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I am at a loss with this. I've been rocking with this amp for a while now and absolutely love it. Everything is perfect with the Plexi except I can't jumper the TMB and Normal channels without a great deal of hum and I cannot seem to locate where this hum is coming from. I could continue to run it the way I am, using only one channel, but it bugs me knowing that something isn't right.

With nothing plugged in, the TMB channel and the Normal channel are very quiet with the normal white noise at full volume. Plugging one cable into either of the TMB inputs alone has no affect, still very quiet. Plugging into either of the Normal inputs alone has no affect, still very quiet. However, when I jumper them, Normal High to TMB Low or Normal Low to TMB High a very loud hum is created. Anything above 1 1/2 on the Normal channel becomes unusable.

I have removed the Normal channel input jacks and rewired them. I have removed and resoldered the ground. I think it has to be a grounding issue but I can't find it anywhere.

A couple of other things. With no jumpering and only one cable being used, when I remove the cable from the Normal High input it creates a hum for a couple of seconds, sometimes longer, and then goes away. When I remove the cable from any of the other 3 inputs there is no hum at all. Also, one of my EL84's is glowing a lavender/purple in the middle of the tube on only one side. I'm guessing this is not a good thing so I am getting another set. I don't know if this would have anything to do with it or not as I would think a bad tube would affect both channels equally.

So any ideas on what else I need to do would be appreciated very much.

Sounds like a ground loop between the two channels when they are connected. If you havent, you can try a separate ground wire from each jack to the pre-amp ground post.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I added the ground wires from the low inputs to the preamp ground post and still has the hum, no improvement or worsening. Does it matter which lug I used on the input jacks? The other grounding wires were installed as drawn here during the initial build.

You cant have two for each jack pair. That would make it worse. Use one per pair and connect it to the jack pair as per the original layout. That's what I was suggesting.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps