I will be changing to power tubes, checking, and setting the Bias on my Fender Hot Rod Deville. Much isn't needed to accomplish this. Everything can be bought online and is fairly inexpensive.
#1 Bias Probe
#3 Phillips screwdriver
#4 Flat screwdriver
#4 a jumper with a 10M resistor installed(for the discharge)
Warning!! do not attempt this or any other servicing of a tube amplifier with discharging the filter caps. Discharging the filter caps is an important step, the caps can carry a lot of voltage in them long after the amplifier has been turned off and unplugged. I have included a link on more information about filter capacitors and the danger they pose.
I soldered a 10m resistor to a jumper, attached one end to the jumper to any pre amp pin number one and the chassis of the amplifier.
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.
Step 1: Getting Started
1 Remove the six Phillips screws and the piece cabinet that covers the tubes and the chassis.
2 There are two large tubes located at the rear of the chassis, locate the power tube furthest from the power transformer.
3 Grab the base of the tube firmly and pull down slowly until the tube is free from the socket.
Step 2: Setting Up Your Bias Probe and Volt-meter
4. Insert the pins from your bias probe to your multi-meter, attach the tube to the bias probe, attach probe to the socket you removed to tube from.
5. The probe measures in DC Milliamps, set your meter to the DCA mode, select the 200mA setting.
6. The amplifier needs a speaker load, plug the speaker out cable to the speaker cabinet. Turn your amp on in standby mode, let warm for 5 to10 minutes.
7. Switch standby off, let value on multi-meter climb for 2 to 3 minutes until it stops.
Step 3: Setting the Bias
7.What is an acceptable level for the bias? The formula for biasing is the plate dissipation of the tube (25 watts for 6l6,EL34,E34L or KT77) divided by the plate voltage(voltage on the power p.c.b.). Most 50 and 100 watt amps using these tubes will have between 450 and 480 plate volts. My amp in particular uses 440 plate volts. Using 440 volts and a 6L6 tube as an example, divide 25 by 440, the answer .0568 or 56 milliamps is then multiplied by .7. That equals .039 or 39 milliamps..7 seems to be the standard for most tube amps, you can go as high a .85, but your tubes will wear out quicker due to the added voltage.
8. If you do a bias pot search on the internet for your amplifier you can find it much easier than using the schematics. The bias pot on my amplifier is the tiny blue knob towards the center of the first photo on this page.
9. turn your bias pot clockwise to the desired setting on your volt/multi-meter. when you reach the proper milliamps, stop for a moment to make sure the voltage has stopped moving around.
10. Once the voltage is set, you can turn your amp off and using the same steps to remove the power tube the first time remove the tube and the multi-meter. Place the back chassis cover back on your amplifier and enjoy your new tubes and/or bias setting.
Play and enjoy to great tone you are getting from your new tubes/bias setting.