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After trying all the might be tried with 6C33C (measurement-wise) I concluded that I know that tube very well …in order do not test it. All that I do is taking a new tube, burn it until the start up current is stabilized:

http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=6140

,then place in amp, set cruse current, turn on my tuner 1000Hz signal generator, set my preamp to reference attenuation level and measure with a dB meter what my MF channel outputs on the horn’s axis  and at mouth plane.  The R and L channels shall be with .25-.5dB (less is better) between each other and within .25-.5dB in relation to the reference point of my next channel. If I have more of less dB then I change the tube to another one. With some experience I can match even non-burned tube but it is tricky.

Laurent, your problem looks to me as you have faulty contacts in the amp sockets. True violently to swing all 3 regulator tube in the socket and to see which of them is faulty.  Yes, one more thing – do not substitute the fuses for larger current – VERY bad practice. This amp was not designed by  an idiot and the currents in there and the fuses in there as they shall be. If the amp blow fuses then it is good – it gives to you an opportunity to search and to find a problem.

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