Wiring up a Plexi 50 - Question about Grounding and PT direction

[QOTSA_Lover]

Hey Guys,

I am working on a 2nd Plexi50. My first build has a strange BUZZ/HUM and I don't want to do the same mistakes again.

1. Larrys's Grounding Scheme:

On my first build I drilled 5 holes along the frontside of the chassis. On the last grounding lug (5th) I attached the HV CT, the EL34 Cathodes and the main filter.
Now I moved the 5th grounding point in the upper right corner of the chassis to have more distance to the other grounding points and also more distance to the PT.

Is it okay to lead the cathode wires that way? (on the upper edge of the chassis direct to the grounding lug)
Is it okay to lead the Screen & PI Cap's ground that way?
Do you have any improvements?

2. PT orientation:

Is it necessary to mount the PT in a certain direction?
For my understanding the mounting of the PT and OT should be done by 90 degrees to each other. With a laydown style PT I already have 90 degrees to the OT so I mounted the PT the way it fits the lead dress / layout better.




Thank you very much
Tom

[QOTSA_Lover]

Here’s a shot from the chassis

[Roe]

the traditional marshall direction works well here

[QOTSA_Lover]

the traditional marshall direction works well here

What do you mean? It's okay that way?
Or do you mean it's better to wire the grounds with the "classic" Marshall scheme? (Bus bar over the pots, filters direct to chassis, etc.)

[Roe]

the original direction of the PT seems fine, but not the grounding

[xtian]

For PT/OT orientation, search "headphone trick".

[Phil_S]

I think 5 ground lugs on the chassis is looking to create a ground loop. People argue/discuss what works best. I've concluded there is more than one good solution, but this doesn't fit with anything I've read or heard. I like to run a buss bar along the edge of the circuit board. Whether you ground at the input jack or the power amp end is open to your taste, but I'd urge you to use a buss bar. I've found that a length of copper taken from ordinary 14AWG Romex is good to work with -- you don't need anything special, but it's nice if you have some real buss bar. I wouldn't solder it to the pots, what for?

[Roe]

Here’s a shot from the chassis

LARRY recommends at least 2" between the grounding points

[QOTSA_Lover]

Thank you very much the replies.

Hard decision how to wire the grounds this time.
I am going to try a slightly different approach this time.

I will use 4 ground points along the front.
1st Inputs and V1 cathodes, maybe heater
2nd preamp Filter, v2 cathodes, volume pots
3rd tone stack, speaker jacks, screen & PI filter
4th bias (I will use the far right one this time to have the 2inch between the lugs)

5th in the upper right corner for Main Filter, HV CT, EL 34 Cathodes
This spot has the most distance to the PT AND!!! the preamp circuit so maybe this is the most quiet spot.


So I hope this lead dress will work

Any complains? What do you guys think?

[QOTSA_Lover]

More pics

[QOTSA_Lover]

On my very first Plexi amp I built a few month ago I copied a real Marshall Plexi layout.
The lead dress was done for the easiest wiring and I grounded the things with no knowledge anywhere to the chassis....

Guess what...?!? This amp is dead quiet
My 2nd build was done with a “better” grounding scheme and it is a little bit noisy.