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All Service Musical Electronics Repair
Quality Service and Commitment ... That's the difference!
617 SE MORRISON - PORTLAND, OREGON 97214 .... 503-231-6552
The following method is accurate and quick to perform. Since the tube bias alters how much current the tube will pass, and since all of this current leaves the power tube directly into the output transformer, you can simply shunt the output transformer with a milliammeter and measure the current directly. Your negative lead on the milliammeter goes to the plate of the output tube (pin 3 on 6L6, EL34,6V6, 6550, and pin 7 for EL84) and the positive lead goes to the center-tap of the output transformer. Now that you can measure the current flowing through the transformer, you need to determine how much is going through the tube. If it's a two output tube amp then the reading on the meter is what's going through the tube. If the amp has four output tubes, the reading must be divided in half. On four output tube amps, two tubes feed each end of the transformer. Since you are only shunting one side, then your reading would be for two tubes and therefore must be divided in half to get an approximation of what one tube is drawing. Now that you know how much current is flowing through the tubes, you need to find the negative voltage supply for the control grids. You need to adjust that voltage so that the output tubes are idling at the correct current. In most Fenders, there will be an adjustment pot that can be accessed with a small screwdriver. Marshalls also have an adjustment pot. Some amps, such as Mesa Boogies and tweed Fenders, do not have adjustment pots. In this case one must find the voltage divider resistors in the negative voltage grid supply. The circuit will have a wire going from the power transformer to the cathode of a solid State rectifier and then will flow through two resistors in series. If the first of these resistors is decreased in resistance, the negative grid voltage will increase making the tube current decrease and if the second of these resistors is decreased, the negative grid voltage will go down, making the current go up. Decide which resistor must decrease and temporarily connect a large value pot, say 250K to 500K, across that resistor. Start with the pot at its highest resistance setting. While monitoring the current, adjust the pot until the appropriate current is reached. Turn the amp off and disconnect that pot. Measure the pot with an ohmmeter, then find a resistor of that value and solder it across where the pot was connected. Re-check the tube current, but it should be the same with the resistor. was with the pot. What is the amount of idling current the output tubes want to see? There is no one correct setting. There are only upper & lower limits. I would suggest not less than 10 mA and not more than 50 mA as a good rule of thumb. Certain tubes (6550, EL34) may like to see up to 75 mA. But generally speaking most 6L6 style amps sound best around 35 mA. Any setting that gives you the tone you like within those limits is correct.. The above method will work in almost any amp . Many silver face fenders have an adjustment pot that looks like a standard bias control however, it is not wired as an adjustment for bias, but as a balance for the output tubes. this circuit can be rewired to operate as a regular bias control using the same parts and just moving a few connections around.
All Service Musical Electronics Repair has been providing quality service on all types of electronic musical equipment since 1981. We repair new, used, and vintage musical gear. We are factory authorized for most brands. We do amplifier repair (this includes both tube amplifier repair and solid state amplifier repair) , CD player repair, CD recorder repair, electric guitar repair, effects repair, electric & electronic keyboard repair, microphone repair (standard & wireless microphone repair), multi track repair ( analog & digital ), mixer repair & sound board repair, power amplifier repair, speaker repair, stereo repair, synthesizer repair, Tape recorder repair (cassette recorder repair & reel to reel repair) turntable repair & DJ equipment repair. Basically if you can plug it in and make music with it we probably work on it. Thanks for taking the time to check out our site.