Sunn 1200s Troubles
Hey all, I'm hoping some of you with more technical knowledge of valve amps can help diagnose some issues I'm having, or at least point me in the right direction. I bought a Sunn 1200s (this is an old one, not the Fender Hybrid) in quite good condition in June and it was shipped up from oklahoma. One of the transformers came loose, but otherwise, it seems to be a great specimen. Used it through the entire summer, no issue. Everything worked fine. Towards the end of the summer I noticed I would get feedback on the reverb if I turned it much past 4. Wasn't too worried, figured it just needed to be serviced. Shipped out East with me for school, but when it arrived, something had gone quite wrong. I couldn't turn it past 2 on the volume knob without a very loud buzz and a significant decrease in output, and the only signal would be very distorted. So I took it into a tech, had him replace a few caps, and the power cable, check the reverb, secure the transformer and see what was wrong. Got it back recently, and it's still messed up. Here's what I've figured out:
-When all controls are set flat, the volume knob can be turned up full with no issues.
-the bass knob can be turned up with no issue.
-the treble & contour controls seem to be causing issues.
-Setting the amp volume at 5, I can turn the treble knob to about 5 before any noise with the contour set flat.
-Same goes for the contour knob. With treble at 0, it can go to about 5
-if I set both treble and contour to about 2, it's fine, but if I adjust either one any further, the volume drops and becomes distorted.
-this occurs regardless of what input is used.
-the reverb issue crept back in after about a day (that might be the tube driving the reverb going a bit microphonic), though I don't think that's related to the other issue.
Anyone experience anything like this before?
I have had a bad (open) contour pot cause similar trouble as one "leg" of the pot should be at ground. The unit did not have reverb as the tank was damaged and was used without so I don't know if it is related.
If it were me, I'd go right back to the tech and tell him to fix it for real; otherwise swap out the preamp and reverb driver tubes(the 12ax7's)as well as the 6an8 to check if there's problems there. Do you actually use the reverb? Sunn 'verb circuits can be fussy, so try unplugging the RCA cables in back to eliminate any additional contributions to the problems. You can also go here:
Oh yes, I'm well aware of the Sunn forum (though I haven't been able to log in in over 2 months). I will be taking it back to the tech, but I'm flying home to the west coast tomorrow for Christmas break, so I don't have a chance to take it back in until January, but the tech said he'd gladly set it right when I do get it back in. I've also emailed Conrad Sundholm about the issue, see what his take on matters are. Thanks for the help guys, any additional thoughts are welcome!
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