Gassy nightmare

Ok, title isn’t very appealing. I know, however it truly reflects what I experienced over the past few days. Let me clarify before I start: a gassy tube developed a nice fault-finding journey. You always learn a lot from all this, for sure. Luckily, due to the holiday season, I had the time to work on this. I’d rather have spent it otherwise, but my 300B amplifier was dead.

I had a gassy 6BY5-GA damper rectifier tube. I should have changed it, but I was lazy and I paid the price for it. When turned on the 300B HT power supply, it will flash blue for a sec or two and the gas ionisation will disappear after a minute or two when tube was warm enough and operated as expected. Problem was that my stash of these damper tubes was up in the storage, who knows where.

On Boxing Day, I was about to enjoy a lovely morning of jazz records (Thelonious Monk was first on the list) with my coffee when the tube failed miserably when I turned the HT on. A quick flash and could notice that a fuse on the Mains HT delay / surge circuit went pop as well. Well, time for a new tube I thought and went up to find them. After some Tetris game with my boxes I found a big box full of them. Grabbed a few and came down to hopefully resume the normal morning with my coffee. But it wasn’t the case. When I tried to fire the amp up, again the fuse went pop. Hmm, it was more complicated than I firstly thought.

Debugging

I took the PS to my workshop and connected a dummy load. I gradually brought the Mains up with the VARIAC and noticed that the mains current was quickly coming over 1A at which point I stopped. Removed the HT filtering PCB, and started checking for faults. Not easy as it’s a PCB and components can’t be easily taken out. That’s the price you pay when you’re not soldering point to point. I found one of the UF4007 rectifiers dead – in short. Worst failure. I then replaced them with a pair of SiC 1.2kV/2A diodes. I suspect that some short developed (not common in tubes) and fried the diode. Perhaps some rectifier tube expert can chip in here to elaborate more.

Replacing the diodes wasn’t enough. When loaded the PS will only deliver way below the output voltage expected. I wired it to maximum levels and again, 330V maximum with a 600V input. Something was wrong:

I suspected some damage on the TX secondary windings due to the short created by the UF4007. I then measured the secondary voltages finding to my relief that they were ok:

The problem was on the rectifier. I could see the filaments running well, but something else was going on. I replaced the board with a spare PCB with a full-wave rectifier formed by 4 SiC diodes. The power supply worked well!

When I removed the Octal socket, I found to my relief that a single core cable connecting one of the cathode was broken. Short peak current damage perhaps?

I proceeded to replace the socket entirely, it’s a NOS ceramic Russian one. Have a few of these good sockets. As I didn’t trust any of the connections, I rewired the socket again using the new one. The power supply then worked back again as expected delivering 390V at full load (estimated to be close to 200mA).

I can now go and enjoy the coffee and the lovely sound of this amplifier. I guess I learnt the lesson.