After testing again, the A tube is still with electric current (bulb is light up) when the amp is "off" (with power switch on) after playing a few songs.
while B , the signal light for power is off when the amp is "off" (with power switch on) at the same time
the another circuit board connecting the main power supply is totally no current (as see from the other 4 6922 tubes, no light up)
I wonder - would it be the capacitor ? i dun know which one, is defective / blow off, so at the start the amp can still play but after a few songs it would die down.
KK has sent me message for help, but need to check after CNY, I just 無聊 and see if anyone can diagnose my tubeamp sickness through these limited info.
Thanks admin