chazlupa

Yesterday while playing, the amp just shut off. The fuse was blown, but no other "obvious" issue. I replaced the fuse, only to have it blow again, and again.

I looked at the boards through a magnifying glass with lots of light and don't see any components that look "charred" or damaged. In other words, I don't see any obvious sources of a short.

Anyone else ever experience anything like this?

stryker59

what cab are you running the head with and are you running the right impedence?

__________________

you've been weighed, measured, and found wanting!

chazlupa

I'm using a Marshall MG412A, 8 ohms.

When I replaced the fuses, I am turning the head on without the cabinet attached, for what thats worth?

stryker59

i know that its not good to run a tube amp without a load attached to it, im not sure about solid state amps....replace the fuse and then plug the cab in before you turn on the amp and see if that helps..if not you may have to take it in for service..

__________________

you've been weighed, measured, and found wanting!

Adwex

SS amps behave the opposite way tube amps behave...A SS amp has no problem running without a speaker load, but they don't like a short. Whereas a tube amp's output transformer can be damaged without a speaker load, but they handle a short relatively easily.

__________________
www.myspace.com/rockerlivemusic

Pete240885

This is not true, see my post in the thread:

really - you can't run a marshall with no speakers?

chazlupa

I narrowed this down to the amp. I found another blog that someone mentioned that the amp sometimes overheats or just gives out. I disconnected the ribbon type cable from the amp to the printed board and powered it up and the fuses stopped blowing. I contacted the original poster who indicated that these things frequently go bad. The good part is that the actual repair is fairly easy and inexpensive (if you know which end of the soldering iron gets hot). They also mentioned that you can buy the small board that the amp is connected too fairly inexpensively too.

Thanks everyone for the help and for the sanity check. I'll post more comments after I receive the part and give it a shot.

stryker59

awesome!! hope that takes care of her!!!

__________________

you've been weighed, measured, and found wanting!

Adwex

Hmmm, according to all the evidence I've read, and from my knowledge of how tubes work, it is true. Why did Fender design tube amps with a speaker jack that shorts itself when nothing is plugged into it? Can you provide any evidence other than one of your posts that says it's not true? I have a degree in Engineering, I'd love to see this math you spoke of in that post.

Meanwhile, here's some info to read:
Tube Amp FAQ 1/20/99 (Frames supported)

Here's an exerpt:
***The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. ***

I'm looking forward to seeing that math...

__________________
www.myspace.com/rockerlivemusic

Pete240885

I have a lack of experience with tubes amps its possible that thier output transformer protects them.

However i know that its true for solid state, as for the maths its quite complex your looking at Standing Voltage Wave Ratio's and the like. Also the power output from the amplifier will have to be dissapted somewhere if there is no load this is most likely as heat inside the amp itself

Pete240885

Have just skim-read a little of that; your right about the tube amps being good at dealing with shorts, he explains it is indeed due to the output transformers impedance, thanks for the info i shall read much of it at the weekend when i have some free time

Adwex

No, it has nothing to do with the transformer's impedance, it has everything to do with the inherent characteristics of a thermionic valve (tube). A tube simply cannot put out enough current to cause any damage when a zero ohms load is applied (short circuit). It's kinda like trying to drown someone by pumping water through a straw...the straw just can't possibly supply enough water, no matter how strong the pump is.

__________________
www.myspace.com/rockerlivemusic

Pete240885

thats interesting as i said i'll do some in depth reading at the weekend, if you dont mind i'd like to check my understanding of what i've read with you