So, after getting the bias sussed out, I set to work on the low output issue. I'd had the thing in an intermittent state earlier (normal volume for a few seconds, then super-low volume and almost no overdrive), which made me think I probably have all (or at least most of) the wires and components where they needed to be - if something were REALLY wrong, the thing would be blowing fuses, smoking, or otherwise NOT putting sound to the speaker.
At that point, I started focusing on making sure all the components were still in spec and checking voltages on the plates, cathodes, and other areas of the circuit. I'd been pretty concentrated on a problem I was picking up with the PI (V3) and the cathode follower (V2), and was convinced that some component must be completely out of whack. I reached out to the facebook group, Mojotone, and finally started a new thread here (
viewtopic.php?f=5&t=54520) and found out that the voltages on the Mojotone schematic were incorrect. Zozobra pointed me to a better diagram with correct voltages, and was able to confirm that my amp is working a lot better than I was thinking.
...and yet, the amp was still not functioning. I got my meter and went through every component I could test, and nothing looked wrong. Grabbed the soldering iron and reflowed every connection on the circuit board, and a lot of the wired connections to the various tubes, etc., and still nothing.
Had someone question my input jacks again, so I decided to wire up an entirely new set of jacks, using new components, and connected it in using jumper wires. The symptoms remained EXACTLY the same, so I determined the jacks I'd been using must be ok, and put them back.
I'd had a lot of people speculating that there must be a short or ground issue somewhere, but I couldn't find anything to confirm that. Finally, after Mort told me that he's sometimes found problems with control pots, I decided to start poking ("chopsticking") all the wire and component connections with a pointy dowel. Fired it back up and still nothing. It was only when I started REALLY focusing on the pots that I managed to hit the one wire that was the culprit, and was able to get the volume to suddenly jump just by moving that wire back and forth.
Played the amp for a few minutes. Kinda sounded like shite, tbh - but, the speaker was turned away from me, the amp was still running on the current limiter, and I had the bias set to the coldest possible setting, so I knew it could only get better from here. It faded out a couple times, but came back by just poking that wire with the chopstick, so I was pretty sure I had the root cause found.
Spent a couple hours replacing that shielded wire (this is the one that goes from the preamp gain to the 2nd half of V1), cleaning up the various wires, etc that I'd moved around during diagnosis, set the bias properly, and got the loop board installed with a new dropping resistor after determining I had better PI plate voltage than before.
And the thing fired up immediately.
Some pics of the final work:
This is the problem wire (see arrows) - turns out I reused a shielded wire from my prior build, which turned out to be a mistake
Closeup of the POS
MUCH better prep of the new shielded wire
Installed
Loop board installed
