What exactly is Bias?  The best analogy of bias adjustment that we can think of goes back to older cars that had carburetors.    The carburetor had an adjustment screw to set the fuel-air ratio.   A lean mixture would have less power, but give better mileage.   A rich mixture would have more power, but would decrease gas mileage.   Most people opted for a mid-range setting that gave sufficient engine power while maintaining decent gas mileage.

In many tube amps, the bias on the power tubes is adjustable.  Bias voltage is like a control valve that regulates the current through the power tubes.   A "hot" bias means that the power tubes are running hard (and hot!), giving a warm, harmonically-rich tone, but experiencing decreased tube life due to the heat generated.   A "cold" bias means that the tubes are running much colder, the tone is cleaner and a bit more sterile, but the tubes last much longer.   Again, the "sweet spot" in between is where most techs set the bias.

Not all tube amps have adjustable bias, but most larger tube amps do (output power of 40 watts or greater, typically).   Most amps with two or more 6L6 or EL34 power tubes fall into this category.   There are exceptions.   Amps without adjustable bias include Mesa-Boogie amps, some Peavey amps, and even some Fender amps (early versions of the Hot Rod Deluxe/Deville and Blues Deluxe/Deville, for example).   Some smaller amps with 6V6 power tubes also have adjustable bias (Fender Deluxe Reverb, for example).

   
 TUBE BIAS CALCULATOR (for push pull class AB1/AB2)

Select Tube Type and the Plate Voltage of the amplifier below.  Recommended range for
setting plate current and/or cathode current will be calculated.  
Note:   The low end of the suggested range is the "cooler" setting, for cleaner sound and longer
power tube life.   The high end of the range is for warmer and thicker sound, but with resulting
shorter tube life.   The higher the setting, the hotter the tube temperature.

Click on photo for larger view Revisit™ BiasEasy™ 200 Cathode Current Measuring Probe For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100,  etc.   Requires separate digital volt-ohmmeter capable of reading current.   This device reads cathode current.  Note:  This adapter will NOT test 7591 tubes.  Quality throughout, with a ceramic socket, gold-plated socket pins and gold-plated banana plugs to ensure solid connections and accurate readings. Not a kit - this device is fully assembled and ready to use!  Complete with detailed instructions.   Used with a basic multimeter (Get Yours Here).   Part #RBE-200 $29.95 Click on photo for larger view Revisit™ BiasEasy™ 800 The Bias Easy™ 800 measures the current in the power tubes of your amp. Includes two 8-pin probes with gold plated pins and socket, power adapter and  operating instructions.  Fully assembled and ready to use.  For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc.   Not for 7591 tubes. The Bias Easy™ 800 allows you to monitor the bias setting safely and easily. While adjusting the bias control pot of your amp, the Bias Easy™ shows the actual current flowing through each of two power tubes.   A switch on the meter allows you to go back and forth between the two tubes.  This lets you set and evaluate the full reasonable biasing range your amp and tubes offer so that you may find the perfect setting.  Complete instructions are included. The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character. Part #RBE-800 $99.95 Click on photo for larger view Revisit™ BiasEasy™ 1200 Quad Tube Amp Bias Tester The Bias Easy™ 1200 measures the current in the power tubes of your amp.   Designed for amplifiers with 4 power tubes, such as 100 watt Marshalls and the Fender Twin Reverb.   (May also be used on amps with only 2 power tubes).   Includes four 8-pin probes with gold plated pins and socket, power adapter and  operating instructions.  Fully assembled and ready to use.  For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc.   Not for 7591 tubes. The Bias Easy™ 1200 allows you to monitor the bias setting safely and easily. While adjusting the bias control pot of your amp, the Bias Easy™ 1200 shows the actual current flowing through each of the four power tubes.   A rotary switch on the meter allows you to check each tube for current flow.  This lets you set and evaluate the full reasonable biasing range your amp and tubes offer so that you may find the perfect setting.  Complete instructions are included. The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character. Part #RBE-1200 $129.95   Click on photo for larger view 1 OHM, 1%, 1/2 WATT PRECISION METAL-FILM RESISTOR FOR INSTALLATION ON POWER TUBES FOR BIAS MEASUREMENT.  ONLY FOR AMPS WITH BIAS ADJUSTMENT CONTROL. Replace the ground wire on each power tube (pin 8) with this 1 ohm precision resistor.  Read the voltage drop across the resistor (in millivolts) with your meter.  The millivolt reading you obtain will be the same as the cathode current through the tube (in milliamps). Part #MF-05-1-1 $0.60 Click on photo for larger view COMPACT DIGITAL MULTIMETER.  Compact, reliable, inexpensive.   Dimensions:  5-3/4" L x 2-7/8" W x 1-1/2" H LCD backlit for low light situations Rubber outer case makes it shock resistant Flip out stand for workbench testing Continuity tone for blind testing Right angle banana jack test leads save space DC Voltage: 200mV/2V/20V/200V/600V  AC Voltage: 200mV/2V/20V/200V/600V DC Current: 200uA/2mA/20mA/200mA/2A/10A  Resistance: 200/2K/20K/200K/2M/20M Ohm  3-1/2 digit, 1999 count display measures AC/DC voltage, DC current, ohms and diode/transistor check.   Works well with our bias probes, and for general trouble-shooting.  Backlit LCD display, easy-to-read 1/2" high digits, Includes test leads, standard 9V battery and owners manual.  Part #A830L $22.95

Compatibility The Revisit™ BiasEasy™, and the Revisit™/Hoffman Bias Checker may be used with any amplifier that provides for adjustable bias and uses 8-pin power tubes with the cathode connection on pin 8, including: EL-34 KT-66 KT-77 KT-88 6CA7 6L6 6V6 5881 KT-100 6550 7027

 

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