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About 6 months ago a friend asked if I could re-tube and bias the new tubes in his DT50 amp, once he bought the tubes. He said that the amp was making a crackling sound and he had not had the tubes replace since he had bought the amp 3 or 4 years ago (I believe that was the timeframe). I said I could do that for him even though I'm not an amp repair tech but I do have an Electronics Technology degree and had done the same on my Carvin amp last year. He finally got around last week to bringing me the amp and new tubes but now said there was no sound at all coming out. Hopefully replacing the tubes would do the trick, he had both the power tubes and preamp tubes. 

   So I replaced all 4 of the tubes last night but still no sound, sort of. Here is what I have observed. All of the new tubes 'light' up except for 1 of the preamp tubes (V4??). The preamp tube furthest from the guitar input, farthest to the right looking from the front of the amp, does not light up at all or even get warm. I warmed the amp up for about 30 minutes and I rotated through all of the switch settings (Class A / AB, Pentode/ Triode, the 4 amp switch settings, above Pentode/Triode ), still the tube does not light up. I can get a very small amount of sound to come out if I turn up all of the knob settings to 10, including the channel volume and Master volume.

   I cleaned all of the tube sockets before putting the new tubes in place. I ohmed the offending preamp tube socket pins, front to back, and I'm getting continuity. I also checked the 2 on board fuses and they are good as well as the power fuse on the back of the amp for grins, it's good as well. The output transformer is cool to the touch, not sure if it gets warm or not.

     Any thoughts as to what might be going on?  Any help is appreciated 

Thanks

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Could be the digital program board. Going that long without a tube change,... may be a volt pull down somewhere.

You've gone as far as you can go. Needs to be serviced by a Line 6 authorized service center.

 

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