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
Time for a TLDR of your own. Here's some safety advice:
Remember it's not just the filter caps (and the nearby connections) which can be dangerous. The filter caps connect throughout the amp, and you could have hundreds of volts just a few cm from the input jack.
I discharge my amps like this: first, connect my special 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. The filter caps are on the bottom of the chassis on my amps, so I can't normally reach those. I wait for about a minute, and then remove the resistor, and I measure the DC voltage at the same point, usually. If it's less than 5v, I consider it discharged.
I have a final safety measure, in case I measured the voltage at the wrong point: I short one of the plate pins at the tube socket to ground with a screwdriver (with an insulated handle!). This isn't a good way to discharge the caps, but it sure beats discharging the caps with my hand. If they're still fully charged, you'll get sparks and flying chunks of metal, and you might hurt the caps themselves.
I've worked so much on live amps lately that I'm probably getting too used to it. Putting the amp on a stable surface is extremely important. If you nudge it, and it falls, you'll either reach for it instinctively and get fried, or it'll fall and break. If you've got really bad luck, it'll fall on you. Ensuring that you'll keep your balance is also very important. I sit down as much possible, and if I have to walk to and from the amp, I do it very slowly and carefully.
You could be doing all the right safety things, and still someone could walk in and touch a live part of the amp for whatever reason. It'll basically be your fault. So, never leave the amp unattended, even for a few seconds. Especially if you have pets. I always warn when I hear someone approaching my room.
I play with the chassis out of the head cab quite often when I'm tweaking the amp: and I'll connect my discharging resistor every time I'm done playing, and keep it connected. I don't rely on the built-in bleed resistors at all.
Following these rules, I've never been shocked. Never had a close call either, from what I can remember.
Thanks Martin. I'll probably add in your procedure for draining the caps. As you can see, I'm not an amp tech or EE. When I read the voltage across one of my bleeding resistors last time I got .01 v or something - but that doesn't mean that they can't fail themselves or that someone else's amp may be different.
PS: I know that was a long read so appreciate your time