deck next to the tube that caused the fault shutdown, to make diagnosis and tube replacement easy. The new
tube is then automatically re-biased to the prescribed level on the next power up, requiring a minimum level
of user knowledge about the operation of the amplifier.
Looking down at the deck of the amplifier through the top cover, there is a green LED visible next to and
behind each 6550/KT-88 output tube. These LEDs are for identifying the output tubes, and if not
suppressed are normally lit when the amplifier is powered on and in the operating mode, and sometimes
blink on and off to indicate bias status.
In the case of a tube fault, whether normal bias LED activity is suppressed or not, the relevant tube’s LED
flashes quickly to identify the tube that needs replacement, and during diagnostic functions these LEDs are
also used to identify the tube(s) being tested.
The top deck tube LED indications are as follows:
Output Tube LED Status
Indication
Amplifier is in idle mode with no signal Output tubes are operating at optimum bias setting,
S-400 Amplifier Owner’s Manual
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