5
Bias current and DC-Balance pre-adjustment procedure
Ensure the AC power cord is disconnected from the unit and the power switch set to the “off”
position: lower half of switch recessed.
Verify that the fan’s air filter is properly installed (bottom chassis plate).
Install tubes as indicated on the bottom chassis plate. The location numbers are marked on each
tube. ( i.e.: V8L or V8R ). Note that the tubes are matched for the left and right amplifier individually.
( Right = Red RCA, Left = White RCA ).
Ensure the speakers are connected to the output terminals.
Set the amplifier to mute.
Set the DC-Bal. control (left knob on the top of the unit) to “Ref” or 12 o’clock position.
With a small screwdriver, adjust V7 and V8 bias control potentiometer (located below the left rotary
dc balanced knob) to the middle of the adjustment range.
Repeat with V9 and V10 (located below the right rotary bias knob).
Connect AC power cord, audio interconnects, then set the power switch to “on” (top half recessed)
and run the unit for 30 minutes. The power indicator light will change from red to green indicating the
end of the one minute soft start cycle.
BIAS and DC-BALANCE adjustments
Note:
the onboard meter is operational in mute mode only.
1. Ensure that the amplifier is in mute.
2. Set the bias rotary selector to
V7
and with a small screwdriver adjust slowly
V7
bias
potentiometer to read
+300mA
on the meter. The normal operating bias current for each
6C33C-B power tube is
+300mA
;
the meter range is
0 to +500mA
.
3. Set the
bias
rotary selector to successively
V8-V9-V10
and perform the same adjustment as
step 2 above.
4. Repeat steps 2 and 3 until
V7, V8,V9 & v10
read
300mA
. Because of the specific nature of the
6C33C-B power triode, this may take some repeating.
5. Set the
Bias
rotary selector to
DC-Bal.
and read the DC imbalance on the meter. If step 4 was
precisely executed the DC imbalance should be less than
+/- 50mV
.
6. Turn the
DC-Bal.
control knob slowly (left knob on the top of the unit) in order to read
zero mV
on the meter. The meter range in this mode is
+/- 250mV
.
7. Set the
bias
rotary selector to off.
8.
After 2 or 3 hours of listening, check the meter readings and readjust if necessary.
ADDITIONAL IMPORTANT NOTES
During the first 50 hours of operation, it may be necessary to periodically check and adjust the bias
current and the DC-Balance after 2 to 3 hours of operation. The 6C33C-B tube takes approximately
50 hours to completely stabilize. After this initial period, the amplifier will remain stable (less than
+/-
5mV
imbalance) even for long periods of time. To obtain maximum tube life and ensure optimal
performance, a monthly check is recommended.
Although Tenor amplifiers are exceptionally stable on low impedance loads, speakers with difficult or
reactive loads below 4 Ohms will require the output stage to deliver more than twice the normal
current and consequently risk to overload and damage the output tubes.
