Mains transformer connected
I spent a couple of hours installing the mains power transformer today, then going over some rudimentary tests. The actual installation was easy enough, though soldering the heavy 5V leads onto the rectifier tube sockets was a bit of a chore. I also took my time to make sure the wires were neatly routed and dressed. Overall, I’m pretty happy with how it turned out.
The chassis with the mains transformer and IRF450s MOSFETs mounted. The MOSFETs will have 300+ volts on the case when the amp is running and will be covered by protective covers even though they will be well inside the head enclosure.
Looking closer at the mains transformer, you can see how much I like wire ties. If you look at the standby switch, just up from the indicator lamp, you’ll see a black/red wire going back to the rectifier and two white wires going toward the filter caps. This is one area I’ve deviated from Nik’s layout. Instead of having the ground pass through the standby switch, I have the B+ go through the standby switch. This puts about 300V DC through the switch, but it’s a heavy-duty switch so I don’t think it will be a problem. And, from what I’ve read, breaking the ground for a standby switch isn’t necessarily the safest route to go. At the top of the picture you can see the two blue TO-6 mounting sockets wired in parallel.
After I had the transformer connected and the wires dressed, I triple checked all the connections and powered the amp up. The 6.3V heater taps worked correctly, as did the high voltage taps. No sparks, no shorts, no drama. Next, I fitted the rectifier tube and tried again. I measured the B+ voltage out of the rectifier tube at 315V. After flipping the standby switch from mute to operate, I re-measured and found the B+ voltage was at 416V across the board. This isn’t what I expected, but after some research I read that the B+ voltage will be significantly higher when the preamp and power section tubes aren’t fitted. Someone from the 18Watt community commented that their B+ measured over 400V before the fitted the tubes, so I think I’m in the right ballpark.
Overall, I’m very pleased that the amp checked out okay so far. Now I just need to wait for some small parts to arrive from Mouser before I wire in the B+ side of the output transformer and re-check the voltages with the tubes in place with a 10 ohm, 50 watt power resistor as a load. If all goes well, I might even try to hook up a speaker next weekend and see if I can get guitar sounds from this thing.