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Biasing is a topic that
always comes up in conversations with players when talking about
amp maintenance. I will attempt to give some tips on biasing Dr.
Z amps.
First, all of my EL-84
amps are “self biasing,” so no adjustment is needed
when installing new EL-84 power tubes. All you need is a very closely
matched set of new 84s, plug them in and start to play. As for current
production EL-84s I prefer JJ/Tesla brand. They are, in my opinion,
the stoutest, longest lasting, well built 84 currently available.
For the last five years I have been shipping new amps with Groove
Tubes EL-84-S tubes, and the reliability has been great. The EL-84-S
is a Tesla EL-84, and the rating I select is a #6.
Before I continue with
simplified bias setting instructions on other Z-Amp models, maybe
an explanation of the need for bias adjustment is in order. The
simplest way to think about biasing output tubes is to use an analogy
of your car’s idle adjustment. To optimize the performance
of your car, the idle speed in RPM must be adjusted correctly. If
it is too high you’ll need to apply more pressure to the brake
pedal when stopped at a light, RPM’s will be too high and
your car will want to race. If the idle is set too low, the car
will want to stall because the RPMs are too low. This is the same
with your output tubes. Bias too high and the amp will sound a bit
harsh and distort too quickly, and will lose all focus at high volumes.
If the bias is too low the amp will sound cold, brittle, and at
higher volumes will have excessive crossover distortion, will sound
dissident, and your guitar will sound out of tune.
The next three amps I
will discuss are “Fixed Bias” amps, meaning the bias
is FIXED by adjusting a pot on the board.
DISCLAIMER: Tube amplifiers
contain potentially lethal, high voltages even after they are unplugged,
that may cause personal injury or death. Do not attempt to repair,
modify, or work on any amplifier unless you are absolutely certain
you know what you are doing. If you are not absolutely certain about
what you are doing, please consider the following a reference sheet
for your amp tech.
The “Route 66”
amp normally ships with Groove Tubes KT-66-HPs with a rating of
#5 or #6. To adjust the bias we will try and set each tube’s
idle current (or plate current with no input signal, Power and Standby
on) into a fixed load measuring from each plate PIN3 to Center Tap
of Output Transformer. This is called the Shunt Current Measurement,
because you are shunting your Current Meter across high voltage.
This is a dangerous setup and SHOULD ONLY BE DONE BY EXPERIENCED
TECHNICIANS. I have found an easier way to dial in the Bias, and
it will bring the correct operating bias point to within 97% of
optimal.
The safer method is to
monitor the Center Tap Voltage, located on the RED and WHITE STRIPED
wire that attaches to the Multi Can Capacitor. Your volt meter would
be set to DC range. Ground your Common lead to the chassis and carefully
measure the RED/WHITE striped wire with your highest DC Voltage
range. Adjust the bias pot till the Voltage at the RED/WHITE striped
wire is at 475 Volts DC. If you are using Groove Tube KT-66-HPs
rated at #5 or #6 with a 5AR4/GZ-34 rectifier in the power supply
then the voltage reading of 475 volts should be the optimum setting.
Remember to have a fixed resistive load, or if not, a speaker will
do to properly adjust the bias.
The KT-45 is the same
basic setup and adjustment as the Route 66, with one small deviation.
KT-45s can have a Solid State Rectifier or a Tube Rectifier; it
is a matter of taste. The Tube will soften the amps response and
lower its overall wattage. With the Solid State Rectifier installed,
adjust the bias pot for 480 Volts DC at the Center Tap. With a 5AR4/GZ-34
Tube Rectifier, set the center tap for 440 Volts DC.
The final Fixed Bias
amp currently in production is the Z-28. It uses a different color
coded Output Transformer so instead of the RED/WHITE striped wire
from the Center Tap, it is a solid RED wire, and it is placed in
the same location. Perform the set-up as noted above, with optimum
voltage set at 375 Volts DC. This will be correct if Groove Tube
6V6-R tubes (with a rating of #5 or #6) are installed with a 5AR4/GZ-34
Rectifier in the power supply.
These methods are used
on my bench. I also spot check for correct current settings, then
“ear test” the amps by playing them to assure correct
operation.
For those interested
in bias current, the settings are in the ranges below:
Route 66 set
between 34 and 38 mA per tube.
KT-45 set between 28 and 32 mA per tube.
Z-28 set between 24 and 28 mA per tube.
DR.
Z
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Straight
From Dr. Z: Bias Notes