Changing tubes? Get great tone by learning How to Bias a Tube Amp!
Inevitably, if you are a tube amp owner, there will come a time when you will start to hear that round, silken tone give way to a weak, frayed signal – just barely resembling a guitar. The answer many a time is a failed preamp tube, both 12AX7 or 12AT7 have been the culprits.
Yet, there may come a time when the larger power tubes of your amp, be them KT66, 6L6, EL34, or 6V6 tubes, will start to Spark. Yes, I mean shoot blue sparks inside of the vacuum tube, ‘arcing’ as it is known in the business. Well my Silver-faced Twin Reverb did just that in ’97, and I didn’t have the understanding to change those tubes, all 12 of them. If your tubes are doing this, change them immediately.
A side note: If your tubes have a bluish glow, there is nothing wrong with them if the glow is steady. Once you see movement in the tube, that’s a bad sign 9 out of 10 times. Also, EL84 amps (like Vox amps) have a fixed bias. No need to bias them!
I am (a little) more enlightened now, and hope to pass on some advice so you can save some money!
- Digital multimeter – No need ot break the bank on this. A simple, cheap one works just fine. See the link for a great suggestion.
- Insulated Flathead Screwdriver – Don’t skimp on this. I once accidentally slipped a screwdriver within the circuitry of my JCM 900 and woke up in the next room with my left arm hurting from the shock I received…or wear rubber gloves.
Step 1. Buy Replacement Tubes
On principle, Stick with JJ/Tesla Tubes. You cannot go wrong with these high quality valves.
Power Tubes: Chances are you may not exactly know what tubes you have. Check on the big tubes, for a number. Some of the more popular power tubes:
They are installed in most tube amps in pairs, which is how you will buy them. They actually come in matched pairs with a small number written on both boxes.
Just click on the above Power Tube numbers to buy a matched pair of tubes for your amp. You may have a Rectifier tube, Usually it’s a GZ34/5AR4. Some amps, like the Mesa Dual Rectifier, or the Mesa Triple Rectifier use two or three respectively.
Here are some links to popular Tube Amps for your convenience in ordering:
- Fender Hot Rod Deluxe/Deville
- Fender Deluxe Reverb
- Mesa/Boogie Dual Rectifier, Triple Rectifier
- Marshall JCM 2000/TSL
Step 2: Find the Bias Point
Depending on the amp you might have, the Bias point might be conveniently located in the back of the amp or on the internal circuitry. My Vintage Sound Amp for instance has both a Bias and Ground point installed into the back of the amp. The Fender HotRod Deluxe/Deville denotes on it’s circuit board where to place the red lead of your digital multimeter. You may have to have to open up the back of the amp or top of the amp in order to find the bias point. Keep the circuitry open as we proceed to the next series of instructions.
Step 3: Test the Bias
- Turn on the amp, first with standby, then with the ‘ON’ switch keeping the speaker plugged in, yet with no instrument plugged in. Volume can be all the way down.
- Turn on your multimeter and turn the knob to the ‘200’ range in the ohm range. Plug in the cords to the multimeter. Black to Ground (or arrow), and Red in to the V(ohm sign)mA.
- Place the black probe onto the metal chassis of the amp and the red probe onto the bias point. You might want to ask a friend to do this, as your concentration and hands will be needed soon to adjust the bias pot in Step 6.
- Read the display, make note of the current numbers on the screen. They will change slightly, but make note of the average. For the tubes previously installed, I found the mA (milliamps) to read around 31.5mA.
Step 4: Remove old tubes
- Allow the tubes to cool down. With a cotton cloth, gently rock the tube in a circular motion while releasing the tube clamp on either side of the plastic cuff of the tube.
- Place the tubes into the box that came with your replacement tubes, yet place them in upside down in order to remember that these are the old tubes. Save these, as you never know when you might need a backup tube.
Step 5: Install new Tubes
- Make note of the slot on the plastic post of the power tubes, this will guide the tube correctly into place. Line the slot and the pins (on both preamp and power tubes)on the tube up with the corresponding socket. With a similar rocking motion, slide the tube with gentle force, seating it snugly in the tube clamps against the socket.
Step 6: Test and adjust the Bias
- Follow the instructions in Step 3 to remind yourself of where the bias is at.
- Find the Bias Pot (potentiometer), it’s usually installed on the inside of the circuitry. Some look like a metal pot, others are a cheaper Blue plastic pot. There is a small slot in the pot, this is where the insulated flathead screwdriver will adjust the pot, and in turn, adjust the bias.
- While reading the multimeter, turn the pot with the screwdriver slowly in one direction, watch the numbers. You guessed it, match the mA on the screen with the mA written on the box/cuff of the power tubes. In my case, I was aiming for 27 mA.
- Blue Plastic Bias Pot on the CircuitboardSome prefer to set the bias a little hotter, feel free to do so, yet keep in mind that this will shorten the life of the tubes…yet give you some extra tone saturation. Definitely do not set the bias number to be less than the prescribed mA. The tone will result in a thin, weak sound.
Congratulations! You did it! You not only saved about $100 in a workshop charge, but you have an amp that is ready to go!
Here are some helpful images!