Class 5 Hum

Marshall Class 5

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Class 5 Hum

Postby 6B4G » 19 Sep 2011, 02:43

Hi all - My Class 5 has, and always has had a hum which does come through the speakers; you can not fail to notice when it is turned on. It doesn't change much with volume, but is always there, I find it very annoying, especially when using headphones. I would like to try and get rid of this hum, but not sure where to start. Looking at the design and circuit diagrams, the designer has tried all they can to reduce/remove hum and noise in general, DC Filament supplies to the Preamp Tubes, plenty of smoothing capacitance, Star Earthing, so I can only assume that the design is pretty good and that the problems have come in during production Engineering. Mine is the MK1, it's been back to Marshall to have the three piece back installing, and it had a faulty speaker which they repaired before that, but the hum has never changed and Marshall allowed it out of their service dept. so I assume it was the same as all the other Class 5 in this respect.

Yes I am being a perfectionist, I have a 1976 Fender Twin Reverb which is a 100 watts, you have a hard job to tell when that amp is turned on, so I want my 5 watt Marshall to be as silent. I should also say that I love this amp on the lower volumes, so mine never goes much above 4, I accept that as the gain goes up, so will electrical noise, so hiss etc. is expected, the hum on mine does not change with volume. Sure when you are playing, it is not that bad, generally you can not hear the hum, but as soon as you stop, there it is, very annoying.

So where to start? As the production engineering was not up to Marshalls normal standards for the Mk1 (The rattle problem anyone), I'm thinking the design is good and there is not going to be much from a circuit viewpoint to correct the hum, I'm wondering if it's pickup from the layout or something. Does anyone have any suggestions, or has anyone fixed their amp?

First things I am going to try is cable routing for the cables that are in the amp, maybe change some to screened cables, maybe install some tube shields on the pre amp tubes, any other suggestions?

Thanks to anyone that replies

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Re: Class 5 Hum

Postby djw171 » 19 Sep 2011, 03:39

I have a Mk1 and cant say I've noticed hum, be it with the humbuckers, single coils or P90's. Sounds crazy and I mean you no disrespect, but have you checked out your guitar leads and the screening on your guitar?

I would maybe look at a different set of tubes, see if there's any difference.

Let us know how you get on.

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Re: Class 5 Hum

Postby BadBluesPlayer » 19 Sep 2011, 03:43

I don't get any hum either.

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Re: Class 5 Hum

Postby 6B4G » 19 Sep 2011, 04:01

Thanks for taking the time to reply - Hum is present with no Guitar plugged in, doesn't change when a Guitar is plugged in, amp is earthed correctly and there is no source of electrical interference nearby.

Have tried new set of valves (Tubes), same result.

Interesting that the two replies posted to this both indicate no Hum present on yours, how old are your amps? Mk 1 or MK 2 ? Mine is a very early MK 1.

Cheers

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Re: Class 5 Hum

Postby djw171 » 19 Sep 2011, 04:20

Mine is an early Mk1 too, November 2009 if my memory recalls. I've had the 3 piece back, did various mods to removed the rattle (posted several pictures on this forum), and done a couple of other mods (output board mod, master volume). I dont have any hum that i would call annoying when the amps just sitting switched on.

The fact your saying it hums with no guitar plugged in would seem to me like there's a faulty component or two in there. Maybe transformer?

Since you've called them valves (I'm from England now in Canada) I'm assuming your in the UK. It could be worth taking the amp into Marshall yourself if you can get to Milton Keynes and let them hear it with you there to point it out. They're a great team and i'm sure will help you.

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Re: Class 5 Hum

Postby 6B4G » 19 Sep 2011, 04:41

Hi djw171 - Thanks for the info, mine is about the same age as yours then, I work the next road up from the Marshall factory, in Bletchley UK. The amp went back the day after I purchased it due to what was probably the rattle problem, they said it was a bad Valve and bad speaker, they replaced both, it had the same amount of hum when it came back, I just assumed that was normal. It then had the rattle problem, I think the original problem probably was the rattle problem, it went back again to have the three piece back putting on, that cured the rattle, but hum was the same, to be fair to Marshall customer service, I never complained about the hum, just thought it was normal, you would think if it was wrong tho, they would have picked it up.

I have modded mine a little changing the input impedance, R20 to 1M, R18 to 1.8K, C8 to 4.7uF, added a 120pf across the volume pot and 470pf across R22, these changes had no effect on the hum, just made the amp more to my tastes, I'm guessing Marshall won't want to know about the amp now it's been modded. I have tried different valves and the hum is the same, wonder what might be different about mine to yours and the others then?

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Re: Class 5 Hum

Postby djw171 » 19 Sep 2011, 05:31

Does seem strange. I have a friend here who has one around the same age as mine and his doesnt have a hum.

I agree with what you say about Marshall though, after you've modded components they may not want to know. They may do though but for a cost. Can't hurt to ask.

Just out of interest have you tried the amp somewhere else? You didn't state but i'm assuming that you just use it at home. Would be interesting to try with a different power souce.

What about taking it to a shop who has one in stock and comparing like for like?

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Re: Class 5 Hum

Postby 6B4G » 19 Sep 2011, 05:54

The problem is with the Amp not the supply or location, my Fender 1976 Twin Reverb is fine in the same room on the same supply, as is my Boogie Mk3, just the Marshall suffers with the hum, I have tried it at other locations and it's the same. I also have a DSL100, but that also is very bad on Hum, both Transformer when in standby and when turned on, so not a good comparision there, the USA are winning on the building silent amp stakes it seems......!

I'll have to find a music shop that has a Class 5 in stock locally and compare the hum levels etc. to mine, but the main local shop where I bought it from is a sort of modern factory warehouse type place, lots of Flourscent tubes and other noise sources, probably why I did not pick up on the hum when I tried the amp out and bought it. I'll keep you posted how I get on - thanks for taking the time to reply.

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Re: Class 5 Hum

Postby BadBluesPlayer » 20 Sep 2011, 04:22

Mt amp is only a couple of months old, so I must have a newer version.

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Re: Class 5 Hum

Postby 6B4G » 20 Sep 2011, 05:08

As suggested I went to a dealer who has a MK1 with the three piece back, so similar to mine, I asked to listen to it with no Guitar, silent, as one would expect, so mine has a fault of some description which has always been present. I'll investigate further, but without a scope to see what is happening I don't think I'm going to get very far.

Will post if I find anything out.

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Re: Class 5 Hum

Postby djw171 » 20 Sep 2011, 06:28

Think I would be tempted to go to Marshall and talk to them. Even with the Mods you have made they may still look at it for you. Worse they can say is No.

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Re: Class 5 Hum

Postby 6B4G » 20 Sep 2011, 06:59

Hi djw171 - Yes I agree, after having the thing in bits today and checking all the grounds, PSU, cabling, ran the soldering iron over it in case of any dry joints etc. again changed Tubes for a known good set, my 'Little Beastie' still hasn't learnt the words and hums along. I changed the route of the cables from the output transformer slighly, cables are short so not many options, it seemed to be a little better, but subjective hearing is difficult with these sorts of things, it most certainly is nowhere near as silent as the one I listened to in the shop that is of a similar period today. I'll contact Marshall via the dealer where it was bought and follow it up. Even if I have to pay to get it sorted it will be worth it as I like this little amp, and especially after the mods (very minor) that I have done to it to tweak it to my personal tastes.

Thanks for your help.

-- Wed Oct 19, 2011 8:02 am --

Well Marshall fixed by Hum problem, sort of, not perfect but much, much better than it was was, however not over impressed with the repair or the fix. The shop where I purchased it arranged for the repair, the information that came back was that some hum and some do not, it seems to be somewhat random......! Guess I had one that was far worse than others, no more information about the fix. I therefore had to take it apart to see what was done. The first thing I found was that the main EL84 tube was not installed correctly, the person doing the repair forgot to put the screw back in the Tube retainer and knocked it putting it back together, the journey home probably rattled it loose in the car. Then I found Pin "A" had fallen off the board with the purple wire still connected, I suspect this happened because they had been soldering around this area, it was sort of in place by the fact that it is a bit cramped where it's located, so it was being pushed on to the board, but fell off when I had a look, not good.

So what were the changes? Well it looks a bit odd to me, R2 and R4 have been removed and re-laid out under the PCB, as far as I can see the component values have not changed, just how they get connected has been changed i.e instead of the following the PCB traces they are now soldered under the PCB with Cable and Heat Shrink. Since on the circuit diagram they are marked as 'Flame Proof', I assume this is because they might get hot, being 2w and 1w resistors dropping power to the pre amp tubes, I'm a little concerned about the quality and reliability of the changes; they were flame proof and mounted off the main PC for a reason in the original design, now on my amp they are mounted on and under the PCB. We'll see how this works out long term, but a major failure could burn a hole in the PCB, so possibly not good.

Did it fix the problem? Yes it does seem to be much better now. there is still some hum, but nothing like it used to be. Interestingly, another unit in the shop that was not modified, but newer, had about the same amount of hum.

I wonder if Marshall have changed/improved the PCB layout since my version? New units seem to not have the hum issue as far as I can see, some older ones too. Still love this little amp, and it is much better now, lets hope it has no more problems.

This is not a modification that I would recommend doing if you have a hum problem as it's difficult to document, best thing would be to get Marshall to have a look if you are concerned.

I still highly reccomend this amp, and I'm sure the new versions in the shops now are greatly improved over my very early MK1, so don't let this put you off if you are considering one, it's a great amp.

Cheers all

-- Thu Oct 27, 2011 11:41 am --

On the subject of my Class 5 Hum and the mods performed recently, on the forum which some do not like is a post as follows that suggests a method to remove the Class 5 hum - has anyone tired this or any comments? If no replies I might give this a go and post back - here is the original persons post:-

OK, here's a photo showing how to get rid of the constant hum in the Class 5:

Rotate the OT 90 degrees. You can use one existing screw and mounting hole in the chassis. I drilled out an additional hole and used an 8x32 stainless steel machine screw and an 8x32 stainless steel nylock nut (my standard practice).
Attachments
OP Transformer mod.jpg
OP Transformer mod.jpg (63.86 KiB) Viewed 6549 times

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Re: Class 5 Hum

Postby mcbrideb78 » 16 Apr 2013, 13:17

I have the C5-01 combo. Mine has a quiet hum when powered on. It does not change with volume. I figured it is normal for a class A NMV amp. I did rotate the OT 90 degrees like the photo above, but it didn't make any change that I can tell. Maybe it's just due to how the internal wires are routed. I can't hear the hum unless I'm within a few feet of the amp anyways.

Edited to add: I also swapped all the tubes with new JJ's and there was no noticable change. I'll be keeping the originals as spares.

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Re: Class 5 Hum

Postby Marshallc5 » 17 May 2016, 22:11

Ciao, I have the Same humm problem on my class 5 in Rome Italy, do you have any other suggestion?
Roberto.

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