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 TOOLS NEEDED: 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
TIME NEEDED: 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 troubleshoot)
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.
10.) Now, pick up your guitar and play through the amp normally. Use the Standby switch if you forget to plug in guitar etc. Watch the bias jump. Play rather earnestly for 15 minutes or so. Take mental notes of how the tone has changed. Do you like it or do you dislike it? Does it sound more full and alive without being muddy or warbly? If it is too hot and warbly then you'll know you want to back down. If it sounds good, then we take it up a bit more next time to get closer to desired measurement. Basically at this point we will be turning the bias pot (VR2) up or down until we find the tone we like the best. Because an amp's bias will drift over time, we want to let it settle in and play through the amp as much as possible time permitting. 15 - 60 minutes maybe. This will make sure your bias is settled in and therefore should be the same the following day. You don't want to bias and adjust once and close it up and walk away without playing it. If you did, you run the risk of all of work, math and numbers being inaccurate and therefore cause tone degradation later. For example from the time I biased the amp 6 months ago until recently my amp drifted from 27 to 23 mA and this is when following this procedure.
When you are done, leave the amp on and set the guitar off to the
side. Let the bias settle back down. Leave the meter on. Take a 30
minute break. Seriously. Go do something else for a bit while the amp
is on. If you have kids or pets in the area keep them away obviously.
Remember, bleeding resistors and such do not protect you from anything
while the amp is turned on.
Note: It is generally not good for a multimeter to be turned on and off while connected like this. True for lots of things in addition to amps. My multimeter is a digital mulitmeter (DMM) and takes some of the guess work out taking measurements. It has a button you can press in the event the multimeter turns itself on hold. Read the manual of your mulitmeter and follow those instructions. You could blow a fuse in the meter at best. Most meters have a spare fuse inside and I recommend getting a few extra fuses before hand. It sucks to go through all of this effort only to have a non-working meter and have to run to the store. Same for the battery. If you get weird readings on the meter there is a good chance the battery is dying.
12.) Here is where some of the magic or mojo comes in when biasing an amp. Because you ideally want to bias the amp to where it sounds the best from here on out you will biasing by your ear. EVERY AMP IS DIFFERENT, and EVERY GUITARIST IS DIFFERENT. It can be distracting and frustrating to focus too much on the numbers. An amp in my opinion sounds best right before it reaches cross-over distortion. I can't explain what that is technically but that is what will cause the muddiness or warblyness described before. (I realize 'Warbly' is not a not the greatest term but I don't know what else to call it.) Also, you have an opportunity to swap some of the power tubes around so they match up better as pairs. For example, when first biased my amp I had readings of say 23, 28, 22 and 32 across all 4 tubes so I paired up the 22-23 and the 28-32 as pairs. If you are installing new tubes, then again just match up your pairs based on manufacturer spec.
13.) Check the cathode bias on the amp and take note. Again, you can now turn the screw (following safety procedures above) either higher or lower and repeat the process of playing through the amp. Because you are on the primary tube for checking bias this is the number you want to focus on. Once you get your bias to a comfortable place on this tube you can turn off the amp as normal and then turn off the multimeter. Unplug the power from the amp. Put the amp in Standby if you like. You will now move onto the second tube of the pair by disconnecting and reconnecting the probe accordingly. At this point we are still only focused on the inner two pair. We will do the outer pair last.
14.) Put your bias probe on next tube with lead connectors in multimeter and turn the multimeter on. Turn Standby off so both power and standby are in off position. Plug the amps power back in. Power amp on as normal. Check amperage of your next tube and follow the exact same procedure as above by following safety procedures, playing your amp (NEVER play guitar and bias amp at the same time), and write down your findings, etc. Remember, the first tube's reading is the most important and it is often the case that the second tube will not have the exact same bias reading as the first. This is normal. As a matter of fact most amps can deal with a mismatch here of to say 4 or 5 milliAmps. Just don't go overboard on adjusting the second of the pair as that could affect the bias of the first if that makes sense.
15.) You can now repeat steps 13 and 14 on the outer pair of tubes. Again, the power tube closest to the power transformer (the one originally on the right when right-side up) is the tube bias you want to focus on. Repeat, check bias, play guitar, adjust up or down AFTER you set guitar down, let it settle in, take a break, come back and make some adjustments up or down. Repeat.
*I actually did this over the course of 2 days on a weekend, but please keep your surroundings and those possibly in it under consideration. It's bad enough to kill yourself, but to kill someone else because of carelessness is worse.
16.) Turn off amp. Unplug the power. Turn off the meter. Unplug the meter. Unplug the probe. Unplug the speaker cabinet and guitar lead. Make sure all the tubes are seated properly. Get your headbox ready. Pick up the chassis and flip it around being careful not to tough the capacitors or the insides of the chassis. Grab the power transformer and slowly and delicately slide the chassis back into the headbox following the guides. Screw the 4 screws back in. Put amp back on speaker cabinet and plug speaker cab back into amp as well as power and guitar lead, etc. Make sure all the tubes are seated all he way down. Power on and inspect tubes for normal brightness. Play your guitar through the amp to make sure it sounds like what you heard before.
The bias probe mentioned in this tutorial is basically just a resistor with built in leads to connect to a multimeter. There are other ways to accomplish the same goal if you have the right tools and knowledge. I find the bias probe to one of the safest. I'm not an electrical engineer or an amp tech. If for some reason your amp has no bias trip pot (variable resistor) you
can also do it without one but it requires some soldering. You remove
the wire from the cathode (pin 8) and add a 1 ohm resistor (a 2W one is
easiest) in line with it (ie. solder one end of the resistor to pin 8
and the other end to the wire you removed). Now you can measure the
voltage across the resistor and the voltage in millivolts is the same
as the current in milliamps. The plate voltage can be measured directly
off pin 3. Also, bleeding resistors on filter capacitors do not guarantee a safe discharge of volts. If you want to take safety to the next step and are already comfortable working with amps and electronics you can use a 'discharge 'tool' (10w 33k resistor with (insulated!) crocodile clamps) between a plate resistor and ground. It's important to always connect the ground side of the resistor first, in case you accidentally touch the other clip. Wait for about a minute, and then remove the resistor, and measure the DC voltage at the same point, usually. If it's less than 5v, I consider it discharged.
*Footnote and Tutorial contributions from Roc8995, Cathbard, Mr Hankey, CECamps