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.
Tube Type
Plate
Voltage
Power W
Plate
Current mA
+/-10%
Cathode
Current mA
+/- 10%
(BiasEasy™
200, 400, 800 and 1200 readings)
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 (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
$139.95
Includes all
tariff costs
Only 1 RBE-800
left in stock! More arriving mid-July.
(Compare with the Tube Amp Doctor of Germany
BiasMaster for 2 tubes, selling for €169 (euros) or $195 US dollars)
7591 Tube Probes
Click on photo for
larger view
Revisit™ BiasEasy™
800 and 1200 Probe Set for 7591 and 7591A Tubes 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, 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
$54.95
(Set of 2 probes for 7591 tubes only)
Includes all
tariff costs
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 (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
$199.95
Includes all
tariff costs
(Compare with the Tube Amp Doctor of Germany
BiasMaster for 4 tubes, selling for €199 (euros) or $230 US dollars)
More arriving mid-July!
Click on photo for
larger view
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.
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
$89.95
Includes all
tariff costs
Arriving in mid-July!
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)
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
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.
All company names, logos and product names mentioned on these pages are trademarks of their respective owners.
WARNING: These products may contain chemicals known to the State of California to cause cancer and birth defects or other reproductive harm.
For more information go to www.P65Warnings.ca.gov
Revisit™ BiasEasy™
800
WARNING: These products may contain chemicals known to the State of California to cause cancer and birth defects or other reproductive harm.