Do you know what the bias setting on your amp is? If your tubes burned out, would you know how to get the same sound back as you had before?
The amps bias makes a critical difference and you really should know all of the bias settings for each amp you own and what the voltage number of the tubes that are in them, especially if you like how your amps sound.
You can check the number of the tubes by looking on the tube box (you did save the tube boxes, right?) it will be written on the end.
But, now you need to know how to bias your amp.
First you will need to purchase a bias probe tester. I own the QuadStage BiasPro from Asharpfretworks.com (it’s metered and easy to use), but a search for “Bias Probe” on Ebay will get you several options to choose from. For a Marshall be sure to select the 8 pin cable option (you will need two of them and they come with the tester, but you have to tell them you want 8 pin when ordering).
You will need to remove the 4 screws on the bottom of your Marshall and slide the chassis out the back to get to the bias adjustment on the underneath side of the chassis. WARNING: Even unplugged, parts in an amplifier can hold enough voltage to kill you. Keep one hand behind you and wear proper gloves when adjusting the bias and you will be fine. Just don’t be stupid and there is nothing to be scared of. I’m only putting this warning up so someone doesn’t get the bright idea to do this after a few beers
1. Turn off your amp, pull the AC cable out and allow the output tubes to cool.
2. Pull the output tubes out and insert the tester probe sockets into the amp’s tube sockets. (The tester sockets get inserted between your amp and the tubes and have a cable coming from them that attaches to the testing meter).
3. Insert the output tubes into the tester sockets (which are now inserted into your amp).
4. Make sure your amp is connected to a speaker and then power up your amp and let it warm up for at least 5 minutes
5. Read the numbers on the test meter screen (switch between tubes on the tester and always use the higher number). This is what your amp is currently biased to with the tubes you have in it. Write down this number and the number of your tubes for reference (if you like the tone) If you have done this reference test previously and are biasing new tubes, then skip this step.
6. Read the numbers on the meter with your new tubes installed. The bias setting will most likely be off from your reference numbers you took from the original tubes. Insert a screw driver into the bias level adjustment potentiometer on your amp (Located on the underside of most amp chassis, usually a little box or round pot sitting sideways or facing up with a screw driver slot, please search the web instead of assuming you have found it). Adjusting this clockwise raises the bias, counter-clockwise decreases. Adjust the bias control on the amp until the test meter shows the value of your reference test (or new bias setting if you are experimenting with different number tubes or raising the bias to increase distortion or lowering it to clean it up, but that’s for another discussion, Search the web to learn about how larger numbers on your tubes increase headroom, smaller numbers distort quicker, etc…).
7. Power down the amp, pull AC cable, let tubes cool down. Remove tubes from tester sockets, remove tester sockets from amp’s tube sockets. Now put the tubes back in the amp’s tube sockets and you are done.