100hz hum in 18w build - larry grounding used

[ACHIEVEIT]

Hey guys,

i recently finished building my Marshall 1974 18w (the original normal/tremolo version). It sounds terrific but i am having some issues with hum. I used the ceriatone layout as well as one that is available in the 18w forum downloads section(have to check which - not at home right now!).

So, the amp suffers from 100Hz hum (after i hit the standby - i live in Europe btw). Other than that, it is very quiet. Even when dimed (either channel), there is no significant noise added, which is remarkable. So, after correcting the 100hz hum, it will be dead quiet. The only different thing i did compared to the ceriatone layout, was add a lar/mar 1M master volume (as found in the 18w forum downloads), which does its job fine by the way.

The transformers used are from Ingo (IGPW) and the pt has a heater ct. I have used shielded cable for the normal ch. input and the master volume.

So, having great success on older builds (of the likes of JTM, plexi etc.), i tried to apply the Larry grounding scheme in the 18w as well. The other builds i did with that scheme came out dead quit, so it was tempting to apply it to the 18w as well. Apart from that, it was easier to separate the grounds, as that would avoid accumulating too many wires in a single point.

So, i divided grounds as follows:

1 -> Inputs, V1 & V3 cathodes, heater center tap
2 -> All pots, 16uf capacitor, PI, V3 anodes, rest of grounds of tremolo channel
3 -> Output transformer common, output jacks, tremolo jack, Master volume (it was really convenient to put it there)
4 -> Power tubes cathodes (through the 500uf cap), 32/32uf filter cap, pt high voltage winding center tap, pt e
5 -> Power plug ground

You can see photos of the build in my photobucket album:
http://s938.photobucket.com/user/johnko ... ibrary/18w

I have also attached a pic of the actual grounding points within the amp for ease as well as a ceriatone layout that i have altered, so that the numbered ground points are more comprehensive.

What i have tried so far:

- Chopsticked the most important and sensitive wires - it really made no difference
- DC elevated the heater center tap - no change and unless i am paranoid, the hum was a tiiiiny bit worse
- Removed each preamp tube at a time - the hum is always present (once i hit the standby of course)
- The power tubes are a matched JJ set from banzai. One of them lights like a light bulb for a second when i first switch the power on. After a second, it goes to normal without red-plating or behaving weird in any way. I swapped the two tubes to see if it was the tube or the socket to blame, and it was the tube. Now, i have read that this might indicate a problem with the tube but i am not sure and i don't have a spare set to test.

With that in mind, if it is not the power tubes to blame, do you think i have done anything wrong in the grounding scheme? Have i created any ground loops? What can i try to tame the hum??

Do you think not having the grounding points in a straight problem (like i did) would be much of a problem?

If you need any further info about the build, i can provide it!
Thanks a lot for your help!
John

[ACHIEVEIT]

A quick update of a couple of things i noticed:

1) Touching the chassis on its side, i can feel it vibrating. Could the hum be transformer related? What could cause the vibration?

2) I joined ground points 2 & 3 (temporarily with an alligator clip cable after i unscrewed point 3 from the chassis) to see if that would make any difference, but it doesn't