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11. TUBE BIAS SECTION – The Tube Selector switch is provided for setting the correct bias for the power
tubes used. EL DIABLO 60 ships from the factory with a matched set of Ruby Tubes™ EL 34 power tubes. The EL
DIABLO will also operate with 6L6 or 5881 power tubes. When using either of these tubes the bias should be set to
the 6L6 position. For harmonic complexity and maximum power, we recommend EL34’s. When switching the tube
types or brand of tubes, your amp should be re-biased to obtain optimum performance, tone and tube longevity.
SETTING THE BIAS – Generally your amp will only need to be re-biased if you are replacing the power tubes.
However, if you replace the Ruby Tubes® EL34 power tubes with the same brand and type having the same “PC’
number located on the label at the base of the tube then re-biasing is not necessary. To re-bias your amp you will
need a small screwdriver and a multi-meter or volt/ohm meter set to measure DC voltage. (If you do not have this
equipment, skill, experience or do not feel comfortable making these adjustments you should have a qualifi ed techni-
cian perform this procedure.)
1. First make sure the Tube Selector switch located in the Tube Bias section on the rear panel matches the
tube type installed. Then with the Master Volume turned all the way down turn the Power switch ON leaving
the Standby switch in the Standby position.
2. Set your test meter to read DC voltage. Then place the negative (black) lead of your meter into the black
Test Point located in the Tube Bias section on the rear panel. Then place the positive (red) lead of
your meter into the red Test Point. Depending on the size of your test leads you may have to hold them in
the Test Points to assure a good connection.
3. After the tubes have warmed up for about a minute turn the Standby switch to the Operate position and
read the voltage on your meter. With a small screwdriver you will need to adjust the Bias Adjust trim
pot located just above the Test Points so that your meter reads about 0.600 volts if you are using EL34
tubes and about 0.450 volts if you are using 6L6 tubes. Turning the Bias Adjust trim pot clockwise
will increase the voltage and turning it counterclockwise will decrease the voltage.
4. After initially setting the bias allow your amp to warm up for about 5 more minutes then retest and readjust
the bias as indicated above. Exact readings are not critical and some fl uctuation is normal. Just try to get
as close to the target voltage as possible.
5. Your amp is now biased and ready to play!
Tube Bias Background Information – All amplifi ers (solid-state or tube) require bias to work properly. In a single
ended amplifi er, the requirements are simple in that the tube should be “biased” or operated so that with no signal
present, DC current should fl ow through the output stage (tube and transformer) at approximately 1/2 the maximum
design value. When the signal transitions “positive” the current increases from the quiescent position and when the
signal transitions “negative” the current decreases from the quiescent position. So you can see that if there was no
bias, there would be no quiescent current, thus no way to operate properly for the negative “going” portion of the
signal, limiting the negative side of the waveform. Too much bias would limit the amount of current you could add for
positive “going” signals, limiting that side of the waveform.
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Pentode/Triode Background Information – The native mode of virtually all power tubes in guitar amplifi cation is
the power pentode. In a conventional pentode topology, fi xed bias amplifi er, the screen grid connects to the screen
supply via screen resistors and the screen potential varies (somewhat) with the audio signal thus increasing the
effi ciency of the amplifying process and maintaining more controlled linear high frequency response. There is a
sonic difference that is added by these additional control elements though, and some may prefer alternate modes of
operation.
Power triodes do have a mellowness and warmth that is a desirable tonal character for many players. A true power
triode has only an anode, cathode and one control grid. Without suppressor (and beam forming) grids there is a lack
of control of the electron stream, which can cause electrons to stray from the intended path and collide with
unintentional targets. This wastes energy and thus effi ciency but provides a unique tonal character that guitarists
seek. Fortunately, several methods exist that allow us to operate pentode power tubes in triode mode by fi xing the
screen grid more solidly to the plate supply (with the TRIODE/PENTODE switch), effectively eliminating the screen’s
“useful” effect from the circuit. This “false-triode” mode effectively makes the circuit (including the tube itself) behave
as a power triode. The high frequency character changes, the output impedance changes, and the circuit’s effi ciency
decreases just as one would expect from a power triode output circuit. This provides the mellower/darker
complexity of the amp in the Triode mode. This is a personal choice that is available to you on the El Diablo 60 and
provides you the fl exibility for further experimentation in tone.