Grounding, again, on GDS 18W combo kit

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bvayling
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Grounding, again, on GDS 18W combo kit

Post by bvayling »

Hi all,

Back a few years after finishing my Hammond AO-35 to Lite IIB, this time to finish the GDS 18W combo kit I started 5 years ago..... all while trying to remember the precious little I learned about tube amps years ago!

I'm reading up on grounding (JMP's Modern Grounding) and using the 2008 revised layout with Ritchie's Tremolo improvements http://18watt.com/storage/18watt_layout ... ents_b.pdf to assemble the kit.

I have the following questions:

1. JMP's comments suggest using shielded wire for all inputs. I did this, then noticed the layout only has shielding for the normal input. Is there a reason for no shielding on the trem input? Should I remove the shielding on the trem input or keep it?

2. The layout also shows shielded wire from C-7 and C-19 to V3, yet note #4 has a question mark with respect to shielding C-19 and C-15 to V3. What's the modern consensus here?

3. JMP's notes also say :" 1 ground near the Input jack covers input, pots, preamp. Ideally the preamp ground bus connects to the input jack(s) and the input jacks connects to an independent ground spot as close to the jacks as possible."
This is confusing to me. It suggests that the input jack is grounded twice, once to the bus and once to its' own dedicated grounding lug on the chassis. So have two separate ground wires coming from the input, one to the bus and one to the dedicated ground lug? Or can I run the ground wire from the input jack to a dedicated lug and then from there run a separate wire to the ground bus?

Then there is the trem input. Same? Or have a dedicated lug in the middle for both inputs?

Thanks for pondering this and forgive me if already discussed. I looked around but couldn't find anything.

Now to assembling the board while I see what you all think!

Thanks.

Bernie
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Re: Grounding, again, on GDS 18W combo kit

Post by JMPGuitars »

Reading the description can be confusing. Regarding the ground schemes, review the referenced layout: files/JMPGuitars_18_Watt_Lite_2b_Layout.pdf

There's one wire that goes from the ground bus to the input jack, and then the input jack connects to ground. The general rule is to keep the ground leads as short as you can. I think it may work out better to ground the first input to the bus, and then the 2nd input to the bus and ground as your ground point for all of the inputs and preamp. That's up to you though, as long as only one of the inputs connect everything to ground.

Shielded wire for all inputs, usually. The only reason there isn't shielding in the trem channel there is because the leads are very short, and depending on your build, probably safe. It doesn't hurt to use shielded wire, just make sure the shielding is only connected on one side, and the other side is taped off with heatshrink so it doesn't short anywhere. For the trem channel you can try the inputs without shielding and see if it's quiet or not. If your build has you running long wires, I would probably stick with the shielded wire either way.

Pins 1 and 6 on V3 are the plates (anodes). I've never built a tremolo amp, but it's probably a good idea to go with the shielding. If the plate leads add noise or oscillations to the tube, it will drive you crazy. I've seen many people on here mention how sensitive the tremolo tube can be.

Thanks,
Josh
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

Thanks Josh, that helps. It's starting to make sense.
Bernie
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

Thanks Josh, that helps. It's starting to make sense again!
Bernie
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

One more thing:

Richie’s layout shows the chassis connection for the board section ground bus on the left, closer to the power section and furthest away from the inputs.

This seems to run counter to prevailing currents. Is there a reason for it? I am building a stock GDS combo tremolo amp from a kit. Even though Graydon says you don’t need it, I am running a bus wire at the top of the board with the chassis connection close to the inputs, per the prevailing wind.

But I can still change it. I know jack but am good at following directions!
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Re: Grounding, again, on GDS 18W combo kit

Post by JMPGuitars »

Follow the ground scheme in my Lite 2b(ish) layout. It doesn't matter what you're building, the ideas are the same.
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Re: Grounding, again, on GDS 18W combo kit

Post by zaphod_phil »

bvayling wrote:
Sat 03/21/20 10:20 pm

Richie’s layout shows the chassis connection for the board section ground bus on the left, closer to the power section and furthest away from the inputs.

This seems to run counter to prevailing currents. Is there a reason for it? I am building a stock GDS combo tremolo amp from a kit. Even though Graydon says you don’t need it, I am running a bus wire at the top of the board with the chassis connection close to the inputs, per the prevailing wind.

But I can still change it. I know jack but am good at following directions!
I use a hybrid approach, which has always worked well in my experience of a good several amp builds.

1 - The power supply and power amp are grounded to the chassis at a common star point tag, at the opposite end from the input jack(s).

2- The preamp and PI have a buss wire which is grounded to the chassis right by the input jack(s). I think of that as a stretched out preamp star point.
3- Then each preamp/PI stage is grounded to the same point on the buss wire, as the power filter cap for that stage. This keeps the ground return currents for each stage in a small, tight local loop, so there's no crossing of ground currents between stages.

From what I've seen of JMP's layouts, he follows the same principles as well.

Since you say you know Jack, have you heard from him lately, or his cousin Cliff? :)
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Re: Grounding, again, on GDS 18W combo kit

Post by JMPGuitars »

zaphod_phil wrote:
Mon 03/23/20 1:59 pm
From what I've seen of JMP's layouts, he follows the same principles as well.
Man, that's so weird! My layouts follow the ground schemes you taught me?! 😂

I guess I can listen sometimes. Don't tell my wife. 🤫
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Re: Grounding, again, on GDS 18W combo kit

Post by zaphod_phil »

JMPGuitars wrote:
Mon 03/23/20 2:31 pm
zaphod_phil wrote:
Mon 03/23/20 1:59 pm
From what I've seen of JMP's layouts, he follows the same principles as well.
Man, that's so weird! My layouts follow the ground schemes you taught me?! 😂

I guess I can listen sometimes. Don't tell my wife. 🤫
I don't recall ever having taught you any grounding principles. :roll: I just basically said it appears we're both doing the same stuff, which is good :)
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Re: Grounding, again, on GDS 18W combo kit

Post by JMPGuitars »

zaphod_phil wrote:
Mon 03/23/20 2:37 pm
I don't recall ever having taught you any grounding principles. :roll: I just basically said we're both doing the same things, which is good :)
We've had a few conversations on the subject. Trust me, I learned at least 90% of my ground schemes from you. So, thanks. ;)
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

I kind of grouped them. I was torn between grouping them tightly but also wanted to keep the wires as short as possible.

Should all the grounds for each stage go to a common point on the bus, with the end of each stage being the cathode of a tube?

I pretty much followed Richie’s layout for this one.
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

Except I did put the chassis connection for the preamp bus over by the inputs!
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Re: Grounding, again, on GDS 18W combo kit

Post by zaphod_phil »

bvayling wrote:
Thu 03/19/20 9:13 am
I'm reading up on grounding (JMP's Modern Grounding) and using the 2008 revised layout with Ritchie's Tremolo improvements http://18watt.com/storage/18watt_layout ... ents_b.pdf to assemble the kit.

I have the following questions:

1. JMP's comments suggest using shielded wire for all inputs. JMP's notes also say :" 1 ground near the Input jack covers input, pots, preamp. Ideally the preamp ground bus connects to the input jack(s) and the input jacks connects to an independent ground spot as close to the jacks as possible."
This is confusing to me. It suggests that the input jack is grounded twice, once to the bus and once to its' own dedicated grounding lug on the chassis. So we have two separate ground wires coming from the input?
No, The input jack is grounded directly to the chassis right there, and also provides a ground reference to the preamp ground buss wire. Without that, the ground buss would just be a floating piece of wire, and would be no use to the rest of the preamp and PI.
I trust that makes some kind of sense.

bvayling wrote:
Thu 03/19/20 9:13 am

Then there is the trem input. Same? Or have a dedicated lug in the middle for both inputs?
A single lug between both input Jacks.
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

I think what I'm getting after distillation:

Input ground goes directly to a chassis ground point and the preamp ground bus gets connected to that same point.

Is this correct?
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Re: Grounding, again, on GDS 18W combo kit

Post by bvayling »

Thanks for the "input." Since I just saw this now and already wired up the amp, here is a photo of what I did. Trying to distill all the information, I decided to ground each input to the bus with the shortest possible wire and then a really short wire from the end of the bus to the chassis. Otherwise, with a chassis ground in between the two inputs, I would wind up with a longish wire from each input to the chassis with another longish wire from the end of the bus to the same point.

I actually have the amp open because I need to adjust the bias (I have Q's about that too, but I'll go to the other thread for that), so I can change it again, but it is quiet with no problems so far.
fullsizeoutput_f51.jpeg
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