Blowing fuses over and over

[Tubes In a Hat]

I sent an email to the customer service link, but I just wanted to post a thread in addition to get as much info as possible.

I had an issue when I first got my RM100 where I thought I may have shorted something with my multimeter leads, but I replaced the blown mains fuse and the two fuses under the midi board. Everything was fine except for the fact that I really had to crank the master volume to keep up in the mix. I didn't think anything of it.

I was getting some microphonic squeal at the last band rehearsal, so i replaced the preamp tubes in two of the modules.

Today I fired the amp up and within 30 seconds of playing at pretty low volumes, the amp blew the mains fuse and the two under the midi board AND the outside two power tube fuses.

I had replaced the outer two power tubes with some brand new E34Ls i had and biased them for 35ma about two weeks ago

I'm just trying to give all the details I can think of.

I'm going to replace the fuses again and see what happens.

What's wrong with my amp?

 

[Julia]

It needs to see an amp tech probably for some pretty major work... It's not tubes. Although a catastrophic tube failure could have caused it. Or not. The mains fuses and the two under the midi board indicate something else. Transformer? I've also had brand new tubes fail and blow fuses straight out of the box. 35 mV is pretty darn hot for the E34L. If you got it new, you should get a replacement.

Do not turn on the amp. Make sure it is unplugged. Use your backup. If you don't have a backup and need to gig grab a Digitech RP500 or one of those Line 6 POD HDs when they come out in a week or so, and go straight to PA.

Did you accidently flip the voltage selector on the amp to European settings?

 

[Mattfig]

Did you accidently flip the voltage selector on the amp to European settings?

Beat me to it!

 

[Tubes In a Hat]

No, it's on 120v like it should be.

And yes, the amp is used. I'm the third owner I think.

I have a backup rig so I'm not really worried.

I just started training at a tech shop, so I'll probably just have my boss look at it when I get some money.

It's weird that it would be a transformer... how would I be able to tell if the OT is blown?

I'm pretty sure the E34Ls are fine. 35ma isn't THAT hot... I usually run my EL34s at 35ma in other amps and have never had a problem at that level.

Thanks for the help. And about the master volume thing before, I figured out that the cab I was using is just really really weak. I need to get a real cab before I start gigging the RM100 I think.

All in all if I have to spend a few hundred to get things in tip top with this amp, I think it will be worth it. It's a pretty sweet box.

 

[Julia]

what are your other amps? Marshall DSL? They will bias a lot hotter there because of a lower rated transformer.

but modern tube consistency sucks. This tube making gear isn't getting any newer. It's about 70 years old now. Like I said I've had a run of four quads fail straight out of the box. I saw them fail right at the tech's. Bam! Snap! Bam! Bam! -- arcing. I had two tubes in a quad of GT 5881s arc in my Lynch Box as soon as I took the amp off standby.

Another possibility: what size cab do you have. 112? 212? 412?

check the ohm rating on the cabinet and measure the impedance. An 8 ohm cab will show up as between 6 and 7 on an ohmmeter. One of the speakers could have become disconnected causing an ohm mismatch. See if you have a 8 ohm 212 and one speaker disconnected you'd have a 16 ohm load. That would be softer and can sound strange. Cranking the amp with that could be bad.

 

[Tubes In a Hat]

Interesting about the tube failures... I guess the best thing I can do at this point is have a pro look at it. It's not really killing me not to be able to use this amp for now.

I really appreciate all the info

I'll be sure to go easy on the bias in the future.

 

[kylendm]

Honestly when it comes to blowing fuses. If it happened more than once, then you might have a bad set of power tubes. I'd replace all of them and buy from a dealer that burns in the tubes before he sends them out. It's the most reliable way. Dougstubes.com does this as well as others.

 

[Tubes In a Hat]

That's the thing. The first time it blew fuses, it was the mains and the two under the midi board and after "investigation" I realized it happened because when I went to measure the bias on power tube 1, the leads of my multimeter were way too long and must have touched something causing a short or something... but I haven't been able to find any scorched areas or compromised components... but I'm no tech.

I replaced the fuses that blew and then the amp was working just fine. I replaced the two outside 6L6s with a pair of E34Ls that were pretty dang new, but HAVE been used in another amp for maybe 3 hours of play time total.

Then after I noticed that my Rectified module was making some microphonic squeal, I replaced the preamp tubes in it with some spares I had. Then when I put the Rectified module back in the amp and fired it up at pretty low volume, it blew the mains fuse, the two under the midi board, AND the two outside power tube fuses after about 30 seconds of playing. Now, all the blown fuses have been replaced, but I haven't turned the amp on.

I've got a couple of 6L6s on the way... I guess what I should do is run the two new power tubes in the inside sockets with none in the outside sockets so the amp will run at 50 watts so I can do more testing.

My current theories are A) the speaker cab I was using has a blown speaker or some other type of malfunction or B) the RM100 has internal damage resulting from a short of some kind OR C) the problem was the tubes and new tubes will solve any problems and everything will be ok.

If you run the RM100 with just two power tubes, does it effect the impedance? I remember hearing something about how I shouldn't use a 4 ohm cab with the RM100 running two power tubes...

Thanks, I really really appreciate all the help.

 

[kylendm]

When you pull two tubes out, cut impedance in half.

 

[Tubes In a Hat]

So... I CAN run the amp with a 4 ohm cab as long as I set the amp's impedance selector to 8 ohms?

 

[Julia]

NO. You run the 8 ohm cab with the switch set to 4 ohms. You cannot run a 4 ohm cab into the amp when you've got two tubes pulled.

 

[Tubes In a Hat]

Thanks again for the help!

I emailed customer service and they said I'm using the wrong values for the fuses. They said to bump the tube fuses up to 400ma slo blo and the two under the midi board need to still be 1.6a but slo blo as well.

Hopefully between new tubes and the right fuses, I'll be back in business.

 

[Julia]

Thanks again for the help!

I emailed customer service and they said I'm using the wrong values for the fuses. They said to bump the tube fuses up to 400ma fast blo and the two under the midi board need to still be 1.6a but slo blo as well.

Hopefully between new tubes and the right fuses, I'll be back in business.

Correction in bold.

 

[eedude]

could very well be tubes, especially el34s. The el34 design the filament and the plate are in close proximity the geometry could be distorted by the tube being dropped or miss handled at some point. The vibration from the amp while played can cause the filament to momentary touch the plate, shorting B+ to the filament supply. In a well designed amp several fuses should blow: B+ and filament in particular. In a badly design amp, Carvin Legacy for example, smoke will pour from the amp as components fry [first hand experience on that one, post mortem lead to conclusive as describe].

 

[Shinozoku]

Necrobumping this thread to make sure the fuse on the MIDI board is 1.6A Slow-Blow.

 

[raz311]

Could you have unplugged the midi footswitch while the amp was on? That happened to me when I bumped the midi cable on the footswitch and it blew the mains fuse and the fuses in the midi board.