Mpjotone JTM45 kit grounding scheme mod - please review

[ThreeChordWonder]

In the quest to remove the quite high 60 and 120 Hz hum from my JTM45 kit, I've reworked the grounding scheme, as shown below.

The idea is to remove the ground bus wire from the pot casings, and to provide one ground bus for all the turret board grounds. It's extended to take in the Presence and Middle Tone pots. The two input volume pots and the input jacks have their own grounds, but I could extend the ground bus to include those as well.

If this should work, either as is or extended, please LMK. If it's a disaster waiting to happen, please. LMK also.

Thanks in advance.

Mototone JTM45 Kit with TCWs Ground Bus Scheme Corrected 27AUG24 2 (1).png

[pdf64]

Attaching the safety earth connection to the chassis using a mains transformer screw is indeed a disaster in waiting.
There's only one safety critical connection in an amp and that is it, it's not rocket science to get it right, just use a dedicated fastener for it.

[ThreeChordWonder]

Fairy nuff. I can do that. Personally, I don't think using a separate bolt, equally (un)likely to come undone, is any different, but I can do it.

Anything else?

[sluckey]

Fairy nuff. I can do that. Personally, I don't think using a separate bolt, equally (un)likely to come undone, is any different, but I can do it.

Look at this youTube video of a Super Reverb restoration. Jump to 15:30 to see a prime example of why not to use PT bolts for any connections.

[ThreeChordWonder]

I've made the change. As it happened there is (well, was) a spare hole in the chassis already. I opened it put a bit and used a #8 machine screw and a keep nut.

Your vid seems to show whoever built that amp hadn't heard of keep nuts, BTW.

A bit of a right turn, but I am yet again reminded of my old friend Sue. Sue was the landlady of a pub I used to pull pints at in Weston-S-Mare. Apart from a lethal right upper cut and a superb Sunday roast, her main claim to fame was that she once worked for Triumph Motorcycles. The old, Meriden Triumph.

Straight out of school, her job was to put the tail-light/number plate holder/turn signal assemblies onto the back mudguards. She was shown a line of bikes, a rack of those assemblies, and buckets of bolts, nuts, and split washers and left to get on with it.

Now Sue had never seen a split washer before. Regular washers, of course, but not split washers.

So, assuming they were all broken, regular washers, she didn't bother using them.

There was about a six month period when all the new owners and the dealers were complaining to Triumph about the tail-lights falling off...

[sluckey]

Your vid seems to show whoever built that amp hadn't heard of keep nuts, BTW.

Fender built that amp in 1968. I don't recall seeing any kep-nuts back in the '60s. The ground solder lugs used have internal locking teeth like this AES link...

https://www.tubesandmore.com/products/s ... le-locking

[ThreeChordWonder]

Yup.

And if you're in a highrise steel-framed building, it's probably being held up by bolts. Without locknuts, without any washers, let alone serrated ones, keep nuts, nyloc nuts...

Just friction.

Comforting that, isn't it

[pdf64]

Fairy nuff. I can do that. Personally, I don't think using a separate bolt, equally (un)likely to come undone, is any different, but I can do it.

This is why electrical safety regulations exist, ie rather than leaving this stuff to the choice of the person / organisation doing the task.
A mains transformer bolt that passes through the lamination stack is the least suitable fastener possible to select for the chassis safety earth fastener. Those bolts are the most mechanically stressed on the amp, and will be subject to magnestriction. Additionally, although it may look and certainly feel solid enough, a lamination stack is not able to maintain its depth dimension when subjected to long term compression and magnestriction, in the same way that an equivalent lump of solid steel can. It'll tend to compress. The net result being that over time the compressive force clamping the earth lug to the chassis metal will reduce, eventually it may become completely loose.
As per the video.
Fitting a lock nut or using a nylock etc nut won't be of any benefit in this regard (I assume that's what is meant by a 'keep nut'?).

Anything else?

The use of the chassis as a conductor for circuit common, although traditional, doesn't seem good engineering practice to me. Rather it seems better to restrict the current flow to wires, then we can properly design and control the path it takes, and the chassis's electrical function is restricted to screening the circuit and maintaining electrical safety.

Here's what I consider a well designed JTM45 layout, done by a TAG member, which seems to follow the principles explained in Merlin's document linked below

https://www.marshallforum.com/threads/j ... st-2348046

http://www.valvewizard.co.uk/Grounding.pdf

[ThreeChordWonder]

I've moved all the grounds off the PT bolts now.

The ground scheme is curretly as follows:

1. The mains ground goes to one #8 bolt and keep nut, near to, but separate from, the PT mounting bolts. This wire should be long enough that in the highly unlikely event that the mains plug sticks in the mains socket and both get pulled out, the live or hot should separate first. 1/4 inch spade connectors BTW.

2. The PT HT and LT (heater circuit) enter taps and the two 32 /32 uF can capacitors share a second grounding bolt, again separate from, but near to, the PT mounting bolts.

3. There's a ground bus along the turret board which starts at the presence pot grounding capacitor, then the the middle tone pot, them along the turret board grounds, ending in a third grounding point near the input jacks. It can be extended, if thought worthwhile, to include the input volume pots and input jacks.

4. The input volume pots currently have individual grounds, using the same grounding bolt through the chassis described above.

5. The input jacks share one separate (but common for all four jacks) ground connection to the grounding bolt through the chassis described above.

6. The EL34 tube sockets are individually grounded on their respective mounting bolts. It would need long pieces of wire to reach the input area grounding bolt.

7. The PPIMV dual pots are separately grounded near the dual pot.[/list]

I could extend the ground bus (item 3 above) to loop in the volume pots and / or the input jacks. Thoughts on this welcomed.

Finally, a quick question. Would moving the cap on the presence pot from the ground side to the input side remove the noise I'm getting when I turn the pot?

[ThreeChordWonder]

PS.

This is a typical "keep nut". No nylon inserts involved. The serrated "washer" on the bottom is attached to the nut and bites into whatever is below:

everbilt-lock-nuts-800618-e1_600.jpg

[martin manning]

This is a typical "keep nut".

"Keps" nut. See https://en.wikipedia.org/wiki/Keps_nut

[ThreeChordWonder]

"Keps", "keep" "lock nut", whatever.

[jabguit]

"Keps", "keep" "lock nut", whatever.

Or simply "kep" nut - which is what it is....


cheers,

[johnnyreece]

"Keps", "keep" "lock nut", whatever.

Or simply "kep" nut - which is what it is....


cheers,

Next you're gonna tell me it's not a Cadillac converter?

[pdf64]

... Would moving the cap on the presence pot from the ground side to the input side remove the noise I'm getting when I turn the pot?

I can't envisage what you mean, maybe sketch a schematic?
FYI as long as there's V DC across a pot in the signal path, there'll tend to be a scratchy rustling noise as it's adjusted.

Later Marshalls wire it differently, but the range of control is reduced and the effect in the frequency response is different.
https://ampgarage.com/forum/viewtopic.p ... ce#p445956