If you're a musician, you'll appreciate the sound of a properly biased tube amp.  Your wallet will appreciate the longer life you can get from your output tubes.  Our bias tools are easy to use, and provides a fast and accurate way to get the best sound from of your amp. Matched output tubes and proper bias are the ingredients to quality sound and longer tube life.   

A bias probe is inserted between an output tube and the amplifier. It monitors the total current flowing through the tube. Current is read on a digital voltmeter (DVM) and may then be easily adjusted to the required settings.

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

Tube Type

Plate Voltage

Power W

Plate Current mA
Cathode Current mA
+/- 10%
- - -


Eurotubes™ Bias Probe
For 8-pin power tubes such as 6L6, 5881, 6550, 6V6, EL34, etc.   Requires separate digital volt-ohmmeter capable of reading current.   This device reads cathode current, but not plate voltage.  Not a kit - this device is fully assembled and ready to use! Complete with detailed instructions.  
Used with a basic multimeter (Get Yours Here).  Note: this adapter, as designed, will NOT test 7591 tubes.  
Part #EBP-1


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The average bias job costs $40 - $60. Build your own bias adapter with our TubeDepot Bias Scout
kit and you'll save money every time you set the bias on your amp. This kit includes all the parts to build a single octal socket TubeDepot Bias Scout. Used with a basic multimeter (Get Yours Here), the finished adapter will allow you to measure both cathode current and plate voltage of a single tube position. We recommend purchasing two of these kits so you can easily monitor both halves of a push-pull circuit.  For 8-pin power tubes such as 6L6, 5881, 6550, 6V6, EL34, etc.  Note: this adapter, as designed, will NOT test 7591 tubes.  

Requires assembly.  Soldering capability and tools required.   Download assembly manual here 



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COMPACT DIGITAL MULTIMETER.  Compact meter features a wide range of measurement capabilities, and is ergonomically designed to easily fit in the palm of your hand. 3-1/2 digit, 1999 count display measures AC/DC voltage, DC current, ohms and diode check.   Works well with our bias probes, and for general trouble-shooting.  Backlit LCD display, easy-to-read 1/2" high digits, square wave output, continuity buzzer, protective holster, meets 600V CATI standards.
Includes test leads, standard 9V battery and owners manual.  5-3/8"(H) x 3" (W) x 1-3/8" (D).
Part #CDM-72-7770


The Eurotubes™ Bias Probe™ and the TubeDepot Bias Scout™ 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


Note:  Some amplifiers offer adjustable bias, and some do not.   Bias adapters will properly measure the cathode current on most all amplifiers, even if they do not offer a bias settings (certain Music Man amps are an exception).  You may wish to have certain fixed bias amps (such as Mesa Boogie®) modified by a technician to provide for bias adjustment.  Vintage blackface Fender® amps from the 1960's offer true bias adjustment.    Silverface Fender®s from the Fender® CBS era offer a bias balance  control, which balances the bias between the output tubes, but doesn't really offer  full bias adjustment.    Most Marshall® amps with 8-pin output tubes offered adjustable bias up through the 1990's.    For other brands of amplifiers, refer to the manufacturer's literature and specifications to see if the bias is adjustable.