Tweed deluxe suddenly blowing fuses

[d. spree]

Need some advice. My self-built mission tweed deluxe has worked really beautifully for the last two months or so since I built it. I stashed it in the closet on its back safely while I took a trip to the UK. Yesterday I took it out and plugged in for first time since I got back - switched on amp, swithced standby to on (meaning standby off) and plugged in the guitar cable and the fuse blew. Replaced the fuse and 2nd fuse blew. Where do I start checking and why would this happen so suddenly after three weeks of inactivity after playing well for almost 3 months.

Tonight I will get iton the table and check for connections. I will have to pick up some fuses which may take a couple of days before I can repower her up. I wondered about a short caused by some leftover tniy fragments of solder or wire remnants that might have happened when I lay it on its back - or I wondered about a probkem coming from the guitar or guitar cable. Wondered if there is a problem coning from the power strip? What advice would anyone offer regarding a checkup?

[mitch8686]

The first place that I personally would look at would be the tubes themselves. A lot of times when I've had a problem where the amp worked one time and not the next, it's been some sort of tube-related issue. That's the simplest place to look first, IMO.

If they all checked out fine, then I'd open it up and start looking for some sort of obvious short. I doubt your guitar cable is the culprit. If there was a short there, it would probably just make a loud hum- kind of like when you touch the tip of the jack with the other end of the cable plugged into the amp.

[Chuck H]

A little chip of wire or a solder blob is a good possibility. The usual culprit is the power tubes trying to draw excessive current. This happens when they either fail (but the amp and tubes are young) or are operating under incorrect bias condition. It's possible that as the amp was sitting the tube pins or sockets oxidized. This can cause an interruption in the bias that results in blowing fuses.

[Enzo]

Everything works until it doesn't...


My vote is tubes, but loose solder bits or even loose hardware are also good possibilities. Pull the power tubes and power up. If it no longer blows fuses, there is a good chance one of the power tubes failed. If the amp was laid down, a tiny fragment of something inside one ot the tubes could have drifted in between two thihngs causing a short circuit. Hold each power tube up in front of your face, tap on the side and gently shake it. Hear anything inside moving around?

[d. spree]

Thanks everyone for responding. Enzo, what do I do about oxidization in sockets or tubes? Do I clean thexpins, and if so with what?

[tubeswell]

Besides a bad output tube, another possibility is a loose nut that has come undone and is shorting the socket pins or something like that. You need to check inside the amp for something that may have come adrift.

[Brad1]

Yeah, even if it is a tube, probably shouldn't power it up again until you've poked around inside and made sure nothing is shorted/loose. Just inspect carefully, and maybe tug a few wires if they look suspect..with it unplugged, of course!

The first thing people usually do when a fuse blows is to try another fuse. A lot of times, it could be fine. Who knows what blew it, right? The first thing you probably SHOULDN'T do when it blows ANOTHER fuse is to simply try a third fuse, or even turn it on again, until you've peeked under the hood. Time to look inside and eliminate things. With it unplugged, of course.

THEN...if all looks OK, I'd try powering it up with no tubes, and work from there. No sense popping in a new tube that could get damaged by another problem that may be obvious, if you had looked.

Brad1

[olddawg]

You should always have a spare set of tubes if you use tube amps. That said (Enzo and others will probably cringe). When I was a young starving musician and aging output tubes started blowing fuses at gigs on my amps, I would sometimes wrap the tubes one at a time in a bar towel and throw them on a carpeted area. It would break up interelemental shorts usually and I could get through the night. Tubes are very mechanical. Do not over rate the fuse btw.

[d. spree]

Yeah, even if it is a tube, probably shouldn't power it up again until you've poked around inside and made sure nothing is shorted/loose. Just inspect carefully, and maybe tug a few wires if they look suspect..with it unplugged, of course!

The first thing people usually do when a fuse blows is to try another fuse. A lot of times, it could be fine. Who knows what blew it, right? The first thing you probably SHOULDN'T do when it blows ANOTHER fuse is to simply try a third fuse, or even turn it on again, until you've peeked under the hood. Time to look inside and eliminate things. With it unplugged, of course.

Brad1

Yeh, after the first fuse blew I immediately tried it with a second fuse.

[d. spree]

Have got extra tubes - have not tried wrapping an old tube in a bar towel! Must be the beer that helps. Maybe I'll just go soak them all in single malt!

[d. spree]

Well I fixed the problem. Took out asll the tubes and plugged into a dim bulb tester just as I did after I finished the build. Switched on and found a thick jet of whire smoke coming out of hole 3 on the rectifier socket. This pin is unused but Bruce at Mission suggests that after tying together the white wire from the power chord and the white wire from the transformer that they get anchored to this unused pin and well wrapped with heat shrink tube etc. . Obviously somehow there was a short here - how this developed after 2 months of playing remains a mystery. It could be that stroring the amp on its side allowed a loose remnant of solder into the hole.

I cut the wires and cleaned off the pin. I reconnected the two white wires and insulated them without anchoring them to the pin (the way I did on my first tweed build) and then started her up and started adding tubes using the dim bulb tester. Now she is playing great again.

I have ordered a new octal socket and when it arrives I will replace the existing one - just in case any damage was done to the component. Oddly enough the tube was fine = which is great as it is an NOS GE tube and cost about $30.