Tube Safety Control; Automatic Status Indications; Manual Read-Out; What Do The Leds Indicate? - Hughes & Kettner TubeMeister 18 Manual

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Tube Safety Control (TSC)
TSC adjusts bias to improve the amp's tonal and
technical stability and extend power tubes' life. It
does this automatically and continuously, so there is
nothing for you to do.
Swapping tubes is more easily, quickly, and safely
done than with a conventional amp. This is a big
help not only in case of a defect, but also when you
want to compare different brands of replacement
tubes.
CAUTION: Replacing tubes is a job for qualified
professionals! TSC merely spares the technician the
biasing effort.

What do the LEDs indicate?

Each LED is assigned to the power tube occupying
that same position. Getting a read-out of tubes'
operating status and bias points is easy using any
standard guitar pick.

3.1 Automatic status indications

All LEDs light up and stay on.
All LEDs remain on for as long as the amp is in
standby mode. They will extinguish when you
flip the STANDBY switch to PLAY after about 30
seconds. If the LEDs remain illuminated, the most
likely cause is a blown anode fuse that needs to be
replaced by a technician. The anode fuse can trip if
a tube is already defective when the amp is switched
on, and TSC does not have enough time to measure
idle current and shut the faulty tube down.
None of the LEDs lights up.
The power tubes are operating normally.
One LED lights up continuously.
The tube assigned to this LED is producing under-
voltage. If the LED does not extinguish after a few
minutes, this tube must be replaced.
One LED flashes constantly.
The tube assigned to this flashing LED is generating
over-voltage. It has been shut down and must be
replaced by a technician. If the second LED lights
up continuously, this indicates it has also been shut
down for safety reasons, but there is no need to
replace it.

3.2 Manual Read-Out

TSC checks tubes' bias points to let your determine
if pairs match. This is easily done by inserting a
pick into the appropriate slot while the amp is on
(rather than in STANDBY mode). The LEDs will
flash. How many times the LEDs flash matters, but
what matters more is the difference in flash counts.
TSC will ensure optimum sound if the difference is
no greater than four flash signals. If the difference
in flash counts is greater than four, the device will
continue operating safely so there is no real need
to install a matched set. However, a matched set of
tubes will improve the tone.