Exemplar-Denon 2900 tube audio output not working

Sometimes, not always, I have problems with the left tube audio output channel of my Exemplar-Denon 2900. I hear noise and not music. Meanwhile the right tube audio output channel sounds fine (in fact, it sounds great). Today I opened the cd-player and I changed the tubes and the problem remains the same. When I opened the player I could notice that the board where the tubes are placed on has two pairs of red led lights, one pair in front of each tube, when connected, one pair of the red leds light is on but the other one isn´t.

One more thing, the solid state left output channel sound OK. In fact, I connect the right channel to the right tube output and the left channel to the left solid state channel so I can get a a part of that great tube sounding John Tucker could achieve.

If anybody has any idea on how to solve this matter, please let me know. I emailed John Tucker, of Exemplar, twice and had no reply at all