GRO100 Intermitant Fuse Blowing

[jcollins]

I've got an old Orange GRO100 on my bench. Brought in with the complaint it's blowing the B+ fuse (1 amp fast blow). Owner was playing amp for half hour and B+ fuse blew. He replaced the fuse and it immeadiatly blew.

I pulled the tubes, popped in a new fuse and it held when I turned the amp on.

I put the tubes back in, one by one and turned the amp on each time. Each time the fuse held. Finally with all four tubes in, I played the amp for a while.

I left the amp on for around two hours while two nights in a row while I worked on other stuff. The fuse held the whole time. I actually used a 1/2 amp fuse as I didn't have any 1 amp fuses on hand. The tubes were fairly old so I advised the customer to replace them. I installed new tubes, biased them to around 70% plate dissapation. Also installed correct 1 amp fuse. Turned the amp off, put the amp back in it's chassis and turned it back on to let it burn in for awhile.

And the B+ fuse blew. Just a second or so after I flipped the on switch. I put another fuse in and flipped the on switch several times.

The fuse held each time. I did notice that there was as momentary hum that went away after the amp was on for a couple of seconds. I also heard that just before the B+ fuse blew.

I plan on letting the amp burn in for another couple of hours and/or see if I can get it to blow by turning it off and on.

Other than that I'm not sure what the next logical step is. I suppose the new set of tubes could have a problem, or maybe the 1 amp fuse was defective but, I'm thinking that there is possibly an intermitant problem that is causing the fuse to blow.

Any guidance is much appreaciated.

[Enzo]

Are you sure it should be a fast blow fuse? The high voltage fuses on the Marshalls are slow blows for example. Uncharged filter caps present a stiff current spike for a moment when the switch is first thrown. That can take out a fuse.

But really, check out the current. REmove the fuse and connect your meter in its place set to current. How much current does the amp draw just sitting there?

If you know the idle current for each power tube, then total them up, add a couple ma for each preamp tube section. The current you measure ought to be about in that area. If it is a lot higher, something is happening, and the fuse is just waiting for something to push it over the edge. But if the current is right where it ought to be, then some impulse is popping the fuse.

What might pop fuses? Power tube first on every list. Power tubes that are working but failing can easily blow a fuse and still work. How old are these? I don't see mention of trying new ones.

The filter cap charging thing - check that fuse type.

Ball up your fist and whack the top of the amp. Turn down any reverb first. If it makes noises when yuo do that, something is loose insde - possibly inside a tube.

[Gtr_tech]

I'd also check for cloth covered wire that is laying across any component leads or tube socket tabs. It is possible for it to become conductve from contaminants over the years. Also, have you cleaned off the tube sockets? Both sides? Maybe it arced over a socket in the past and it was never changed.

Got a meter with min/max function? Now's the time.

[jcollins]

OK. I got back to it. Monitored current across the fuse holder with my multimeter at turn on.

Just like Enzo suggested, there is a big current spike. The height of the spike varies but it definitely goes over 1 amp sometimes. Next to the B+ fuse it definitely says fast blow.

Now I'm thinking that one or more the caps should be changed out.