Grounding

[Snake5150]

Is it better to ground the pre amp filter cap to the bus wire in a jtm 45 build. I’m using an aluminum chassis and saw a grounding scheme that made me curious.

As I understand ground everything to the pt center tap via a screw and solder lug.

The scheme I saw saws to ground the pre amp filter cap to the buss wire on the pot?

The power tube grounds, run wire to pt center tap or ground the resistors to the chassis.

Line ground will get riveted to a solder log thru the chassis.
Considers die electric anti oxidizers on all connections.

Pics attached.

[sluckey]

Line neutral will get riveted to a solder log thru the chassis.

NO! Neutral does not connect to chassis.

I prefer Hoffman's ground scheme.

[tdvt]

Line neutral will get riveted to a solder log thru the chassis.

I think you are loosely using the terms "neutral" & "ground", they are NOT the same thing.


The AC neutral in the layout you attached is the blue wire (labeled "N") on the IEC AC inlet, & that runs directly to one side of the PT primary. The AC ground is the green wire from the IEC inlet that is connected to the chassis.

The drawing has the blue wire kind of passing over the IEC mounting nut, but there is no connection. Not well drawn.

The AC ground is meant to have it's own dedicated attachment point to the chassis (as shown), & you would connect your main circuit ground  to the chassis in that immediate area with it's own attachment.

[Snake5150]

Line neutral will get riveted to a solder log thru the chassis.

NO! Neutral does not connect to chassis.

I prefer Hoffman's ground scheme.

I mis typed. I meant line ground

[Snake5150]

Line neutral will get riveted to a solder log thru the chassis.

NO! Neutral does not connect to chassis.

I prefer Hoffman's ground scheme.

I mis typed. I meant line ground

I’m confident in wiring the iec jack and plug was mainly referring to grounding the caps, also the screen resistors

[Merlin]

Is it better to ground the pre amp filter cap to the bus wire in a jtm 45 build. I’m using an aluminum chassis and saw a grounding scheme that made me curious.
As I understand ground everything to the pt center tap via a screw and solder lug.
The scheme I saw saws to ground the pre amp filter cap to the buss wire on the pot?
The power tube grounds, run wire to pt center tap or ground the resistors to the chassis.

No ground scehem is perfect in practice. The designer has probably done the best he can by modding the atrocious original ground 'scheme' found in the original JTM45. His diagram shows non-ideal heater wiring just like the original, and he has a recto-killing standby switch position before the reservoir cap, so beware.
This vid has some really good examples of what different grounding points do in that amp. Sure, it would be better to rip it up and start from scratch, but most amp techs work by cut-and-try than by hitting the textbooks.

Line neutral will get riveted to a solder log thru the chassis.

 Hoffman's preferred wintage grounding for reference:

[pdf64]

... Hoffman's preferred wintage grounding for reference:

Using a mains transformer fastener, especially if it goes through the lamination stack, seems a rather unsuitable choice for fastening the chassis safety earth lug?

[passaloutre]

... Hoffman's preferred wintage grounding for reference:

Using a mains transformer fastener, especially if it goes through the lamination stack, seems a rather unsuitable choice for fastening the chassis safety earth lug?

Could you tell us why? I frequently use the transformer bolt for ground connections, and I see it recommended often.

[sluckey]

Could you tell us why? I frequently use the transformer bolt for ground connections, and I see it recommended often.

Look at this video by one of the forum members. Jump ahead to 15:25 and you will see a good reason to use a dedicated ground bolt rather than a transformer bolt...

[Snake5150]

Is it better to ground the pre amp filter cap to the bus wire in a jtm 45 build. I’m using an aluminum chassis and saw a grounding scheme that made me curious.
As I understand ground everything to the pt center tap via a screw and solder lug.
The scheme I saw saws to ground the pre amp filter cap to the buss wire on the pot?
The power tube grounds, run wire to pt center tap or ground the resistors to the chassis.

No ground scehem is perfect in practice. The designer has probably done the best he can by modding the atrocious original ground 'scheme' found in the original JTM45. His diagram shows non-ideal heater wiring just like the original, and he has a recto-killing standby switch position before the reservoir cap, so beware.
This vid has some really good examples of what different grounding points do in that amp. Sure, it would be better to rip it up and start from scratch, but most amp techs work by cut-and-try than by hitting the textbooks.

Line neutral will get riveted to a solder log thru the chassis.

 Hoffman's preferred wintage grounding for reference:

So specifically would you do different that the ground scheme layout I provided. I’m not above drilling specific grounds.

[Merlin]

So specifically would you do different that the ground scheme layout I provided. I’m not above drilling specific grounds.

It's basically the same as the Hoffman grounding, so I would leave it as it is. But I would certainly move that standby switch so it comes after the reservoir cap!

EDIT: Do use a dedicated screw for safety earth and the common grounding point, don't use the transformer bolt, as pdf pointed out.

[pdf64]

...
Could you tell us why? I frequently use the transformer bolt for ground connections, and I see it recommended often.

Try servicing amps and you'll find it's not uncommon for mains transformer fasteners to be not tight / loose / snapped / completely missing.

It's been illegal in most regions to sell stuff with its chassis safety earth attached like that for decades.

I've encountered 70s Twin Reverbs whose mains Transformer was hanging on by their wires.

Anyone that opens up commercially made mains equipment made in that time can see that a dedicated fastener has been used.

Yet DIYers and companies that supply and advise them promulgate this bad info, such that this 'worst practice' has become sector wide and self reinforcing.

[passaloutre]

Ok, I'm convinced!

[Snake5150]

So specifically would you do different that the ground scheme layout I provided. I’m not above drilling specific grounds.

It's basically the same as the Hoffman grounding, so I would leave it as it is. But I would certainly move that standby switch so it comes after the reservoir cap!

EDIT: Do use a dedicated screw for safety earth and the common grounding point, don't use the transformer bolt, as pdf pointed out.

Stupid question time.

Which large cap in the attached layout is the preamp cap, and what do you mean by reservoir cap?

How does one move the standby switch to be after said reservoir cap.

I’m pretty confident in most things amp related by some of the lingo still has me stupid.

As it stands I’m gonna use the Hoffman ground layout. Except add  a dedicated lug for what would have be attached to the pt leg, and obviously a dedicated lug for line earth.

Am I missing any thing? I also have bias test ports from the power tubes , I’m assuming that that ground also goes to the pt leg or added lug in my case. Same with the outboard bias pot ground.

Thank all

[sluckey]

Which large cap in the attached layout is the preamp cap, and what do you mean by reservoir cap?

How does one move the standby switch to be after said reservoir cap.

The dual 33/33 cap on the board are the preamp caps. The reservoir cap is the one connected to the rectifier tube pin 8 via red wire "A" in the attached pic which correctly places the reservoir cap before the STBY switch.