere from 350V to 600V or higher, give or take. Write that number down.

Next, you'll measure the CURRENT draw from the bias probe (if you have 1 ohms in the amp put the BLACK probe on ground and the RED probe on PIN . If you have a seperate meter for the probe, cool. If not, unplug the meter probes and plug the bias probe into the meter. Again, you're measuring in VOLTS. This will be a small number though, in the MILLIVOLT range. You'll see a number anywhere from 10.0mV to 35.0mV or so. This is the only time Ohm and his stupid law will come back to haunt you. He wants that reading in VOLTS, not millivolts. So if you have an auto ranging meter, you've got to convert it. Easy enough, just divide by 1000...move the decimal to the LEFT 3 places. If it reads 28.5mV, that's actually .0285V. And guess what, remember all that anything times 1 is itself nonsense. .0285 VOLTS is automatically equal to .0285 AMPS. Now we have CURRENT draw in AMPS, write it down.

Here's where STATIC PLATE DISSIPATION WATTAGE comes back to haunt you.

So you've measured the plate voltage (PIN 3). Let's say it was 503V for our example. And you got 28.5mV on the cathode (through the bias probe or the 1 ohm...PIN . 28.5mV is .0285V or .0285A.

STATIC PLATE DISSIPATION WATTAGE = PLATE VOLTAGE x CURRENT DRAW

STATIC PLATE DISSIPATION WATTAGE = 503V x .0285A

STATIC PLATE DISSIPATION WATTAGE = 14.3355 WATTS

14.3W ? What the heck does that number mean? Well, look at the bottom of the page and look up the MAX STATIC PLATE DISSIPATION WATTAGE for your tube type. This is the MAX that you would want to run the tubes at. So if you have EL-34's in this amp, the MAX being 22W, you're well within limits. Whoever set the amp last time didn't set them too hot. If you like the sound of the amp now, maybe you'll want to set it for that number again...or maybe you'll want to experiment. It's your wild west, cowboy. Keep in mind, that's just the wattage for one tube. The numbers may be slightly different for the others. But you're not keeping those tubes in the amp so who cares. When you put the new ones in though, you will measure all of them...even if it means shutting the amp off 4 times so you can switch the bias probe over to each tube in a 100W amp. May God help you if you have a Mesa 400+.

So now you know where the amp has been running. And you wrote it down so you don't forget. Let's pop the new tubes in and have a go at those. Turn the amp off and take them out. Wait for them to cool if you want, if not use a glove or oven mit and BE CAREFUL....They're burning hot glass.

Pop the new tubes in, with the bias probe on one. You will measure all of them so it doesn't matter which. If you have 1 ohms in the amp, no need to worry, you can make the measurements quicker...though you had to solder them in and take them out (I recommend that), so you'll make up the time again later.

Again, amp in standby (if your amp doesn't have a standby switch you may want to do the following neg voltage reading before you put the tubes in, just a precaution). Again, you'll measure the negative voltage from PIN 5. BLACK probe on ground, RED probe on PIN 5. Here's where it's different. What you want to do is adjust the neg voltage, using the trim pot so you achieve MAX negative voltage. This will insure that the tubes and running their coldest when you flip the amp off standby. Yes, MORE neg voltage means COLDER bias. So if turning the little pot one way gets you -28V, and the other way gets you -60V...you go with the -60V side.

NOTE on locating the pot. Some amps have more than one pot. Some amps have a small pot that looks like a bias pot, but is really a hum balance for the heater lines. Some have 2 bias pots that control the neg voltage for 2 seperate power tubes, and some Fenders and other amps have a bias BALANCE pot, which BALANCES the neg voltage between the tubes, as opposed to adjusting it overall. It's impossible for me to give a cure-all way to find the bias pot in every amp. If you're unsure about what the pot does (if there's more than one), don't turn it! Simple enough. If you turn it and the neg voltage moves up and down, it doesn't always mean it's a bias pot! It could be a balance. Again...know what you're doing, or find someone who does.

Moving on...Where's the bias probe plug? Not touching anything bad? Ok...flip the amp off standby. Go turn the lights off in the room and look at the tubes. Wait a minute while they settle in and look for any redplating. Redplating is when the large metal pieces inside the tube glow red. This is an indication that something is wrong....but it shouldn't be, especially with the tubes running as cold as they can in the amp. If everything is ok, move on. If not stop and find someone who can figure out why.

Now measure the PLATE VOLTAGE (PIN 3) and CURRENT draw (1ohms off PIN 8, or bias probe) again, just like you did before. You're not concerned with the negative voltage number anymore. Say you get something like 507V on PIN 3, and 12.3mV for the current draw. Do the math like above and you'll get about 6.2W. VERY cold. You're not running anywhere near too hot for any tube type. So from here you'll turn the bias up by adjusting the neg voltage with your little screwdriver. Start in small increments and take readings to get a feel for how quickly the numbers ramp up. Err on the side of caution, don't risk cranking the tubes up too high so they redplate and risk one popping.

I can't tell you where to set your tubes. Most techs will set between 60-70% of the max, give or take. Some a little lower, some a little higher. There is no magic number that will sound best. The most important thing is how it sounds to you...While staying within the limits set by the guys who made those tubes, again, look at the MAX STATIC PLATE DISSIPATION WATTAGE numbers below. If you go higher, you shorten your tube life. Different amps will respond differently to higher or lower bias, but generally if the amp is colder it'll sound more crisp but maybe more brittle, break up later but maybe feel too stiff, and sound more defined but maybe a bit flat. Hotter bias will generally sound more fluid but maybe a bit mushy, break up earlier but maybe feel too flubby, and feel more responsive but maybe a bit less defined. Find a healthy medium or find the sound you like the best within the limits. Keep your eyes on the tubes for redplating.

Check the bias of EVERY tube in the amp. You will use the HIGHEST STATIC PLATE DISSIPATION WATTAGE number as your benchmark for weather or not they're running too hot. If one tube reads 17.5W, and another reads 18.1W, you set the amp according to the higher one. This is also a good indication of how well the tube are matched. Again, everyone has different preferences...a lot of us like the sound of a slight mismatch. Some of us don't.

Summary

So, in short what you have done is:

-Connect the bias probe to one of the old tubes to practice taking measurements and find out where the bias was originally set.
-Measure the plate voltage from PIN 3
-Measure the current draw from the cathode (1 ohms off PIN 8 or bias probe)
-Did the conversion and punched the numbers in your calculator to get STATIC PLATE DISSIPATION WATTAGE.
-Take the old tubes out, put the new tubes in.
-Set the neg voltage to the max so the tubes run as cold as possible their first time in the amp.
-Taken readings and eased the bias up until you got it in a good range
-Checked the other tubes and fine adjusted it according to the sound and the highest drawing tube (keeping in mind the MAX dissipation wattage for your tube type).
-Wrote the info down for future reference, if you're smart about things.

And one more time...if you're unsure about ANYTHING written here, just don't do it. Or find someone to show you how. It's not worth getting hurt.

[Jondog]

ditto