FIF : Set Your Bias With The TAD Bias Master

Written by
Logan Tabor
Published on
January 21, 2021 at 10:50:06 AM PST January 21, 2021 at 10:50:06 AM PSTst, January 21, 2021 at 10:50:06 AM PST
Welcome back everybody and thanks for tuning in.  This week on Fix It Friday we are going to be using our TAD Bias Master to do a couple of things: First we will be using it to verify that our power tubes, in this case a pair of 6V6s, are well-matched.  Once we have determined our tubes are a good match, we will use the TAD Bias Master to set the bias on our amp. 

The first thing to note is that I am changing tubes on a Deluxe Reverb clone which has an adjustable bias potentiometer on the top side of the chassis.  This potentiometer allows the user to adjust current draw on the power tubes -- all one needs is a flat head screw driver and, of course, the Bias Master.  

As you can see in Image 1A below, I have removed my chassis from the cabinet, but still have my speaker plugged into the speaker jack on the amp.  This is because we will be turning the amp all the way on and we certainly don't want to fry anything by not having a load on the circuit.  I've also gone ahead and removed my old power tubes.  

Before we go any further, let's take a look at our Bias Master.  It comes with a control unit, and 4 tube sockets that are wired out to an RCA style plug.  In this case, I'll only be using 2 of the sockets, as I only have 2 tubes to worry about.  Insert the RCA style plugs into the first 2 inputs on the controller and then insert the tube socket ends into the tube sockets on your amp (See Image 2A).  Next, insert your new power tubes into the TAD Bias Master tube sockets (Image 2B).

Now we can flip the power switch to the "On" position and let the amp warm up for about 30 seconds.  Once we've warmed up, let's flip the standby switch so that our amp is all the way on, as if we were getting ready to play through it.  Then, we can turn our Bias Master knob to "1."  This will show us the current draw on the tube that is plugged into the first socket on our Bias Master.  Make a mental note of the number on the display and then turn the knob to "2."  If we have a good match, our tubes will have a reading within 1mA of one another.  It looks like my tubes are pretty well-matched so I'm happy about that (See Images 3A and 3B).  

Since we know we have a good match, we can start adjusting our bias.  Grab a flat head screwdriver and slowly turn, but try not to go too far too fast, just enough to make sure it's working.  As we turn we should be able to see the milliamp draw creep up or down depending on the direction in which we are turning (See Image 4A).  

If you don't know the appropriate bias range for your amp/tube configuration, it is best to go online and find a chart showing different bias ranges for different amps, or you can even hop on a forum to find good recommendations.  It looks like the recommended bias range for my amp is between 18 and 30mA.  I like to dial mine in a little "hot" so I'm going to stick around the 26-27 region (See Image 5A).  

Once you've got your bias set where you like it (and definitely within the recommended range), you should turn the amp off and let the tubes cool for a minute.  After the tubes have cooled enough for you to touch them, you can remove the tubes, remove the Bias Master sockets, and reinsert the power tubes directly into the sockets on the amplifier chassis (See Image 6A).  

Badda bing badda boom, folks!  Wasn't that extremely easy?  Now that you know how to use the TAD Bias Master, you may feel a little more confident in adjusting your amp's bias to see where it sounds best to you -- everyone has a unique preference so get out there and figure out what works for your rig!  Thanks so much for tuning to this week's Fix It Friday and I sure hope to see you again soon.