Anode Loading Chart Note that this loading is for grounded cathode designs of class A or AB amplifier (ie: fixed
bias). It cannot be applied to class B designs.
Enter the anode voltage used 440 V
Maximum design anode rating 25 W e.g. 25W for EL34
10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110% 120%
Heat Meter 6 11 17 23 28 34 40 45 51 57 63 68
Warnings….
If you don't know how to alter the bias in a valve amplifier, or have not worked with valve or other
high voltage equipment, please seek skilled advice from a repair technician or electronics
engineer. If you're still not sure, walk away….. Preferably in the direction of a qualified repair
technician.
The information above is a guideline only, and is based on "classic" designs from valve
manufacturers. Individual implementations may vary, and the manufacturer of the amplifier
should be regarded as the definitive source of bias current information for their product.
How to use…
Type the anode voltage (B+) of your amplifier into cell G4. Type the maximum anode wattage (plate
loading) into cell G5.
What you see…
The Heat Meter shows the percentage of loading at idle for various bias currents in a loading range
from 10% to 120%. Most designs are biased to work in the green area.
Operation in the ice blue area will lengthen the life of your output valves, but may lead to crossover
distortion. Operation in the red area exceeds the manufacturers specification, and will cause your
amplifier to run hot, potentially causing damage to valves and other components.
Try to keep out of the orange area, as line voltage fluctuations may result in variations after the
bias values have been set, although many class A designs target this area in order to maximise
output power.
Further information from:
http://www.eden.com/~keen contains the Tube Amp FAQ with lots of useful tips on measuring
bias current.
Comments to: duncan@muffy.demon.co.uk
Duncan Munro - 29/5/97.
ode designs of class A or AB amplifier (ie: fixed
mA
anode wattage (plate