Larry Grounding scheme

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screamindemon

Well-known member
Hi guys,
I'm building a Mojotone British100 watt marshall clone. It's going to be hot rodded with 4 gain stages and a plate driven tone stack.
I would like to impliment the Larry star gounding in this amp.

Larry grounding described as follows.
1, Input, V1 cathode, heater center tap, gain pots
2. Volumes pots, V2 cathode, preamp filter.
3. Mid and presence pots, PI filter, output jack, OT common
4. Bias Circuit, screen filters
5. Mains filter, HV CT (via Fuse) V4 and V5 Cathodes
6. Power cord

So I have a couple questions if someone wouldn't mind helping clarify this for me ?

So for the Bias circuit, star lug #4 The PT has a small white and green wire labeled 95v Bias. The green wire should go to #4 correct ?

And for star lug #5 it says to ground the HV CT to lug 5 Via fuse. Per illustration below that would be the thin yellow wire Correct ?
The mojotone layout shows this going to one of the mains filter caps not ground, I am confused.
Could someone please explain where the HV CT should go.

Thanks in advance !
7020810_02.jpg

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I always use Larry grounding in my Marshall mods. Takes a lot more time but it’s worth it.

Yes, green bias wire goes to #4.

I never put the HV CT to a ground lug. I always run it to the mains filter cap. I do run the HT rectifier ground to lug 5 though. In fact, the print out I have in the folder on my work bench shows just that, the rectifier ground to lug 5. Not the HV CT.

Anyway that’s the way I do it and it works great.
 
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Thanks for the reply !
So the HT Rectifier ground is the black wire in the pic going to the ground lug at the mains filter above the yellow wire, right ?
 
Also do you bother with putting cathodes V4 and V5 at ground #5. I thought I read where Larry said it didn't make much of a difference.
 
screamindemon said:
Thanks for the reply !
So the HT Rectifier ground is the black wire in the pic going to the ground lug at the mains filter above the yellow wire, right ?
Click to expand...
Yeah, that’s it.
screamindemon said:
Also do you bother with putting cathodes V4 and V5 at ground #5. I thought I read where Larry said it didn't make much of a difference.
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Usually, no. I prefer running 1ohm resistors off the cathodes so I just run them to their respective ground lug next to the socket.
 
are those all chassis connections? If so, abort and look at other grounding schemes: TUT3, Aiken, and Valve Wizard (which are all very similar).
 
V2a said:
are those all chassis connections? If so, abort and look at other grounding schemes: TUT3, Aiken, and Valve Wizard (which are all very similar).
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What are those ground schemes like? Are they the type that busses everything together and only connects to ground at one point?

I've seen both work and I've seen people poo poo the Larry grounding scheme just based on a "fear of multiple ground points" but it seems to work. I never see anybody really explain why one is better than the other tho.
 
I've tried it. And I've tried just one single chassis point (other than mains ground, which gets its own). And both work fine. I have a preference just based on neatness and ease of build. But both they work either way.
 
Good point about mains ground. Those can introduce hum when coupled directly to other grounds further upstream in the amp.

I hide most of the ground lugs under the front of the circuit board with the Larry scheme. It’s actually very tidy.

The best thing is eliminating the factory buss on the back of the pots. I hate the way those look.
 
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RedPlated said:
The best thing is eliminating the factory buss on the back of the pots. I hate the way those look.
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I like running a bus bar above the pots and connect everything (most things) to that. But not soldered to the pot casings. Basically, what Soldano does in the SLO. And similar to what Ceriatone does in some of their amps, just instead of hanging off the turret board, I use the relevant pot lugs.
 
When you guys do the Larry scheme, where do you physically put the ground points? And have you found that it matters?

IIRC Larry suggests a line from input jacks toward the power end for most points and then the dirty ground for main filters in the back corner. Or something like that. I’ve done it a few different ways (like combining the first two) but I usually ground the power tube cathodes at their socket base (thru 1ohm bias resistors) and the speaker jacks to chassis (thru a Switchcraft-type jack) or a power tube ground lug if it’s a Cliff-type jack.
 
I put mine in line across the front of the chassis, below the front edge of the PCB board.

Mains filter ground goes to #5 along with either the HV CT on a 50w style circuit or the rectifier ground on a 100w style circuit. The earth ground from power cord/ IEC goes to its own lug at the back corner of the chassis.

I run 1 ohm resistors from power tube cathodes to ground lugs next to the sockets. Works fine.

I also run the AC heaters CT to #4, not #1. Larry says you can do this. It has worked great for me. #1 is too far. On an older amp you almost always have to extend the wire to make it reach, even to #4.
 
All good, running like a dog with his butt on fire..
Calls are coming in faster than I can knock them down :doh:
Hows the amp coming??
 
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