MichaelvanWeert

Thanks for the tip! It's quite a drive from my place, but they supply a 2 year warranty on their services, that rocks! "He would have to be an authorized service center to get access to the service manual." I Googled it, found a few schematics. Not sure if those are the official technicians guides or anything of that order, but there are some hefty scheme's and algebra in them.

I'm not sure if there's such a thing as a 'Line6 certified/authorized tech' in the Netherlands, we only have like have a dozen ampdoctors as it is. I'm not a technician myself, I only know what the tech told me. He fixed my amp so I'm sure he knows what he's doing. If his conclusion was wrong however, then I'm just the messenger who doesn't know any better :) The powertubes had been replaced by tubes suggested in the service manual. I assume they are biased (I wasn't in the room when the tubes were replaced).

Hi guys, I recently ran into some issues with the power section of my MKI 212. The tubes had been changed less then 10 playing hours before this issue occured. My master volume was all over the place, any slightest touch of the potmeter would blow the volume in any direction. Even with the channel volume turned almost down, the (master)volume would fluctuate from dead silent to earbleeding loud with hardly moving it. So, I brought the unit to an ampdoctor for repairs, turned out the masterpot was physically worn out...because of the channel volume pot. He explained to me what has been explained in this thread already; channel volume should be low at all times, so that the masterpot can be turned way up. The thing is, the channel volume is a 'digital' setting, it's part of the moddeling infrastructure. The masterpot however gets an actual load to bear, the more it's turned down, the more tubepower/current it has to 'block'. So turning the masterpot down will wear it out sooner. It was an expensive repair, the tech had to build a new potmeter by hand (no official Line6 replacement part available in the Netherlands unfortunatly). Not sure if this direclty relates to OP's issue, (probably irrelevant since it's been 3 years) but maybe my case helps on other mastervolume-related issues :)