DC-3 issue

Hi all. I currently have a DC-3. Great amp but it has had an issue the last month or so. The problem is two of the power tubes keep lighting up far more than they should. This causes the volume to drop back and is also causing amp to blow fuses. This is intermittent. I gigged with this amp last weekend without any problems, then sitting down at home to practise for a while it happened after about half an hour. I can switch it to stand by and issue disappears temporarily. I did this twice in the space of about 15 mins then turned amp off then back on again and it blew a fuse, it has done several times over the last month or so. If anyone has any idea what is going on your help would be appreciated. I have changed tubes and had amp in shop to check for dry solder joints without any luck. Cheers J

I know you just said you replaced your power tubes. The shop gave you a shitty set if they are already red-plating. Based on your description, they sound like the are not matched. Your power tubes are failing and need to be replaced. If you did not replace all of the tubes (pre-amp too), that is a good idea. I have had a few different sets of brand new matched Quads red plate in my DC3. Mesa biased this amp VERY hot from the factory. I would take it back to the shop and make them fix it.

I had a local shop re-tube my DC3 a few years ago (last time I let them touch my amp) with a Mesa matched quad of EL84s. The inner left, and outer right started to red plate badly along with a loud whooshing sound. They had to use a set of JJs and a JJ P/I tube to get it working right again.

Matched quad of JJs hold up the best. If you perform the 100K -bias mod (replace the 120K bias resistor with a 100K, or piggyback a 470K on the 120K) you can get away with a lot of other brands of tubes as this will lower the -bias supply and run the tubes a bit cooler. I have been using Mullard re-issues and I like them a lot. Smooth and really creamy.


Also, replace your P/I tube (V6). I use a JJ 12AX7 here and have never had any issues. Balanced triodes are a good idea here.

Hey thanks for that mate, what your saying makes sense. Bias mod sounds like a good idea as the last of tubes i had in amp were JJ's and it cooked them so will sort that. Thanks again

look like grid current fade sometimes , this makes power paired power tubes glowing and blowing fuse because over tension!