HELP!! problems with MA100C

[lukepat619076]

I just bought this amp a few days ago. I plugged into play like usual, and im playin for a bit. Stop for a second, hit one power chord, and theres a flash and a pop from the back of the amp. Now theres no sound, Ive checked all the preamp valves and they seem to be fine. Can somebody please tell me what is wrong with my amp and what i need to do?

[thephantum]

Did you buy it new or used? If it's new, take it back as it's under warranty. If it's used (not under warranty) and you are not comfortable opening up the amp and trying to fix it yourself, then take it to a tech to have it checked.

Just a guess, but it sounds like a power tube let go and smoked an internal HT fuse.

[lukepat619076]

So i just did the extremely nerve racking and scary task of taking the amp apart to see if I could figure out the problem. I went thru all the power and preamp valves now, and they seem alright to me. Still no sound. When i took the actual amp out (or whatever you call it, the part with all the electornics that the tubes hang from) and I found that one of the fuses is blown. I havent replaced it, and i hope thats the only problem, but also while i was checking tubes, one of the power tubes (the first one on your left if you look at it from the back) was getting hotter, and brighter then all the others. Is this a potential problem to?

[surfnorthwest]

thephantum is right, sounds like a classic case of a power valve failure. If it is new your tubes are under warranty for 90 days. If not you just need to get another EL34 tube, it will be easy to look at the tube and determine which one blew. Also take a look at the fuses for the amp because if the tube went chances are one of the fuses did also.

Also it sounds like your new to tube amps... that's OK but please be aware not to touch any of the components inside the amp, even with it turned off the amp can have lethal amounts of stored voltage. You are OK to grab the tubes when the amp is shut off, just don't go poking around in there.

Let us know how it goes.

[thephantum]

Yep. Sounds like you have a bad tube. The plate is glowing brighter because it's drawing too much current, which blew the fuse (the flash and the pop).
Assuming it's a used amp: Replace the power tubes, replace the fuse and re-bias. Then you can get back to rockin'.


EDIT - If you plan to do it yourself, there's a thread in the forum here on retubing an MA100H. The MA100C is very similar. And like Surf said, be careful.

[lukepat619076]

Thanks for the help and advice. Will I definetly have to rebias if i buy new tubes? The amp is brand new, so could i just get a set of the marshall tubes and skip the bias or what? I would like to be able to do the repairs myself, and i am new to the tube amps, so any pointers you guys could give would be really appreciated

[surfnorthwest]

If you have a authorized Marshall service center close they will replace them free and set the bias for you at no cost. If not you could get a matched quad of EL34s and keep the others as spares.

Biasing a amp is not hard but please fully understand what exactly you are doing before you do it. Best to pay someone to do it ($30) and watch him to learn how for the next time. Also contact your retailer and see how they can help you. Tubes can blow anytime, doesn't mean you got a bad amp it just happens and that is a big welcome to the tube amp family for you.

[lukepat619076]

I would love to have someone do it and show me how, but there are no amp techs around where i live. Im fully confident i could do the bias, i just need to know where i need to test, and what i need to set it to. It needs to be 76mA a side right? And i adjust that by turning a set of screws?

[surfnorthwest]

Read this thread

http://marshallroadhouse.com/viewtopic. ... hilit=bias

[lukepat619076]

Hey guys
I just bought a new set of shuguangs to replace my tubes, and that got me thinking. Do ineed to replace both pairs of tubes, or just the pair with the bad tube? And also, what kind of fuse do I need to use to replace the one that went in the amp?

[thephantum]

As long as you put a matched set in you can get away with replacing one pair. Different brand tubes do have different tonal qualities though...just something to keep in mind.

The fuse is a GDC 630mA 250v slow-blow fuse. You might also see it listed as a T630mA 250v.