The amplifier came with a note that it doesn't turn on. Control lamps remain dark after flipping the switch. The reason for that was damaged switch. It made a click noise but there was no connection. Establishing reliable connection requires force to applied on the lever in side direction, then the control lamps lit but there was no signal coming through the amplifier. After flipping the standby (Vollbetrieb) switch, one could hear that the power stage was working because it wasn't complete silence, it was typical quiet noise of a working tube amplifier.
For this amplifier there is no circuit diagram available, but there is one very similar (Superem Luxor 40
https://elektrotanya.com/suprem_luxor_40_sch.pdf/download.html ) which except for number of inputs and tubes reference designation is the same. I checked heater resistance of all tubes but they all appeared fine.
Keeping in mind that tube amplifiers require connected speaker I measured all supply voltages including bias and they were correct.
I observed that one of the ECC83 tubes look different, see the middle tube on the picture:
See milky deposit in the top part of the tube which lost vacuum, it should be uniform, shiny surface.
In a normal tube there is shiny surface on the inner part of the glass, above a getter. (see
https://en.wikipedia.org/wiki/Getter ), the middle tube on the picture has no shiny surface but a milky deposit. This is a sign that the tube has lost vacuum, and that particular tube drives the end stage of the amplifier. This explains why the amplifier was mute. I have no vacuum tubes in my workshop so I shuffled other tubes in the way that there was no signal for vibrato but all input channels should work. And it works. It sounds pleasant with a bass guitar. I ordered replacement tube and will build it back together soon.
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