Help Implementing Star/Larry Grounding scheme

[Roe]

the wiki layout suggests the bias supply

[arledgsc]

I finished my recent Metro JTM45 build with Larry grounding. The amp is quiet as a church mouse relatively speaking. I can only start to hear AC hum far in the background at moderately loud levels which could be due to the light filtering. By comparison the electron hiss of the circuit is louder. Definitely no gound loops, weird buzzing, or oscillations as the controls are maxed and minimized. With no guitar plugged in and the normal and bright loudness controls on 0 and PPIMV on 10 I hear nothing from the speakers. The loudest thing heard at this point is slight mechanical hum from the PT (out of the head box) and that is only because nothing else is making noise. So, I've had great success with this mod! Thanks and happy camper! Scott

[sfo_pete]

I am looking at re-wiring some of my JTM45 and this is a great idea, thanks for the thread and info. Couple of questions. 1) Are the lugs insulated from the chassis? 2) I was always taught that a star ground takes the local gnds back to a single point. If the lugs are not insulated, then wouldn't you have ground loops in the layout drawing at the beginning of this thread? Thanks!

[arledgsc]

The Larry grounding method is really modified star grounding broken up into local ground nodes. The individual circuit grounds connected to each node are selected so that one node doesn't interfere with another and, in theory, remain quiet. Thus, the ground currents are localized to their part of the circuit. The chassis looks like a giant low impedance conductor where the ground potential (0v) does not vary much from one of the chassis to another. In a true star ground the circuit ground currents have to travel through wiring to get back to one ground usually near the power tranny. Since wiring has higher resistance than a big hunk of metal would mean the ground voltage at say the preamps may not be 0v due to ohms law V = I x R. And this is compounded with larger circuit layouts and has more potential for lifting ground potential when using a wired star ground because the circuit is physically bigger. So what if one ground is slightly higher than the other? Well, electrons want to move when there is a potential difference and that would be a ground loop.

So to answer your question about the Larry ground chassis lugs - yes, they are connected to the chassis and not isolated. Ground loops are minimized since each ground node is "isolated" from the rest of the circuit as much as possible.

My biggest head scratcher was when I built my JTM45 and the shared 16uF capacitor that filters B+ for the V1 preamp stage and V2 cathode follower. So does this cap connect to Larry ground 1 or 2? The cap supplies current for V1 which is the most critical of gain stages and you want to be quiet but also to V2 as well. I connected to Larry ground 2 but have been wanting to try ground 1 just to see if it makes a difference.

Electronics is really funny in that the sloppiest layout and wiring can be dead quiet while a neat job is noisy as hell. Sometimes you just have to give something a try to see if it actually works. I have wired three amps with modified grounding and quite happy. Give it a try. You can always rewire if it doesn't work out. Scott

[sfo_pete]

Great! That's what I thought, analog electronics is as much an art as a science, grounding is always tricky. Thanks Scott

[thehoj]

I don't think this was covered.. If I wanted to use shielded cabling for all of the v1a and v1b signal wires, should I ground those shields to lug 1 as well?

Also, I was thinking I will shield the cables to the PT grids. Should those shields be grounded to lug 3?

[tonemerc]

I have to admit I was a little skeptical about this grounding scheme, not anymore. Normally, every amp in this room hums like crazy this JTM45 is dead quiet.

TM

[PCollen]

I'm happy to see that this "Larry" grounding scheme is VERY close to that which I used in my amp. Variances in mine are:

1) Mid pot ground tied to #2 star, vs. #3, as I associate the tone stack with the V2 stage, not the PI.
2) Heater winding c/t grounded with secondary HV winding c/t.

[wharris]

I am unable to figure out how the #1 node "attaches" to the PT. (Stock Metro 1202-55)

[PCollen]

I am unable to figure out how the #1 node "attaches" to the PT. (Stock Metro 1202-55)

The PT secondary heater/filament winding Center Tap connects to the #1 node in this grounding method.

[wharris]

Please forgive my ignorance, but can someone tell me which wire that represents on the PT? I am assuming it is the Green/Yellow. I haven't even started my build yet, and I feel like a pre-schooler.

[tonemerc]

Please forgive my ignorance, but can someone tell me which wire that represents on the PT? I am assuming it is the Green/Yellow. I haven't even started my build yet, and I feel like a pre-schooler.

Correct

[wharris]

next question... does the 1 ohm resistor at V4 & V5 still get grounded from pin 1 to ground lug (as described in the kit directions)?

[Roe]

next question... does the 1 ohm resistor at V4 & V5 still get grounded from pin 1 to ground lug (as described in the kit directions)?

the best is to ground the 1 ohm resistor at ground point 5 (mains filtering etc). but the kit instructions also work fine

[pvan339]

If the bus wire is not soldered to the back of the pots with the Star/Larry grounding system, does the back of the treble and bass pot need anything done to them?