found a chassis in my storage place :D

[Guy77]

Great looking cabinet Marcus! Did you use pine? I like how light pine is . I am also curious to hear your differences between grounding the output transformer near the spk jack or the main B+ location.
I have also had success in grounding the spk jack on the bus wire at the point where the global negative feedback is implemented .

Here is something I dug up from a previous post here.
https://ampgarage.com/forum/viewtopic.php?t=2009

****************************************************
Aiken writes:

"the output transformer secondary "common" should be run directly to the shield connection of the output jack (preferably using an isolated jack), not to the buss. Then run a second wire from the output jack shield connection over to the buss at the point where the global negative feedback is implemented (usually the phase inverter ground point). If the amp is not using global negative feedback, just run the wire over to first filter cap ground. This keeps the heavy output stage currents flowing in a loop from the secondary of the output transformer to the speaker and back, keeping them out of the sensitive preamp or buss ground circuits, and off of the chassis. The wire back to the phase inverter carries no significant current, but provides the "reference" ground for the feedback loop to work properly."

Tdale wrote:
Also..when we talk about the GNF resistor being grounded to the bus.. we mean the grounding of the presence pot, right? The wire from the output jack would have to go to the place on the bus where the presence pot is grounded, right?
yep!

*****************************************

Cheers
Guy

[norburybrook]

Guy,

Thanks, yes I did use pine. It's incredible how light it is compared to 18 mm plywood, also it's easy to work with regarding cutting and especially routing.

I've got a new Celstion B300s speaker to try in it too. A 300w speaker that's supposed to be like an EVM12 but is neodymium so only weighs 4lb's, so I'm hoping this will be a reasonably portable combo I can throw in the car with a guitar and be self contained with just one delay/reverb pedal that will power from the footswitch in theory

just bought one of these for the purpose

NUX.jpg

I'm going to tolex today.

The grounding is interesting so let's see how that goes. Normally the NFB grounds at the loop jacks, however in this case as they're now a separate amplifier stage so that's no longer relevant..perhaps.....I've never had to think about grounding before as the standard ODS layouts are fine, so again, I'm learning some new stuff.


M

[norburybrook]

No tolexing was done today......

Transformers arrived from Italy so they're now firmly installed.....

IMG_20191108_160355.jpg

IMG_20191108_160322.jpg

perhaps I will tolex tomorrow...


M

[martin manning]

Normally the NFB grounds at the loop jacks, however in this case as they're now a separate amplifier stage so that's no longer relevant..perhaps.....

NFB ground and PI ground are one and the same. There will be some AC current from the OT secondary flowing in the NFB circuit, so grounding the PI and speaker jacks close together makes sense.

[talbany]

Guys
Aiken lays out all star gnd points including output transformer here. I highly recommend reading the whole paper on the subject
https://www.aikenamps.com/index.php/grounding
"THE OUTPUT TRANSFORMER"
The amount of current flowing in the secondary is huge compared to the signal currents in the rest of the amp, so the utmost care should be taken here. The secondary current goes only to the speaker; it isn't used anywhere else in the amp, unless there is a negative feedback loop, in which case a small portion of the secondary voltage is fed back, usually through a large resistor, so the current in that path is small. In either case, feedback or not, the secondary should be wired directly to the output jack. Do not ground the output transformer common to the chassis and then ground the output jack to the chassis. This will create a heavy ground current path through the chassis, which may run through a preamp section, depending upon the location of the output jacks and the output transformer.Do not ground the output jacks to the chassis at all, they should be isolated from the chassis. Also, do not route these output transformer secondary wires anywhere near the preamp stages, they should be routed as far away as possible, around the edges of the chassis to the output jacks.

If the amplifier uses global negative feedback, there will have to be a ground path from the secondary common back to the phase inverter tail ground (or wherever the feedback is returned to). This should be done via a wire from the ground of the output jack back to the ground of the stage the feedback is applied to. The idea is for the feedback amp to amplify the difference between the feedback "hot" lead and the feedback ground lead, but nothing else. Note that there is very little current flowing in this ground wire. The total secondary current only flows in a loop around the secondary and the speaker, not back through this wire, so it is a voltage sense wire only. Even if global negative feedback is not used, it is sometimes necessary to ground the secondary common side to prevent noise or oscillations, and to prevent any chance of mild shocks when touching the jack sleeve and ground while running a signal through the amp.



Tony

[martin manning]

Aiken writes in his white paper: "If the amplifier uses global negative feedback, there will have to be a ground path from the secondary common back to the phase inverter tail ground (or wherever the feedback is returned to). This should be done via a wire from the ground of the output jack back to the ground of the stage the feedback is applied to."

Or, you can do as Dumble did, i.e. let the speaker output jacks ground the OT secondary to the chassis, and run a wire from the PI ground to a location near the speaker jacks.

It has been mentioned several times already that running the OT common directly to the speaker jack sleeve keeps the OT-speaker current loop off the chassis.

[norburybrook]

Aiken writes in his white paper: "If the amplifier uses global negative feedback, there will have to be a ground path from the secondary common back to the phase inverter tail ground (or wherever the feedback is returned to). This should be done via a wire from the ground of the output jack back to the ground of the stage the feedback is applied to."

Or, you can do as Dumble did, i.e. let the speaker output jacks ground the OT secondary to the chassis, and run a wire from the PI ground to a location near the speaker jacks.

It has been mentioned several times already that running the OT common directly to the speaker jack sleeve keeps the OT-speaker current loop off the chassis.

Martin,

But with the loop being located less than a couple of inches away from that point now, does that not mean the OT-Speaker current is now going to interfere with the loop as they're grounding more or less in the same place, hence isolating either the loop or the speaker(just one jack) and grounding it away from the loop?

Did Dumble ever build a loop into an ODS? if so I'm sure he'd have sorted out the ground points well, do we have a layout or schematic to show this?



M

[10thTx]

Or, you can do as Dumble did, i.e. let the speaker output jacks ground the OT secondary to the chassis, and run a wire from the PI ground to a location near the speaker jacks.

I am not sure that I am understanding this??? I looked at 4 layouts in the Dumble files and it appears that that PI ground is run to a ground on a FX jack? Can someone more knowledgeable clarify this please?

https://ampgarage.com/forum/download/fi ... &mode=view

https://ampgarage.com/forum/download/fi ... &mode=view

https://ampgarage.com/forum/download/fi ... &mode=view

https://ampgarage.com/forum/download/fi ... &mode=view

Not trying to be contentious at all, just trying to learn and make sense of this? Perhaps I'm misunderstanding something?

IF Dumble did ground the PI on the passive FX loop jack, would that still be done if there is an active on board FX loop?

With respect, 10thtx

[martin manning]

But with the loop being located less than a couple of inches away from that point now, does that not mean the OT-Speaker current is now going to interfere with the loop as they're grounding more or less in the same place, hence isolating either the loop or the speaker(just one jack) and grounding it away from the loop?

Did Dumble ever build a loop into an ODS? if so I'm sure he'd have sorted out the ground points well, do we have a layout or schematic to show this?

I do not know if HAD ever did a built-in loop, maybe someone else can comment on that.

OT-speaker current is not flowing through the chassis if you run the OT secondary common directly to the speaker jack sleeve lug. If you still have non-isolated speaker jacks installed, I would try grounding the PI to the chassis at the loop jacks, or nearer to the speaker jacks and see if one place is any better than the other. If you have isolated speaker jacks installed, I would still run the OT secondary common directly to the speaker jacks, and then ground the speaker jacks and the PI to the chassis nearby. Don't forget that Erwin kept the normal PI ground location and didn't have any issue.

[martin manning]

Or, you can do as Dumble did, i.e. let the speaker output jacks ground the OT secondary to the chassis, and run a wire from the PI ground to a location near the speaker jacks.

I am not sure that I am understanding this??? I looked at 4 layouts in the Dumble files and it appears that that PI ground is run to a ground on a FX jack? Can someone more knowledgeable clarify this please?

Correct. The loop jacks are at a location near the speaker jacks.

IF Dumble did ground the PI on the passive FX loop jack, would that still be done if there is an active on board FX loop?

Erwin did it that way and it worked fine. Marcus can try it in different places and see if it makes any difference.

[norburybrook]

a day of tolexing and cabinet work.


All done. Pleased wit this one, probably my best cabinet to date from a technical standpoint. Its square and the tolerances are good all round. Snake skin looks great

IMG_20191109_170847-01.jpeg

IMG_20191109_170211-01.jpeg

IMG_20191109_170152-01.jpeg

IMG_20191109_135408.jpg

Soldering tomorrow......


M

[martin manning]

Really nice looking cab there! What’s the design on the grille cloth?

[erwin_ve]

Marcus, very nice! I love snakeskin and you always draw attention with that kind of tolex

[norburybrook]

@Martin

It's fender black grill cloth.

https://www.tube-town.net/ttstore/grill ... 90-cm.html


@Erwin

attention from Pythons.......


thanks guy's , yes, as I said I'm pleased with this one. Lets hope the important stuff turns out as well. I may return to the studio this evening after dinner and crack on with the soldering iron..........................not sure that's a good idea after half a bottle of wine though

M

[martin manning]

Thanks, I guess the light reflection is making it look like there is a pinstripe in it. Looks good!