Bias Testing Products

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, by applying a DC voltage to the tubes' control grid.   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 cooler, 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).

>>> To Go Directly to the Bias Products Section, click here <<<
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  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.



Tube Type



Plate Voltage



Power W

Plate Current mA
+/-10%		 Cathode Current mA
+/- 10%
 (BiasEasy™ 200, 400, 800 and 1200 readings)
- - -


>>> To Go Directly to the Bias Products Section, click here <<<

THE TABLE  BELOW OFFERS SUGGESTED REVISIT BIAS METERS
FOR VARIOUS NUMBERS AND TYPES OF POWER OUTPUT TUBES:

  TOTAL NUMBER OF POWER TUBES:
TYPE OF TUBES: 1 2 4 6
EL84, 6BQ5, 6N14N or 7189 Cathode biased - not adjustable RBE-800 or
RBE-1200 with
RBE-EL84-PROBESET
(for Blues Junior only)		 Cathode biased - not adjustable N/A
6V6 Cathode biased - not adjustable RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
6L6 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 RBE-200*, RBE-400*,
RBE-800*, RBE-1200*
5881 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
EL34 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
KT66 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
KT88 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
KT100 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
KT120 N/A RBE-200*, RBE-800,
RBE-1200		 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
6550 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 RBE-200*, RBE-400*,
RBE-800*, RBE-1200*
7027 N/A RBE-200*, RBE-400, RBE-800, RBE-1200 RBE-200*, RBE-400*,
RBE-800*, RBE-1200		 N/A
7591 N/A RBE-800-7591, or
RBE-800 or RBE-1200 with
RBE-7591-PROBESET 		N/A N/A
  * INDICATES THAT THE BIAS PROBES MUST BE MOVED BETWEEN TUBES IN ORDER TO MEASURE ALL TUBES.   THIS REQUIRES POWERING DOWN THE AMP AND ALLOWING THE TUBES TO COOL BEFORE MOVING THE PROBE SOCKETS TO NEW TUBE LOCATIONS.



 

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larger view

 Revisit™ BiasEasy™ 200
Cathode Current Measuring Probe
For 8-pin power tubes such as 6L6, 6V6, 5881, EL34, KT88, 6550, KT66, KT100, KT120, and 7027.   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


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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, EL34, KT88, 6550, KT66, KT100, KT120, and 7027.  Not for 7591 tubes (see below for special probes for the 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.

We also include "quick-start" instructions so that you can start using your new Bias-Easy right away!  See the quick-start instructions here.

The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character.
Part #RBE-800

$129.95

(Compare with the Tube Amp Doctor of Germany BiasMaster for 2 tubes, selling for €169 (euros) or $195 US dollars)

SPECIAL MODEL, ONLY USED IN A LIMITED NUMBER OF AMPS.   READ DESCRIPTION FOR IMPORTANT DETAILS!  



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larger view

 Revisit™ BiasEasy™ 800-7591

Important!   This unit only works with 7591 or 7591A power tubes, found only in a limited number of amplifiers!  Ampeg J12R Reverbojet, Gibson GA35RVT Lancer, Epiphone EA16RVT Regent, Premier Twin 8 (1961 through 1964 models), Hammond E and H series organs (including the H-100, H-200 and H-300), Leslie models 122 and 142, as well as a few other boutique amps.

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 7591 or 7591A only!     May be upgraded to work with other 8-pin power tubes by adding the RBE-PROBESET.

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.

We also include "quick-start" instructions so that you can start using your new Bias-Easy right away!  See the quick-start instructions here.

The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character.
Part #RBE-800-7591

$129.95

(Compare with the Tube Amp Doctor of Germany BiasMaster for 2 tubes, selling for €169 (euros) or $195 US dollars)



7591 Tube Probes

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larger view

 Revisit™ BiasEasy™ 800 and 1200 Probe Set for 7591 and 7591A Tubes ONLY
As noted, the Bias Easy™ 800 and 1200 models with standard probes cannot measure the bias current in 7591/7591A power tubes, due to a difference in the pin assignment (cathode on pin 5 instead of the usual pin 8).  With the RBE-7591-PROBE SET, you can now use the Bias Easy™ 800 and 1200 units to measure the current in 7591 type tubes power tubes, and properly set their bias.

Very few bias meters offer a 7591/7591A tube option.

Includes two 8-pin probes with gold plated pins and socket.  FOR USE WITH 7591 TUBES ONLY!   These probes are clearly marked '7591'.

A short list of a few amplifiers which used 7591 tubes in the output section:   Ampeg J12R Reverbojet, Gibson GA35RVT Lancer, Epiphone EA16RVT Regent, Premier Twin 8 (1961 through 1964 models), Hammond E and H series organs (including the H-100, H-200 and H-300), Leslie models 122 and 142, as well as others.   New replacement 7591 tubes are still being manufactured for these amplifiers, and are available from both Electo-Harmonix and JJ Tubes.

The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character.
Part #RBE-7591-PROBESET

$47.95

(Set of 2 probes for 7591 tubes only.  These probes work only with the RBE-800 or RBE-1200 models.)


 

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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, EL34, KT88, 6550, KT66, KT100, KT120, and 7027.   Not for 7591 tubes (see above for special probes for the 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.  We also include "quick-start" instructions so that you can start using your new Bias-Easy right away!  See the quick-start instructions here.

We also include "quick-start" instructions so that you can start using your new Bias-Easy right away!  See the quick-start instructions here.

The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character.
Part #RBE-1200

$168.95

(Compare with the Tube Amp Doctor of Germany BiasMaster for 4 tubes, selling for €199 (euros) or $230 US dollars)


Tube Probes for 6L6, 6V6, 5881, EL34, KT88, 6550, KT66, KT100, KT120, or 7027

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larger view

 Revisit™ BiasEasy™ 800 and 1200 Probe Set for 8-pin power tubes such as 6L6, 6V6, 5881, EL34, KT88, 6550, KT66, KT100, KT120, and 7027.  Not for 7591 tubes (see the RBE-7591-PROBESET for special probes for the 7591 tubes).

Includes two 8-pin probes with gold plated pins and socket.  FOR USE WITH 6L6, 6V6, 5881, EL34, KT88, 6550, KT66, KT100, KT120, and 7027 TUBES ONLY!   These probes are clearly marked 'EL34 6L6 KT66'.

The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character.
Part #RBE-PROBESET

$47.95

(Set of 2 probes for 6L6, 6V6, 5881, EL34, KT88, 6550, KT66, KT100, KT120, and 7027 tubes only.   These probes work only with the RBE-800 or RBE-1200 models)

EL84 Type Probes

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larger view

 Revisit™ BiasEasy™ 800 and 1200 Probe Set for 9-pin power tubes such as EL84 or 6BQ5.  

As noted, the Bias Easy™ 800 and 1200 models with standard probes cannot measure the bias current in EL84, 6BQ5, 6N14N or 7189 power tubes.  With the RBE-EL84-PROBE SET, you can now use the Bias Easy™ 800 and 1200 units to measure the current in EL84 type power tubes, and properly set their bias.

Includes two 9-pin probes with gold plated pins and socket.  FOR USE WITH EL84, 6BQ5, 6N14N, OR 7189 TUBES ONLY!   These probes are clearly marked 'EL84'.

Note:  Most amps which use EL84 type tubes are designed for cathode bias of these tubes, which is a design that allows for a more-or-less 'automatic' biasing of the tubes.   The only common amplifier which uses EL84 outputs which are biased by applying a voltage to the tubes' control grid is the Fender Blues Junior.   Although the Blues Junior does not have a bias adjustment control from the factory, the bias can be changed by changing the value of resistors in the bias supply section of the power supply.   An even better idea is to install a bias adjustment pot to the Blues Junior.   See this page for the pot and instructions.

The bias setting has a dramatic impact on the amp's tone, response, break-up and dynamic character.

Part #RBE-EL84-PROBESET

$47.95

(Set of 2 probes for EL84, 6BQ5, 6N14N or 7189 tubes only.  These probes work only with the RBE-800 or RBE-1200 models.)


Arriving in mid-July!

 

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 Revisit™ BiasEasy™ 400 Dual Tube Analog Amp Bias Tester
The Bias Easy™ 400 measures the current in the power tubes of your amp.  Designed for amps with two 8-pin octal power tubes.  Includes two 8-pin probes with gold plated pins and socket, and dual analog meters which measure 0-100mA of current.  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.

Well-built, rugged and dependable, the RBE-400 is an accurate, economical method to measure tube bias on amps with octal power tubes.

The Bias Easy™ 400 has the advantage of dual simultaneous readings on both power tubes ... no switching back and forth between tubes to compare readings.   Comparable to the Alessandro Bias and Matching Meter, which sold for $149.95.

The Bias Easy™ 400 allows you to monitor the bias setting safely and easily. While adjusting the bias control pot of your amp, the Bias Easy™ 400 shows the actual current flowing through each of the two power 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-400

$74.95


Click on photo for
larger view		 OCTAL TUBE KEYWAY, SET OF 2
If you have a cherished bias probe with a broken keyway, here is a great solution. By cleaning the remainder of the broken keyway from the tube base and properly installing this keyway replacement (a little dab of silicone sealant works great to hold it in place), your tube can safely be reinstalled into your equipment.   These great tube-savers are sold in sets of 2.   Note:  When using an adhesive to secure the keyway to the tube base, DO NOT use a water-based glue.   Silicone sealant (RTV) which is acryllic-based or butyl-based is fine.   Latex sealant or hot-glue is NOT.
 Part #OTK8-2		 $8.99
(for a set of 2 tube keyways)

 
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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 RBE-200 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

     

    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 Fenders 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.  To modify these silverface Fender™ amps to the blackface standard, see our blackface modification kit here.  

    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.
     

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