Ashdown CTM-200-RACK, User Manual

Page: 4 / 22

ASHDOWN ENGINEERING - CTM-200-RACK
USER GUIDE USER GUIDE
CTM-200-RACK
THE VU METER
The VU meter fitted to the CTM is a dual function meter detailed as follows:
By using the push button selector switch marked “AUDIO / BIAS” the functions are
accessed in the following manner:
1/ “AUDIO” - In audio mode the meter is set to read the audio output of each of the
six power tubes and each can be measured by utilising the rotary selector switch
on the front panel. Whilst playing this is the normal mode of operation of the meter
and the reading obtained will be a direct indication of the power being produced
by each output tube, the meter is calibrated so that when the needle reaches the
start of the red section the power tube being measured will be producing maximum
clean volume.
The switch can be rotated to select any of the power tubes and its relative output
will be indicated on the meter, note that when performing this test you will need to
play at equal volume for each test.
2/ “BIAS” - In this mode the meter will glow RED.
To check the bias of the output tubes proceed as follows:
Run the amplifier for at least 10 mins to allow the output tubes to fully heat up with
the amplifier set to mute. Standby on. Select.. Tube 1 and with an insulated jewelers
screwdriver gently adjust the trimmer situated on the rear panel. The reading to aim
for is the needle just at the start of the red ie 0db.
Note that although each tube can be adjusted individually it is possible to get tubes
that cannot be adjusted to the correct reading, these can still be used providing the
reading is not in the red, a lower reading is acceptable and will note impact audibly
on the power output or sound quality of the amplifier.
Please also note that the trimmers are 25 turn types to make adjustment easy, This
type of adjuster has no end stop so if you reach a point where the adjuster will
not give any more adjustment then the output tube will have to be replaced if the
reading is to high ie in the RED.
Repeat this for each of the four output tubes in turn.
RETURN THE METER TO “audio” MODE AFTER BIASING. DO NOT RUN THE
AMPLIFIER IN BIAS MODE WHILST PLAYING.
Note there is a protection fuse fitted to each of the output tubes and if no reading
is obtained on the meter this fuse will have failed. The protection fuses are situated
next to the output they are protecting the correct rating is 315Ma quick blow, in the
event of a blown fuse they MUST be replaced with fuses of the correct rating and
type.
A continually blowing fuse would indicate either an incorrectly biased output tube
or a defective output tube.