JVM Noise and electrical interference

moz91

Okay so my JVM210c is ridiculously noisy! I get a constant electrical hum which is increased by the master volume but not affected by the preamp. Im aware theres a fair amount of hiss on the gain channels without a noisegate but this is a seperate issue. Its my second rig and has never been abused; just rehearsals and the occasional gig.
Source of power definitely has an impact; the noise is massively reduced when using a power conditioner but still not completely silent compared to another amp!
Same can be said for putting pedals infront of it - if i take a feed from a power conditioner for my pedal board then there is a noticable difference in background noise and interference from the JVM.
In anticipation of your responses, i've tried many times going straight in the front of the amp and ive used a tonne of different power sources, IEC's, extensions etc in many different locations.

Amp has recently (last 3 months) had a full and thorough service, and new valves in both pre and power amp. Its completely stock other than the 'Plexi Cap Mod', which has no effect on the power amp or transformer what so ever.

I'm stuck for ideas, tried allsorts to get rid of the issue! Its getting to the point where i'm considering getting rid of it which i really dont want to do; other than all the noise problems i'm happy with the tones and versatility i get from it

Hope someone can help!

ecc83

Check from the earth pin on the IEC socket to chassis. Should read less than one Ohm. 

If it indeed have a PROPER service it should carry a PAT label and that includes a high current earth continuity test. 

But you never know! 

Maybe the Top Men will tell us they are just noisy fekkers?

Dave.

midlifecrisis

not much help, but I had the jvm 210c combo, the gain channels were very noisy, the high gain one unusable for me. I was advised to try a noise gate, but I felt this was unacceptable on a £1200  amp, plus it was too heavy for me so I sold it.  I believe the latest satriani models have addressed this.

ICBM

Possibly a long shot but since the noise is definitely coming from the preamp (since it's from before the master volume) -

Try changing the third preamp valve from the input jack end - make sure the replacement is not a Russian-made (Sovtek, EH, Tung-Sol etc) or JJ valve - or if that fails, the fourth one. A standard cheap Chinese one will be fine for the test. The reason is that these positions are cathode-followers, and these can sometimes cause some of those valve types to hum (or fail). Even a new valve might do the same thing if it was the same type as the old one.

If it doesn't fix it you've lost nothing other than gaining a spare valve.

Chinese 12AX7s have a distinctive double chrome strip on each side when you look into the valve.

moz91

Off the top of my I think I've got EH's in all positions apart from 3; ive got a TAD (Chinese; most likely premium selected shuguang or something) in pos 3. Oh I'm lying, I've got an old harma in pos 2 as I got a dodgy EH.
Is there any chance it could have anything to do with the plexi cap mod?
So, by process of elimination, the fact that the hum and noise is 'controlled' increases or decreases) with the master volume, that confirms the issue is in the preamp?

ICBM

Yes. If the noise is affected by the MV and not the gain, it's in the middle stages of the preamp - where the cathode followers are. I think that only the first preamp valve has a DC filament supply on this amp, so it could be getting in from the AC supply to the others. (Also a problem with some valves.)

I'm not sure what the Plexi cap mod is exactly, so I don't know about that. I have to admit I wouldn't want to mod an amp like a JVM, just too big a can of worms if it doesn't work out right. Especially now the JS version is available which seems to have all the previously popular mods done as stock. You wonder why they didn't get it right the first time really… except that this is the modern Marshall company and they seem to be unable to .

moz91

Must say a satriani JVM is definitely a consideration...
That mod is just a 1uf cap parallel with the 2k7(iirc) resistor on the 1st stage of the 2nd preamp valve. Subtle effect on tone, not a serious mod or anything.
So if it's a fault with the AC power supply getting to the valves, should I be looking at dodgy valves or looking deeper in to the amp? Tested the valves in another amp and had no issues and all come up okay on a tester (practically brand new)

ICBM

moz91 said:

Must say a satriani JVM is definitely a consideration...
That mod is just a 1uf cap parallel with the 2k7(iirc) resistor on the 1st stage of the 2nd preamp valve. Subtle effect on tone, not a serious mod or anything.
So if it's a fault with the AC power supply getting to the valves, should I be looking at dodgy valves or looking deeper in to the amp? Tested the valves in another amp and had no issues and all come up okay on a tester (practically brand new)

That doesn't prove anything unless the other amp has cathode followers - what was it? Even the tester result doesn't mean anything since it won't subject the valves to the same sort of cathode-to-filament voltage as in the amp, which is where this fault occurs. (If it is the cause here.)

I doubt the cap mod has anything to do with it since that will increase midrange and top-end, rather than bass.

moz91

Good point, I just put them in a jcm900.
I've got a few spares knocking around somewhere; I'll try mixing and matching different Chinese tubes in the v3 and v4 positions, see if that achieves anything

ICBM

moz91 said:

Good point, I just put them in a jcm900.

That might mean something - depending on the model. Each version has either one or two cathode followers:

Dual Reverb - V1

High Gain MV - V2

SL-X - V1 and V3

moz91

High gain dual reverb, usually has a set of JJs in it. Tried various valves in all positions (labelled so went back where they came from int jvm) and all seem fine!
I'll get some spares from my gig case tomorrow and try them in the jvm over the weekend.
Anything else worth trying that I've missed??

moz91

Gigged this last night with the amp, g major 2 and my pedal board running from a conditioner and it was silent. Bliss!
Then some silly bollocks frontman decided to plug his wireless mic psu in the 2gang extension from my power conditioner that was running my pedalboard and it was noise central! Horrible digital-like electrical hum and interference ont dirt channel.
Unplug the psu. Silent again.
So confusing!!!

ICBM

Did it still make the noise when the PSU was plugged into another power socket?

It sound like it might be getting in via the earth connection, if the noise only happens when they're on the same circuit.

moz91

Nope, noise stopped when the wireless mic was on a different circuit.
Had one feed from power conditioner going to back of stage; 2 gang for amp and g major 2.
Another to front of stage for pedal board. This is the one that the wireless mic psu was put into by mistake (desk was on my side, easy done). When that was put on to a different circuit, all was silent as it should be!
Had the same problem before; if either the amp or pedalboard shares a circuit with anything else (usually regardless of what it is), then it gets fucking noisy!!

ICBM

Sounds like the solution is simply not to allow anything on the same circuit. Replace your 2-gang for the pedalboard with a single socket extension cable.

moz91

Aye that's the plan, going to get a power conditioner to put in the 2u rack that goes with amp; currently got g major 2 and a blank space. Run a 1gang out to pedalboard, done.
I'd love to know how/why there's so much interference though. You say it could be some sort of earth leak?

ICBM

moz91 said:

I'd love to know how/why there's so much interference though. You say it could be some sort of earth leak?

Not a leak, but possibly a consequence of the partially lifted ground in the JVM. You can test that by connecting the JVM to any other piece of earthed equipment - an older amp will do, it doesn't even have to be switched on as long as it's plugged in to the wall - via one of the unused jacks. (eg, connect a spare guitar cable from one of the unused speaker jacks to the input of another amp which is not switched on, so it doesn't cause any damage.)