Disclaimer - Biasing amps can be dangerous!
This is for my amp using EL34s, a single tube bias probe, a standard digital multimeter, and a bias range recommended by the manufacturer - your amp MAY BE DIFFERENT. My amp is a Splawn Quick Rod amp head running at half power into a 212 cabinet. My amp is a Fixed Bias amp and does not have external bias pots or testing points. It runs in Class A/B push-pull operation. I will walk through some safety guidelines, things needed, how to get started, what to look for, and how much time you need. A pictorial is posted below the Safety Guidelines. See footnotes for additional contributions.
Here are some things to reduce shocking yourself. Filter Capacitors can store deadly voltages for days on end even after the amp is turned off. In order to bias an amp where the bias trim pot is inside the chassis and accessible from only the underneath you need to be working inside the amp WHILE THE AMP IS TURNED ON. Very Dangerous. If you do not feel comfortable then do yourself a favor and take it in to a qualified amp technician.
-Determine work area in advance and notify others around you to stay away.
-Find a wooden desk, tables, platform as your work space that will more than clear the tubes and transformers while the amp is upside down. I actually recommend measuring the length of the chassis and to get a feel for how much space you'll have on each end and also to clear the tubes and transformers while providing a safe platform. Remember, you'll be pulling tubes upside down with the amp's electronics exposed and facing up. I happened to find the perfect solution by using my 212 cab and another amp as two 'tables'. This provided a nice snug working top to where the amp would not slide off by accident. Brace it right up next to the power transformer. There is not much space between the power transformer and the edge of the amp so you don't want it to slip off. Remember the chassis is fairly heavy. If your amp falls while you are working on it the only thing you should do is step back and safely unplug it. Don't reach for it in these types of situations. It is usually the unexpected things that get people into trouble so having a safe work space is very important. Also make sure you can reach the power, standby, and volume knobs. You will be turning the amp on and off a fair bit.
-Wear rubber gloves, but only if they do not impede your work. I actually do not but it is certainly not a bad idea.
-Have good thick regular gloves for pulling tubes and such.
-Wear rubber soled shoes with no steel toe.
-If you have rubber floor mats or the like you can lay those down on the floor as well, especially if you are not on a carpeted area.
-Do not work near other major appliances or old pipes that conduct electricity.
-I also suggest to not drag your feet on the carpet and walk up and touch the amp. Discharge any static electricity on something else first. Anti-static wrist bands are good to if you have them.
-Find, in advance, the smallest flat head screwdriver you can find (1/8") that has a rubber or plastic insulated handle. The type you would use for eyeglasses will work. You will also need a standard phillips too remove the chassis.
-Know in advance what the filter capacitors look like and where they are located.
-Never touch a filter capacitor on the inside of the chassis. There are also traces to these filter capacitors that can be dangerous as well so it is best to keep your hand out.
-You can touch tubes, transformers, etc outside of the chassis. You can even touch the outside of the capacitors on the outside of the chassis but never the insides. There are 4 good sized cylinders behind the the power tubes on my amp. They are a bit shorter and fatter than a roll of quarters. These are the filter capacitors. Mine are black, they are often blue. Be aware of where they are when you first flip the chassis upside down. Mine has bleeding resistors on them to bleed off deadly voltages but you never know. You can see these resistors attached to the bottom of the capacitor from the underside. When I read mine with a multimeter they showed .01 basically but I do not suggest checking yours unless of course you know what you are doing. Just stay away from them. If for example your screwdriver falls over onto the caps or into the amp while the amp is on, DON'T REACH FOR THEM instinctively. Very important to remember. Turn off the amp and remove the screwdriver with wooden tongs and rubber gloves.
-Never disconnect bias probe leads from a multimeter while the amp is on.
-Never put two hands inside an amp at the same time unless you have to.
-One hand on screwdriver One hand in back pocket.
-Don't leave your amp unattended while it is opened up as kids, pets, employees, etc may not know how dangerous this is and want to touch the amp out of curiosity. Put a cardboard box over it and lock the door if you need to leave. Or drain the caps.
-Never play a guitar and touch the inside of the amp at the same time. Set your guitar down to adjust bias. This can be hard to remember as it is easy to want to play guitar and fiddle with bias at the same time without following the steps. Your guitar can make you the middle of a full circuit.
*If you follow these basic safety procedures you can bias your amp safely and save yourself some money while gaining knowledge and satisfaction.
The following 15 steps can probably be broken down into 5 but for those that are new to this I feel it is important to be methodical. If one person learns one simple thing that avoids getting shocked then I've accomplished my goal. Again, these steps are an example ONLY using MY AMP. Different amps may have different characteristics/procedures. Fenders for example are completely different. This tutorial is based on a Splawn Quick Rod (Marshall based amp).
Refer to your owners manual or contact the manufacturer if you are not sure. ALSO, I AM NOT AN AMP TECH - so utilize other resources and combine that with what you learn here.
Here are some pics I took that can be used as an aid. I will put the rest at the end of the 2nd (Continued) Blog:
I've number the tube positions in the order I do them in
18" flat head screwdriver with rubber or insulated handle, standard phillips screwdriver, multimeter, bias probe, rubber gloves, regular gloves, deoxit contact cleaner, chopsticks or wooden tongs, and extra fuses and battery for multimeter
2 hours minimum
1.) Unplug the power from your amp and turn the Standby Switch to on (play) position. This can help drain any deadly voltages. Take off the back plate of the amp with a screwdriver. Your amp should be right side up, we'll flip it upside down later. We will now check the tube plate voltage in milliVolts. The inner two and outer two power tubes work in pairs. Start with the inner 2 pair. The tube closest to the power transformer in any given pair is the one we focus on (the one on the right if you are facing the back of the amp). Remove the tube retainer by gently lifting the springed top up and off to the side of the tube. It is best to hold it there so it doesn't scratch the tube when you pull the tube. Gently rock the power tube back and forth while pulling strait up and out being careful not to bend a pin. If you have contact cleaner like DeOxit it is good to use it once on each socket or the tube pins to clean and lubricate. The socket is the white base with 8 holes in it. (You may not see all of this in my pictorial because I removed all of the tube retainers and pulled the outer two tubes as I run at half power).
2.) Find the 'key' on the bottom center of the black tube post of the tube you just pulled. Align that key to match the bias probe that you have. In this example we will assume a single bias probe. Other probes out there can eliminate some of the steps in this tutorial. Push tube into probe and then probe into amp following the same key alignment. Make sure it is nice and snug (seated). You can let the retainer rest against the tube.
3.) Connect the red and black leads from the bias probe into the corresponding connection points on the multimeter. Black is Communication. Do not use the red 10a unfused option, use the other one labeled Amp/Volt. Turn multimeter on and set to milliVolts (mV). Always set a multimeter to the parameter above what you expect. You will be expecting roughly 500 millivolts for this amp so set to 2000 mV. Now plug your amp power back in and turn on the amp after turning Standby back off. Inspect power tubes for consistent or normal brightness. If one or both tubes looks overly bright or not lit at all turn the amp off and investigate and take meter off and put back to starting point if you need to. No sense going further if something is amiss. (NOTE: Similarly, if you are putting in new tubes and have issues, remember you always put the old tubes back in to
4.) Let the amp warm up. Write down the milliVolt reading after 2 or 3 minutes roughly. Mine was 475 mV. It may climb slowly so just let it sit there a bit until it stabilizes. Once you've done this you can move on to the other power tubes (this may not be necessary as it should be the same). Reverse the process and move on to the second half of the pair. You should get a similar reading here but the first one is really the one we will focus on. It is also normal if the outer pair are different than the inner pair but not by too much. Maybe 10 mV. Remember, to turn the amp off, then multimeter, then let the tubes cool down, and use gloves if needed when moving on to next power tube. If you have new power tubes to install, then obviously follow manufacture spec and match pairs as closely as you can.
5.) It is also a good idea to write down the current cathode bias on your main power tube so you know what you will be dealing with and out of curiosity but you will find out soon enough anyway. It is sometimes recommended to turn the amp off while changing the multimeter. Set the multimeter to milliAmps (mA) or .000 amps and again set it to highest level above what you expect. Because in my example I am expecting something between 20 and 40 milliAmps set multimeter to 200mA.
*You've accomplished the first phase of biasing an amp by determining its plate voltage and its cathode current.
6.) Make sure all power tubes are back in there original places and seated well with the amp turned off. With amp power off and Standby ON (play position) you will help amp drain lethal voltages more quickly. Turn off multimeter and disconnect it.
(Review safety procedures)
7.) Make sure amp power and standby switches are off. Unplug power, guitar lead, footswitch cables, FX loop cables, attenuators, speaker cabinet, etc and set amp head down on the floor or work surface. Set the amp face down with feet pointing toward you. Carpet or Towels is highly recommended as to not scratch anything. Remove the 4 screws underneath that hold the amp to the wooden headshell. Slide the amp head up and out. I find it easier to just grab the power and output transformer and get a good hold. You will want your work surface ready to go because the chassis is a bit heavy and you need a place to put it quickly. Some people just set their amp down on the transformers themselves but I don't recommend that. You'll have a fair amount of surface area to work with on the preamp tube side (4") but not on the power transformer side (1.5") so be prepared. As mentioned earlier, I recommend measuring the chassis and having a safe, secure, place to set amp chassis upside down once you get it out. Also make sure your surface is secure so the amp won't slide off.
*Again, be aware of where the filter caps are and keep the amp clear from yourself as you spin amp upside down. You wouldn't want your belt bucket, for example, to brush up against a filter cap or other internals.
8.) Flip the amp upside down and set the amp down on your work surface lining up the chassis to your predetermined platform and make sure the amp is secure. Repeat the process above of attaching your bias probe to your first power tube (starting with the inner pair) in the same order as before. Your power tube closest to the power transformer (in each pair set) will be the tube you focus on. Make sure everything is seated. Connect leads to multimeter. Turn multimeter on and set meter to milliAmps as above (200mA). Plug in the amp power source, guitar lead and speaker cabinet. Don't run your amp without a speaker cabinet plugged in. Power amp on as normal. Again, inspect tubes for normal brightness. Power off if not normal. Strum guitar and check that things sound normal. Power off if not normal. Fix problem if there is one.
9.) At this point you should get a similar reading in milliAmps as you did before. This is your Cathode bias which is variable by adjusting a variable resistor that you'll find soon enough. Let the amp warm up as it may take a bit to climb and settle in. Here is where we introduce a simple formula to determine a 'rough' bias measurement that is healthy on paper. In my amp, I've found it sounds better on the 'cool' side of the bias range (60%). Many people do. We will learn in a minute how to bias to tone, but it is always good to start with a safe range and general idea of where you think you want to be. Here is where plate dissipation comes into play. Tube amp power tubes dissipate a certain amount of electrons during operation. Generally, anywhere from 60% to 75% is an acceptable range. If you like things on the 'hot' side you may want to use 70%. If you like things on the 'cool' side you may want to use 60%. Running tubes 'hot' should not be confused with better or 'hot-rodded' or anything like that. Running tubes too hot will create a muddy and 'warbly' sound. It will also cause you to burn through tubes more quickly. Many amps are run on the cool side out of the factory. Peavey for example is notorious for having a cold bias. Splawn HQ also recommends a cooler setting. The numbers we will be focusing on is our plate voltage, cathode bias in milliAmps, plate dissipation %, and the wattage of the tube in question.
MilliVolts is a constant that we captured earlier. (475)
Cathode bias is a variable that we will be adjusting.
Plate dissipation % we decide on up front but only as a guide. (60%)
Tube wattage is a constant. EL34s are 25 watts each. (25w)
If you take your tube watts and divide it by your plate voltage you will get your cathode reading in milliAmps. You then take this number and multiply it by your desired plate dissipation %. This will give you your cathode bias 'ball park' starting point. We will adjust the variable resistor up or down from this number but more importantly - we will adjust it to the best tone.
Here is the formula with my readings:
25 watts per tube / 475 plate voltage = .052 amps
so 52 mA x .60 plate dissipation = .031 or 31 mA
bias amp to 31 mA to start
So, as you can see - I want to shoot somewhere in the 30 - 34 mA range. My first power tube is reading 23. So, we want to adjust that upwards. Roughly in the center of the green PCB board is 2 small blue boxes. They are roughly close to the Gear or Gain knob
Make sure to see the 'Continued' blog where I continue this and show more pictures.