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68.12.20.39
In Reply to: Death of a 6C33C-B Power Tube posted by PNZ on March 30, 2006 at 20:03:41:
I have enjoyed an vk-60 for four years and have had three-four tubes 'go out'. Your report of intermittent faint popping for a month or so preceeding the loud thump and bias light going off mirrors my experiences perfectly. With a replacement the light comes back on and the filaments light up. I have discussed this with Mr. Khomenko of BAT and he stated that if the bias lights come on then generally all is well with the amp. I did have a socket go bad/wear out and the bias light would come on but only 1/2 of the 6c33 filament would light up. You should be OK :)FWIW, When the amp arrived (2nd hand) with worn out tubes I witnessed one glowing BRIGHT orange and that socket would not stay biased, which is how people describe failures in other tube types, but this seems to be the exception for me.
Follow Ups:
Thank you for reply -- but I am now beginning to doubt if there is not another underneath problem or 'collaterial damage' involved. Last night, I played the amp for about 5-7 minutes. Bias light is on and fine -- but the bass-like, 'thump'/two came back/re-occurred and with a very slight acrid smell. This tells me that perhaps there were either two separate occurances/problems or collaterial damage when the 6C33B-C failed. All in all, I believe that it is likely that a resistor was stressed previously, started to fail over time and with the tube failure?, the outside covering of the resistor started to wear/burn, causing the smell. So, I think that I may have a bad/worn resistor problem also -- which should be localized on the board in the area of the bad tube/bias light etc. What are your thoughts? I turned the unit off immediately -- I did not want to risk further problems if indeed I had a bad resistor.
Bummer. I am not sure exactly what is going on but would suggest replacing the other output tube on the side of the 'thump' (if you can localize the problem to one channel) and see where that gets you. When a tube fails in this design extra stress is placed on its partner to drive that channel so you may have fried both power tubes in a side. It is possible that a resistor burned or something worse but the amp has fuses to protect other components and that it plays and biases seems promising. If that does not work then it probably should go into BAT (they are great and very fast on service), but the expense and difficulty of shipping such a beast has always made me a little more adventurous about troubleshooting it at home first. Best of luck.
I think it's possible that the failure of the first tube damaged some upstream circuit component. One could speculate all day long re what might have happened exactly, but in the end you are probably going to have to have the amp serviced by a competent tech, best of all by BAT.
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