by corndog71 Sun Apr 08, 2018 10:09 pm
Well, whenever you go off script you’re bound to discover new problems. When rearranging a tube amp into a custom layout it’s ridiculously easy to make mistakes. Before putting any power into it, I thought I had everything down. I’d let it sit for a day or two and then take another look. I also keep a drawing that I compare to the schematics because it helps me wrap my brain around everything.
Nevertheless I still made mistakes. Missed a wire here, and connected a wire or cap to the wrong tube pin there. Rinse and repeat. This morning I was feeling confident and decided to give it a test. I only have three 6550’s so half-power it is. I checked the heaters with the driver tubes first and things seemed to be ok. I even checked one side of the octal sockets with my meter and they were good.
I shut it off and plugged in a pair of the 6550’s and turned it on. One side started lighting up but not the other side. Then all kinds of noise poured out of my test speaker and I shut it off. Let it cool down a bit, swapped the tubes to make sure it wasn’t a bad tube. Same side wasn’t lighting up and then the noise came again and this time it blew the cathode fuse. Shut it down, pulled the power tubes and flipped it over to see what I missed. Ugh! Totally miswired one of the power tubes. I forgot to rotate it in my drawing from an earlier version. I can’t believe I missed it! I would’ve caught it had I check all of the sockets during the heater test.
I fixed the socket and decided to give it another shot. Started it up and checked the bias. Bias was good but then more loud noise kicked in. Out of curiosity I flipped it to triode mode and the noise stopped. Checked voltages and everything seemed good. I plugged my iPad into the input and played a song and it worked! Flipped it back to ultralinear and loud noise again. Triode mode works fine. Voltages from the output transformer plate and screen taps are right on. I suspect I damaged the tubes. Have you guys encountered this? I was going to get new tubes anyway.
For what it’s worth, the drivers seem unaffected by their proximity to the output tranny. In triode mode with the input shorted there’s just a hint of noise from the speaker which I have to put my ear up against to hear.