26
SECTION 14 – Set the Bias of the Power Tubes
Please refer to Drawings 23 - 25. You should insert all vacuum tubes into their respective sockets at this
point.
Step 1 – Measure the resistance of the cathode resistor that will help us bias the amp
Drawing 23 shows this being done.
A) Make sure the amplifier is turned off and unplugged.
B) Connect the DMM alligator clips to the two points indicated in the drawing.
DMM “COM” connects to the negative lead of any one of the 22µF @ 500V filter
capacitors (or any other ground point).
DMM “Ω” connects to V3 cathode.
T14
V3
cathode
C) Use your DMM to measure the resistance here and take note of it for the next step.
R = _________ Ω
Step 2 – Set the Bias Point of the Power Tubes
Drawing 24 should be used for biasing 6L6GC’s.
Drawing 25 should be used for biasing EL34's.
A) Do all of the following:
Connect the amp output to a matching load for the impedance selected.
Turn the Bass, Treble, and Volume controls to zero.
Turn the bias pot all the way to cold.
Plug the amp in.
Turn power on, and wait a minute before turning the standby switch on.
Keep the DMM alligator clips connected to same points as in step 1, but now set
it to measure “V”.
B) Use your DMM to measure the voltage in milli-volts here after about 10 minutes
and take note of it.
V(cold) = _________ mV
12AX7/
ECC83
V1
12AT7/
ECC81
V2
V3 V4
6L6GC or EL34 6L6GC or EL34
If you smell or see smoke, hear something pop, or the chassis becomes too hot to touch, turn off
power and unplug immediately.
To Next Page
Loading...
