“Don’t Stop Til you Burn Enough!”
Merry Christmas to you all! Christmas Eve ended up with some smoke and the party was over a bit early. We played music really loud last night and got carried away with some dancing around whilst playing “Don’t stop til you get enough” from Michael Jackson. And literally, the 814 SE didn’t get enough!
Let me explain briefly what happened. The 814 output stage has a crowbar protection circuit. It is configured to trigger around 200mA. The crowbar works brilliantly well, however, what is tricky on my design is the fact that you do get proper grid current in A2 operation. This grid current adds to the cathode current and flows through the crowbar sensing resistor. Well, volume was so loud so am sure that when the drum or bass kicked in, the crowbar was triggered. Interestingly enough, the shunt resistor is a 330Ω / 50W piece which should (in theory) blow the 500mA fast fuse. Well, it didn’t. The current peak wasn’t big enough before the output voltage of the 600V supply dropped significantly. Bear in mind that 330Ω should take serious current out of a 600V supply in theory!
The result was evident in a couple of seconds. The shunt resistor went madly hot, burned the plastic stand-off isolator quickly and fell over one of the current meters and burned the plastic cover badly as you can see on the image above. Luckily only one channel crowbar got activated.
When I rushed into the 600V supply mains switch I saw the secondary pair of dampening resistors (100Ω 7W wire-wound) melting and red hot. They were acting as a fuse and obviously preventing the supply to deliver further current.
Of course, the 814 was intact as it was the OT. At least the crowbar did it job, but far too early 🙂
Fixed it this morning after doing the Christmas present stuff. I couldn’t end up without music in Christmas!
Lesson learned here. Crowbar needs proper testing! I will buy smaller fuses – probably a pair of 200mA slow burn will work fine next time.
Happy Christmas!
Hi,
i just love this one. This is – for me – the essence of being british 😀
All year you develop sopjhisticated circuits, cultivate your image as a beau vivant listening to sublime musical finesses unvealed by delicate thoriated DHT-filaments. And then finally, on christmas eve you are throwing a smashing party, roaring along to cheesy hits from the 80-ties until you fry the amp. I stand in awe, you just rock!
Indeed Harald. Is funny now, but a bit embarrassing on Christmas Eve when “The Music was over” 🙂
if the dampening resistors were glowing hot, wouln’t that state that not only the fuse used in the crowbar circuit but also the (probably slow) fuse in the power supply are way to big too? Because 100 Ohms glowing red must have been quite a current 😉 …
Fuses for +650V are tricky. I have only a 700mA 3kV fuse for microwaves in place to protect the oil capacitor bank. It has blown in the past when I had an annoying interference when switching speeds in a cheap fan. However, when there is a proper short ( the active crowbar will act like one), you would expect the crowbar fuse to blow properly and before the microwave fuse. A steady 600mA current means 36W across the dampening resistors, more than enough to bring them cherry red even before the microwave fuse will notice that the party was over 🙂