100 Watt MV Shorting out?

rcol1325 · Post by **rcol1325** » Fri Mar 26, 2010 1:53 pm

Please help,

I completed my 100Watt MV amp last week, and everything was working great for the first few days. However, the last couple of days the mains fuse keeps blowing as soon as I turn it on. I ruled out bad tubes, as it does this with no tubes insalled. Also, I have checked my solder joints (note: I thought I fixed it when I resoldered the yellow and black wire connection underneath the board, it worked fine after that, then I turned it on today and blew the fuse again). Anyway, before I disassemble the whole thing, is there anything I can do to rule out bad filter caps, bad transformer, etc...Please help me somebody, I don't want to go back to my solid state amp...

Sincerely,

Bummed in Cleveland

Post by **VelvetGeorge** » Fri Mar 26, 2010 2:02 pm

You've ruled out bad tubes, that's always first on the list.

Next, try removing the 1A high voltage fuse. If the mains fuse continues to blow then it's down to:

heater wiring or PT primary wiring.

If it doesn't blow, look closely at all the high voltage wiring. It takes a lot current flowing to blow the 1A fuse with no tubes installed. look for shorted or partially shorted wires (like squeezed between the board and mounting bolts, for example).

george

rcol1325 · Post by **rcol1325** » Fri Mar 26, 2010 3:45 pm

George,

Thanks for the quick response. And thank you for the information. Since my last post, I resoldered the red and white wires attached to the underside of the board (I'm not sure they needed it but I did it anyway; couldn't solder from the bottom, so I did it from the top). After that, I reinstalled a new 4amp mains fuse, pulled all the tubes, and pulled the 1amp H.T. fuse. Powered up - no problem. I reinstalled tubes, and 1amp H.T. fuse - and I'm rockin' like Dokken. This seems to be an intermitent problem, so if it happens again I will hopefully be able to rule in or out the heater wires, and proceed from there.

Thanks

Rockin' in Cleveland

rcol1325 · Post by **rcol1325** » Wed Mar 31, 2010 4:08 pm

Well it happened again. I seem to be blowing the mains fuse every couple of days. I put in a new fuse, and pulled the h.t. fuse, no problem. I put the h.t. fuse back in and things are fine again. I will check the connections again, but I am wondering about slo-blow fuses. Should I try these?

axeman · Post by **axeman** » Wed Mar 31, 2010 4:11 pm

Post close up pictures so we can help.

rcol1325 · Post by **rcol1325** » Wed Mar 31, 2010 7:43 pm

Sorry, can't get pictures onto the forum from my computer.

mad-axeman · Post by **mad-axeman** » Sat Apr 03, 2010 4:32 pm

rcol1325 wrote:Sorry, can't get pictures onto the forum from my computer.

Do you not have a means to take photos, or simply don't know how to get them to post onto a forum?

Stu

rcol1325 · Post by **rcol1325** » Sun Apr 18, 2010 10:15 pm

Just a follow up. After putting in the slow blow fuse, everything has been fine. I'm new to this amp building thing, and tube amps for that matter, and it can get a little frustrating at times. But overall I'm happy with my 100MV, and I just finished a 50W Plexi for my friend. Looking forward to my next build...

Thanks George

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100 Watt MV Shorting out?

100 Watt MV Shorting out?

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