Metropoulos Forum

Well I have essentially gutted my JTM45. Removed all the tube sockets turret board ect. All that is left is new sockets and the pots as well as the big blue caps.

Anyone have a guide or a basic writeup for the alternative grounding scheme as I would really like to incorporate it into my "re-build". What I have gathered is that instead of running a wire from the across the pots and attaching the black grounding wires you re locate them by the rectifier tube. and use the grounding lugs.

Any pointers or advice?

Thanks

I haven't started it myself, I'm busy, but the point is to have the loops spread out, and be as close to a line in their positioning as possible. I don't know all of the theory behind it, but there are numerous posts by larry which go into greater detail. The diagram from the thread you posted(the last diagram posted) seems to have worked for other people, so I'd stick with that for a JTM45.

You'll need to drill 1/8" or so holes for the grounding lugs, and might need to order some extra lugs if you don't have enough left over from your build(5 or 6).

The only thing I'm unsure of is grounding the HT CT. Is it possible to do this with a stock metro tranny because the terminals are not exposed like marstran's? I planned on trying out brian's tranny anyways b/c of the optional higher B+ tap, but am unsure if this is possible w/ George's tranny.

Doesn't seem too bad I have 4 more grounding left so I think I might give it a try. I'm basically trying to ground everything close to one point rather than spread out correct?

Hi mate!

"but the point is to have the loops spread out, and be as close to a line in their positioning as possible."

Cheers,

Dennis.

Perfectly pointed out, WHAT WHERE has to be grounded in a JTM45!

When the ground lugs have a tooth (lock) washer between the lug and the chassis (!)* and are tightened properly, these amps must be dead quite!

(*) Marshall always did it wrong! They've placed the lug directly to the chassis and the lock washer between the nut and the lug.

The correct way is: Stick the screw through the chassis' hole from outside, from inside first the lock washer comes, then the solder lug and then the nut. Tighten the ground connections properly, but don't overtighten it! You know - right after very tight comes very loose, what might happen very unexpected with the brass screws

Have fun with your dead quite JTM45's

Larry

novosibir wrote:Perfectly pointed out, WHAT WHERE has to be grounded in a JTM45!

When the ground lugs have a tooth (lock) washer between the lug and the chassis (!)* and are tightened properly, these amps must be dead quite!

(*) Marshall always did it wrong! They've placed the lug directly to the chassis and the lock washer between the nut and the lug.

The correct way is: Stick the screw through the chassis' hole from outside, from inside first the lock washer comes, then the solder lug and then the nut. Tighten the ground connections properly, but don't overtighten it! You know - right after very tight comes very loose, what might happen very unexpected with the brass screws

Have fun with your dead quite JTM45's

Larry

Hey Larry,

I think I've got a handle on it, but am confused as to what to do with the Heater CT.

Do I need a different transformer with external terminals?(Metro's are not exposed)

Larry's grounding scheme puts the heater CT at point #1.

If the wire is not long enough, which I'll bet is the case, solder an extension to it and put heat shrink tubing on the joint then run the wire to #1.

So basically all I have to do is drill the holes marked in the picture and use the grounding lugs from Metro. Doesn't seem too bad. I never had a problem with hum but I hated soldering the back of those pots. Plus it made the build look a tad messy IMO.

LJay942 wrote:So basically all I have to do is drill the holes marked in the picture and use the grounding lugs from Metro. Doesn't seem too bad. I never had a problem with hum but I hated soldering the back of those pots. Plus it made the build look a tad messy IMO.

+1 it's a challenge for beginning solderers to solder to the back of pots, and i don't like running wires to the buss for grounding. it never looks totally clean.

I had no problem getting the solder to stick it just never looked neat.

thats a great pic. will that grounding scheme differ much for a 50 watt? majority looks the same

clarkydaz. wrote:thats a great pic. will that grounding scheme differ much for a 50 watt? majority looks the same

AFAIK it's the same.

Cheers,

Dennis.

How would the Lar/Mar fit into this scheme?

Kramer wrote:How would the Lar/Mar fit into this scheme?

nicely, are you thinking about grounding the shielded cables to point 3?

That's what I'm thinking. Using the Belden coax I would just connect a wire to the shield, heat shrink over it and the conductor wires, and ground the shields at point 3. What I'm wondering is where I should ground the 2.2m resistors..point 3 too?

Since I'll be making a 2203 circuit next I might make and post another drawing with all the grounds on the master volume amp with optional Lar/Mar PPIMV grounds also listed. Should you ground the shielded wire feeding PreAmp Gain to Point 2?