Faulty Marshall JVM advice

Bod

My amp, a JVM215 combo, went all snap, crackle and pop this morning when playing at low volume.  It's six months old and has only been gigged once, spending the rest of its time perfectly stationary in my back room.

I popped it open, and found that internal fuse FS6 had blown.  I replaced it and it went again within 30 seconds.

It's covered under warranty, so all should be OK there, but what's likely to be the problem?  A dodgy valve or something more serious?  If it is a dodgy valve, is it a reasonably common occurrence?  Also, what's Marshall's warranty repair service like?  Am I likely to be without it for an extended period?

It's the first "serious" amp I've bought, and the first ever that I bought new, so it's disappointing to be experiencing a failure so early given that all the other used amps I've bought have had no problems whatsoever. 

ICBM

FS6 is the HT fuse, so the most likely cause is a faulty power valve.

If you have another spare, remove both power valves, replace the fuse and power the amp up again. If the fuse doesn't blow, it's a power valve fault.

You should probably replace both valves, and if you can identify the faulty one of the originals, you can keep the other as a spare. If it's not obvious from looking at them (sniffing the valve bases can sometimes tell you!) and you have more spare fuses, replace just one of them - leave the other socket empty - and if the fuse blows, the faulty one is in the amp.

If the fuse *does* blow with no power valves, contact Marshall. They're pretty good with customer service and turnaround time usually.

Bod

Thanks ICBM as always!  I'll try those tests tomorrow.  I've got plenty of fuses.

If I determine that there's a dodgy valve, would Marshall just supply a replacement for me to fit/bias (which I'm reasonably happy with)?  I'd rather they footed the bill without going to the effort of re-boxing and returning.

ICBM

Bod said:

If I determine that there's a dodgy valve, would Marshall just supply a replacement for me to fit/bias (which I'm reasonably happy with)?  I'd rather they footed the bill without going to the effort of re-boxing and returning.

I doubt it - they only guarantee valves for 3 months so they will charge you for them even if you want 'official' Marshall valves.

If it does prove to be a valve fault you're absolutely right not to send it back though, it's not worth the cost and risk of couriering it for something so simple.

Bod

Thanks ICBM.  What should the valves be biased at?  35mV?

ICBM

In my opinion less - 25 to 30mA. Marshall bias all these amps too hot at the factory, although less bad than the DSL/TSLs, which were set at 40mA. It doesn't help with reliability or tone, really.

Bod

Right, I did what you said and found one of the power valves was indeed responsible for the HT fuse blowing.  I've replaced both valves with a new matched pair, biased them and all is working perfectly so far.

Thanks once again!  You've saved me a LOT of grief. Marshall wanted me to return it via the retailer, which would have taken ages.

Cheers,

Bod.

ICBM

No problem, happy to help .

And glad that it was much easier and cheaper than the fuse-blowing Marshall I have here, which sadly has turned out to need a new power transformer...

Bod

Looks like I spoke too soon.  It's popping randomly now.  Better get it back to Marshall now I reckon

Bod

This is what it's doing now...

EDIT : Aaaaaaand it's stopped doing it.

EDIT 2 : It seems to start again sometimes when I'm not playing anything, but when it starts I can make it stop by playing...odd.  Preamp valve problem too perhaps?

ICBM

That's very odd. Too regular to be a valve problem I think.

Which controls is it affected by, if any?

Bod

None, changed channels, adjusted master and channel volumes, loop on and off, reverb etc still continued.

ICBM

So it is definitely a power stage or power supply problem. It's an odd one - the popping sounds very regular in the clip, which is not like something randomly failing. It's also not clear whether it's related to the valve failure or just a coincidence.

I would try replacing each power valve in turn with the good one from the old pair, in case it's a fault with one of the new ones.

If it isn't, I think you'll need to send it to Marshall since it won't be a valve fault, I don't think - although you should probably replace the phase inverter valve to be sure. If you don't have a spare 12AX7, just swap the two that are furthest away from the input jack. It won't be any of the other preamp valves if it isn't affected by the controls.

normula1

noisy mains wiring?

ICBM

I wondered if it was some sort of external noise being picked up too, but I don't think it would then stop when you play.

Bod

OK, I swapped one of the power tubes last night for the good one from the old set and all is good so far, no noise whatsoever. I haven't tried putting my delay in the loop yet, which I suspect *may* be what I did prior to getting the initial failure, but my memory is a little hazy about that.

So, as one of the new valves appears to be faulty, should the shop replace or do you think I could get some resistance due to the fragile nature of tubes?

Nerine

Just buy some new valves.

ICBM

Nerine said:

Just buy some new valves.

He did…

Bod said:

So, as one of the new valves appears to be faulty, should the shop replace or do you think I could get some resistance due to the fragile nature of tubes?

If one of the new valves is faulty - I would make really sure it is that before going any further though - the vendor should replace it. They may want both of the valves back and to supply a new pair.

Although that can depend on which dealer it was. Some are more willing to honour their warranties than others, in my experience.

RiftAmps

I see you're in Leicester...

Milton Keynes (Bletchley) is only an hour down the M1 from you and Marshall's service dept. offer while-you-wait appointments so might be worth giving them a call on 01908 375411.

peteri

Random thought.

Are you sure that the regular noise isn't external (for example a boiler spark), and the noise stops when you're playing because you're touching the strings so giving a better earth?

Sorry if that's stupid

Bod

RiftAmps;1110951" said:

I see you're in Leicester...



Milton Keynes (Bletchley) is only an hour down the M1 from you and Marshall's service dept. offer while-you-wait appointments so might be worth giving them a call on 01908 375411.

I contacted them via the website support and was told to return it via the retailer. I'll give them a call on that number (thanks) if I have further problems.

@ICBM I bought them from Hotrox as they're reasonably local. Never used them before so we'll see.

@peteri the noise wasn't random but the start of it was. Once it kicked in, it was metronomic and would continue until power off if I didn't hammer the strings a bit, almost like it was a capacitor charging and discharging.

Anyway, thanks all for your input folks. I'm keeping everything crossed that it's fixed now but will do more testing tonight.