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Amplifier Tube Grommes 20LJ hums when powered on

Hello people!

I have an Amplifier Tube Grommes 20LJ, condition when the power is turned on, the left channel (L) is HUM very loud, the condition lasts about 3 minutes, then it stops, the sound is also smaller.

I have repositioned the EL84 and 12AX7 tubes, i have replaced the 0.047uf cascade capacitors, and also the volume control potentiometer, but the problem is still not resolved.
What should I do, please advise me!

Thank for all.

Sorry, I'm not fluent in English!
 

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I would check for a high resistance soldered joint in other words do a bit of re-flowing the components including the capacitors , that's a start especially capacitors connected to the left channel.

Looking underneath the chassis there are quite a few capacitors including a high voltage /high capacitance one .