Ground question (part deux)...

[Paul-in-KC]

All,

After doing some reading from Aiken and Blencowe (The Valve Wizard) to better understand sound grounding principles - I am afraid.

I say that only half kidding. I have built several tube amps. Most from kits (JCM800, JTM45, Princeton Reverb), one (TrainWreck Express) from scratch - but with the documentation here, that was similar to a kit build. I can honestly say that all turned out very nice - sound great and have been 100% reliable (other than a bad filter cap that failed in the JTM45).

But after reading the two discussions of grounding listed above, I don't think I was doing anything correctly (other than following a known good circuit and layout - which I realize is ultimately most important).

So, now I am afraid of all of my output jacks, potentiometers, switches, and half of my input jacks.

It seems that none of these (except the input jack) should be allowed to come in to contact with the chassis.

I am presently building a hybrid 3rd Gen Low Plate (with master volume and presence) - and using one of the chassis from Amplified Nation (hence the master volume and presence).

I haven't purchased isolated jacks, or washers, or anything of the sort. Taylor's chassis are thoughtfully made and beautifully finished (powder coated) - and he masks off several sections of the interior for the specific purpose of making good ground connection to the chassis (at least that seems the only possible reason for doing so).

So, for my Dumble project, should I assume that it's reasonable to make direct connection to the chassis at all of these points?

I am (almost) sure that it must be - otherwise Taylor wouldn't have masked the areas in the first place. But if I make contact with the chassis at all of these points, it's going to be very difficult for me to understand exactly what's going on - on the ground side if the circuit.

Do I need to isolate all of the jacks and pots (doesn't seem likely) - and make only one connection from "circuit ground" to chassis? (in accordance with best practices)

Probably more likely - allow direct connection with chassis at these spots and follow a "typical/traditional" grounding scheme for the ODS.
Something like the "Ground Map" that illustrated in the schematic for the 1984 version of #124?

Seems like it would make sense to follow (at least the essence of) an established good grounding scheme for this family of circuits.

Not sure if this reads like a question, but any thoughts, opinions, guidance would be appreciated.

Kind Regards,
-Paul

[pompeiisneaks]

I think the general reason to have isolated jacks is because jacks are notoriously bad as grounds. The work great when the amp is first built, but via vibration and/or corrosion they lose ground. I may be wrong, so take that with a grain of salt, but I've always grounded to non isolated jacks AND put a small ground wire soldered into the ground bus directly to chassis at that same point. I've never had a problem so far. So long as you ground your higher current higher noise sources far away from there, you'll be fine.

That's my 2c.

~Phil

[M Fowler]

We each have our own way of getting the noise knocked down and mine is this:

I either use Cliff Jack input or isolate the Switchcraft jack from the chassis.

Run all preamp grounds to a chassis bolted solder lug.
This includes the filter cap ground for the preamp and the above choice of input jack.

The filter caps for the PI, screens, and main filter cap go to the same ground lug as
the Power Transformer (PT) Center Tap (CT) bolted to chassis near the PT.

The IEC receptacle or power cord ground wire has it's own ground solder lug bolted near the incoming AC cord or IEC receptacle.

Mark

[Paul-in-KC]

We each have our own way of getting the noise knocked down and mine is this:
...

...The IEC receptacle or power cord ground wire has it's own ground solder lug bolted near the incoming AC cord or IEC receptacle.

Mark

Mark - thanks for that info! It is helpful.

Yeah, I think I will use the grounding map from the #124 schematic as a general guide and throw in what I have read, been told, and intuition.

It is fun coming to understand more of the circuit details. In the past, I have never really been forced to think about grounding as the "other half" of the circuit (just following known good layouts). Definitely interesting. I enjoy learning more than almost anything else.

Thanks again!
-Paul

[Paul-in-KC]

So far - no one is pitching the "single connection to the chassis" thing.

But Aiken and Blencowe seem to be pretty emphatic and unambiguous on this point.

I suppose from the academic perspective, it removes all "luck" from the equation (since you aren't using the chassis as a pseudo random ground buss). And also makes modifications easier (with respect to understanding the grounding).


If one was going to subscribe to the single connection, then these "ground-nodes" (not connected to chassis) would be connected together (through buss or star) and then connected to chassis (at the input). Yes?

Having multiple chassis connections is effectively a buss that is hard to interpret? No? (probably other implications too, but ignoring those)

-Paul

[norburybrook]

I've never had a noisy Dumble amp so I wouldn't over think this.

Blencowe states that screen grid resistors should be at least 1k and Dumble doesn't adhere to that either.



M

[martin manning]

Seconded! The Dumble hybrid ground scheme works fine, and it is very quiet. Why mess with it?

[Paul-in-KC]

I've never had a noisy Dumble amp so I wouldn't over think this.

Blencowe states that screen grid resistors should be at least 1k and Dumble doesn't adhere to that either.

M

Right - probably guilty of overthinking (many things) - but in my case, really just trying to understand.

Good to hear that noise isn't of particular concern - presumably I should be fine using the grounding map/scheme from the schematic of #124.

Thanks,
-Paul

[Paul-in-KC]

Seconded! The Dumble hybrid ground scheme works fine, and it is very quiet. Why mess with it?

Martin,

Wasn't my intention to second guess Dumble's work or mess with his scheme. I was just trying to sort out how I am going to handle grounding my build.

I think the docs posted on #124 should be a good guide.

-Paul

[sluckey]

The learning will really begin when you actually build something.

[Paul-in-KC]

The learning will really begin when you actually build something.

Indeed. And though moving at a glacial pace on this build I am actually getting some things done.

And I do really appreciate those who take the time to answer questions and share their knowledge (and Steve - you are certainly one of those). Apologies if this question/post somehow offended you.

Most of the hardware mounted - PT primary and heater wires; Relay transformer primary; OT secondary wired; Heater circuit works on all sockets. Main power supply filter board assembled; and misc other stuff...






Here are a couple of pics from my TrainWreck Express build - turned out very nice...

[sluckey]

No offense was intended and none received. It was just a joke that didn't land well. Should have used a smiley. Good to see you actually got it off the ground.

[norburybrook]

looking good. I personally like to do elevated heater wiring last, there's quite a lot to get in there and having the heater wiring can get in the way if you're not careful, which I'm not particularly....


M

[stelligan]

looking good. I personally like to do elevated heater wiring last, there's quite a lot to get in there and having the heater wiring can get in the way if you're not careful, which I'm not particularly....


M

Too late now, but I second this. My first builds all suffered from melted heater insulation. We likely have all been there *Looks very nice so far, BTW.

[M Fowler]

Just to followup on my previous post because I didn't realize the OP was in the Dumble section.

Dumble ground wiring is very quiet and I have never had any issues.