Class 5 hum Mod/Fix?

[6B4G]

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 the amp 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 than that. 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, or made changes to the amp layout in general since my version of the MK1? Newer MK1 units seem to not have the hum issue as far as I can see, some older ones too, not listened to a MK2, but no one commented they had a problem so I assume MK2's do not suffer this problem.

On a forum that is not so popular on this forum (relating to certain Gibson Guitars), (Sorry SteveD) a poster who posted some interesting mods, but with less than Marshall friendly comments also posted a suggestion as to how to resolve the Class 5 hum - has anyone tired this or have 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).

[Crunchifyable]

Although I don't own one of these (... yet ;) ), it seems like pretty common thing to do (rotating 1 transformer 90 degrees whenever they are close to each other), and appears to be a recommended practice in building and creating audio tube amps (at least from the one book I read on the subject...or tried to read...far too much over my head for one once over). Most amps I've seen that have the two transformers close tend to have one at 90 degrees as well (including most older 50/100w Marshall heads).

I've been reading about it these mods too. As long as you make a good connection to the chassis (I believe it grounds the transformer), and don't mind a few extra holes in it, I think the worst that would happen is you would have more holes in your chassis and the hum would still remain. At best you have no hum and two more holes in your chassis.

I would unscrew and twist the transformer to make sure the wires have enough play to be repositioned 90 degrees. That would be my only concern.

[6B4G]

Well I'm confused by the picture I posted from the other site, as as far as I know this was for a Mk1 type amp, not the Mk2 with the valves on a different board and facing down, so this picture seems to be of a Mk1, but mine has the mains transformer under neath and the output transformer is in a similar position to the picture, my valves are much more in the middle of the amp compared to this one. How many versions are there of the main amp? Have Marshall reworked the amp since mine was released I wonder and before the Mk2?

The hum is related to the output transformer, if I undo the mtg screws and move mine around when powered up, I can make the hum louder by moving it closer to the mains transformer, no real surprise there. I need to cut the cable ties and move it around some more to see if I can find a place where the hum is less than the position it is currently located. I wonder if the cable paths also have an effect on hum? I will do some more experiments at the weekend and see if I can improve it. I think mine was one of the very first Class 5's, which might explain why a Mk1 in the shop where I bought mine from does not hum as much as mine does, but clearly was not a Mk2.

[JCM900MkIII]

Looks like a modded Class 5 head (picture in the first post).


(My MV modded Mk 1.2)




Silly question.
Would it be possible to just make a shield or cage around one of those transformers to stop the hum??? Like a Faraday's cage???
I only have a slight hum, which is hardly noticable when playing at regular volumes b.t.w. (Mk 1.2)

[6B4G]

I agree the first picture is modded, but is it a head unit? I'm not sure, very different to a Mk1 or Mk2 combo from the ones I have seen but who knows, I'm not an expert or claim to be one where it comes to these products.

I was thinking of making the cable a screened cable, it's the primary that is picking up the noise and creating the hum, problem is I'm not sure if I can find the connectors for the end of the cable to go on to the PCB if I changed the cable to a screened cable.

I'm still going to experiment with cable layout, but what I am now convinced of is that it is the White cable (EL84 Anode) that picks up the noise and is creating the hum in my unit.

Thanks for taking the time to reply.

[acefreecell]

That has to be a head layout, look at the screen print on the front, and valves are on the top - the combos are the other way around.