Bias Measuring Devices

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.  The bias measuring devices on this page are easy to use, and provide 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.   

Bias measuring tool are inserted between an output tube and the amplifier. It monitors the total current flowing through the tube. Current is read on a digital meter and may then be easily adjusted to the required settings, which are provided in data tables and formulas found in the owner's manuals.

Bias King
As tubes age, their bias point will shift. By using the BIAS KING™ you can maintain the sound you want by periodically adjusting the bias on your own amp. This will keep your tubes in top shape throughout their life and will keep your amp sounding its best. Use the chart provided to find out what range of current flow is acceptable for the tube type in your amp. It works on most octal socket tube amps, and requires no power source other than the amp it's plugged in to. It can also be used to match tubes into pairs. Features one tube socket allowing the bias to be checked one tube at a time.



Bias King Pro
The Bias King Pro has the same display as the regular Bias King, but with two tube socket plugs. You are able to take bias readings for two tubes by switching between the two inputs - biasing an amp is simpler and quicker than ever.

Please note that the Bias King requires AC power on the tube's heaters. Some tube home audio amplifiers use DC power for output tube filaments; consult your manual to confirm how your amp works.



The BiasPlug™ is made in the USA of the highest quality components available.   The octal plug is made from bakelite phenolic resin to provide the highest electrical insulation and heat resistivity.   The cable is double-insulated Belden 8443 20-gauge copper for low resistance and high durability.   The banana plugs for the meter connection are USA-made high-grade.   All connections are hand-soldered, and cables are provided with strain-relief to ensure long life.  Each BiasPlug™ is thoroughly tested by one of our technicians on a guitar amplifier before it is packaged and shipped.   All you need is a digital voltmeter!
The BiasPlugPlus™ is the latest addition to the BiasPlug™ product line.  The BiasPlugPlus™ is made in the USA of the same quality components as the original BiasPlug™, but also contains a switchable option to read the output tubes' plate voltage quickly, safely, and easily.  Plate voltage is a necessary component to calculating proper bias.   Once you know the plate voltage for a particular amplifier you can record it and you won't have to take that measurement again ... however, if you work in the industry and set bias on a lot of different amplifiers, the BiasPlugPlus™ is an indispensable addition to your tool box.   The BiasPlugPlus™ uses a precision resistor network to divide the actual plate voltage by a factor of 1,000 so that the voltage which appears on your test leads is safe and will read on the 2V scale of your meter.   A 450mV reading equates to an actual plate voltage of 450V, for example.

With the direct-reading Plate Voltage Option.  Complete with detailed Pilot's Manual which gives complete instructions and tells you everything you need to know about amplifier biasing.



BiasPlug™ Compatibility

BiasPlugmay 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
  6550 7027  

Note:  Some amplifiers offer adjustable bias, and some do not.   The BiasPlug 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.    (This can be changed - see our Fender® Blackface modification kits here).    Most Marshall® amps with 8-pin output tubes offered adjustable bias up through the 1990's.   The Marshall® TSL and DSL series of amps introduced in the late 90's have adjustable bias, but that bias is set by using a special procedure rather than by using the BiasPlug.  For other brands of amplifiers, refer to the manufacturer's literature and specifications to see if the bias is adjustable.

BiasPlug Specifications

USA Made, bakelite phenolic resin
Heavy-duty plastic with long-life contacts
Double-insulated Belden 8443
Strain relief, tube base:
Neoprene heat-shrink boot
24 inches
Made-in-USA banana plugs.
Spiral-Bound Pilot's Manual
1 Ohm Cathode Shunt, reads mV/mA
Shunt Accuracy: