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My amp stopped working yesterday. I just took off the back and powered up my amp, all the tubes are glowing. Theres absolutely no sound though, once i blew a fuse and these crazy feedback sounds were happening, but theres nothing now. Theres a very quite hum, but it doesnt increase or decrease when i mess with the volume.
Theres absolutely no sound when i plug in. I was under the impression there would be some noise, or a tube would stop glowing if it was busted.

Any ideas on what could be wrong?

Its a marshall JCM2000, DSL401 combo.

Tubes:
4x ECC83 Preamp
4x EL84 poweramp

Thanks.

unfortunately valves glowing arent a reliable sign that theyre still working. only that their heaters are working. if they pulse in time to the signal passing thats a different matter... but since youre getting no signal that doesnt help much.

The hum would suggest that the power amp is at least functioning, maybe if not correctly. The ecc83s shouldnt normally glow with the signal they receive in marshall amps if i remember correctly, but its probably more likely to be something like the transformer running power to the preamp valves than the valves themselves.

if youre happy messing with the insides, try getting continuty and resistances off the transformers first, the primary and secondary connections should be fairly obvious from where theyre connected to. Also try and check the power caps for burst insulation.

Because often the power and pre stages are powered differently, the pre can totally fail but because the power stage is there with the hum coming from the amp people get confused and freak out a bit.

Of course, there should me internal fuses on both stages if marshall have been fastidious in their design (unlikely), so check that if theres an internal fuse for the preamp its not blown, should be one of those clear glass slo-blow types if its there.

Afraid i havent got much experience with more recent marshall stuff so cant be more helpful. hopefully someone else can...

Thanks alot for the reply, I checked both fuses and one definitely appears to be blown. Its not the main fuse, but the "HT Fuse", whatever that means.

Sorry for my naivity, this is the first time i've dealt with faulty fuses (lucky me).

It appears to be just a reguler ol fuse, could i just pick up a replacement at an electronics store?

well HT usually means 'high tension' comes from automobile electronics. Probably just the high voltage side of the transformer. There should be values written around the end of the fuse embossed. As long as its the right rating anything should be ok, but make sure if its a slo-blow type you use another one. Some types of fuses will blow when you turn the amp on from the spike that comes out of the transformer.

And be weary, unless you were doing anything crazy with the amp, the fuse probably didnt go for no reason. So be aware the transformer or part of the power stage before the fuse might be doing something mad.

If the fuse goes as soon as you put it in, its probably the transformer thats shorting somwhere.

as far as getting fuses goes, youre lucky in the US that electronics stores actually know what theyre talking about, theres more than one per city, and they actually like to help. Chances are if you ask someone theyll probably be able to point you the right way.

If its labelled HT it may require a 400v fuse or something high like that, maybe not though. Sorry to be vague...

Danm-

Make sure to get a handful of those fuses, and it wouldn't be a bad idea to get a couple backups for the mains fuse as well.

First, replace the HT fuse. Turn on the amp, let it warm up, and take it off standby. If the HT fuse blows again, you'll know it by the same symptom you have now.

If the amp works fine and the HT fuse doesn't blow again, this is where it gets tricky. Maybe you were playing very loud, for a long time, or had some other extenuating circumstance cause the failure. Or maybe not. In any case, it wouldn't hurt to take some time putting the amp through it's paces, to make sure you're not on the verge of blowing the fuse again.

Assuming the HT fuse blows after being replaced, the next step is to turn the amp off, remove all of the power tubes (EL84s), reinstall another HT fuse, and see if it blows yet again. It's more than likely that it won't, as most fuses blow due to a problem in the power amp, and in many cases this means a bad power tube. If this is what happens, you'll need to replace the power tubes.

If you want to isolate the one (if it is just one) culprit, you could play a little game of mix and match with the new stubes, to see which combination blows the fuse again. This will require more fuses, of course.

If the amp still doesn't work with new power tubes, then you either have another problem upstream, or perhaps one of the other internal fuses has blown. You amp appears to have two additional fuses: one for the filaments (which is fine if the tubes are lighting up), and one for the bias supply. The latter could blow in the case of a tube failure, but unless you're comfortable being electrocuted, I wouldn't recommend digging inside the amp without a little more information.

If you're interested, here's a schematic:

http://www.piller.at/music/dsl201/dsl401schematic.pdf

best of luck,

Tom

I bought a new one, and it works, but when I crank up the volume, it sounds really weak, it sounds clean at low volumes, but when drums and bass come in, i can barely hear myself.
I assume the power amps are going bad, but it was fuctioning perfectly before the fuse blew.

I've read that if you're going to replace the power amps, you have to replace all of them, then bias your amp and what not. My main reason for not doing this is... i have (almost) no money. Is there a way to replace just one power amp, or change the order, or do something without it costing an arm and a leg (more than 40-50 bucks)?

I'm sure its not the speaker, I hooked the head up to an external cabinet (crate 4x12) and it still sounds very weak.


I'm about to try tgavin's advice, this is just an update.

Thanks!