Silvertone 1448

[lowell]

This is a great thread. Main thing to take away is the isolation of the mains from the secondary circuitry grounding - EARTH. Customer didn't wanna pay $60 to fix it. Ah well. I made it very clear that the thing is potentially deadly.

[nickb]

My understanding of US NEC (National Electrical Code) has Ground and Neutral "bonded" at the service box. IOW, tightly connected within the breaker box, and nowhere else.

What I've done with "widowmakers" is have neutral go to the negative end of the audio & power supply circuits. Ground continues from the AC plug, around the iso transformer, and connected to the metal box that contains the amp. It may be a bit tedious putting insulator washers on each input jack, providing separate circuit grounds instead of using the chassis as "common", etc. but it's what you have to do.

Have I understood this correctly? In other words, neutral is connected to the sleeve of the jack and the chassis is at safety earth. Along comes wrongly wired /lead/plug/outlet and now your guitar is LIVE. If I did understand it right, this is a horrible idea and undoes all the good of the isolating transformer.

Just sayin...

[nickb]

..and just to add a bit of info on why a low resistance earth path is important. Consider there is a live to earth fault on your amp chassis. You could have a 100A fault current (for a little while) . You touch the chassis and now the current has two paths, one through the safety earth and the other through you. The earth conductor is say 1 ohm are your body resistance is say 10k ohms. That means that 10mA will flow through you. Reduce the earth conductor impedance to 0.1 ohms and now it's only 1mA through you.

[rjb]

Have I understood this correctly? In other words, neutral is connected to the sleeve of the jack and the chassis is at safety earth. Along comes wrongly wired /lead/plug/outlet and now your guitar is LIVE. If I did understand it right, this is a horrible idea and undoes all the good of the isolating transformer.

I read it that he isolates the jack sleeves, removes everything else that uses the chassis as common, and runs it all to the "neutral" side of the transformer's secondary. Then tie ground from the AC plug to the chassis.

EDIT:
OK, I just read post #36, and it all finally sank through my thick skull.
Thanks, Juan. All others, please ignore this post.

[Randall]

I'm really confused now. What is the difference between this amp with an iso and a green chassis ground, and a typical Fender with grounds going to chassis all over with a green chassis ground? Aren't the same perils there regarding a reversed outlet? Or is it because most Fenders have a center tapped PT?

The Danelectro I am now working on has the HT grounds elevated off chassis, but some of the preamp shunted to chassis. Should I go to the trouble of isolating the input jacks and pots? In both cases, the Dano and the Fender if there is a reversed outlet, the breaker will blow, right?

[J M Fahey]

neutral is connected to the sleeve of the jack and the chassis is at safety earth.

NO, neutral is not connected to ANYTHING inside, period.
It only goes to one end of the PRIMARY winding which is floating respect to chassis earth.

Enzo said it clearly:

Grounding is simple. The point of the isolation transformer is that it, well, isolates the mains from the 120v inside the unit. The reason you cannot ground with a three wire cord a stock hot chassis amp is that you;d be grounding one side of the mains. With an iso, the chassis and wiring HAVE NO connection to the mains, so you are free to wire a third prong ground direct to the chassis. Yes, it goes around the iso. The ground itself was never the issue, the issue was the mains wiring being coupled to the chassis.

neutral is connected to the sleeve of the jack

NO

Along comes wrongly wired /lead/plug/outlet and now your guitar is LIVE.

It does not happen because neutral is NOT connected to chassis.

If I did understand it right, this is a horrible idea

Fortunately you understood it wrong.

I read it that he isolates the jack sleeves, removes everything else that uses the chassis as common, and runs it all to the "neutral" side of the transformer's secondary.

A non solution to a non problem, mains neutral does not connect to anything inside and no PT secondary is called "neutral" , why would it?

What is the difference between this amp with an iso and a green chassis ground, and a typical Fender with grounds going to chassis all over with a green chassis ground?

If properly wired, none.

Both have a floating primary connected to mains and nothing else, and secondary(es) with at least one winding end grounded ... or it wouldn't provide voltage referred to chassis/circuit ground.

The Fender usually grounds the secondary center tap; the converted widowmaker one end of Iso transformer secondary ... in fact it could be modded a little more (cost difference would be nil) and get a proper PT with full wave rectification, plus a proper filament winding (yes, it can be 100/120VAC .... what's the problem?)

The Danelectro I am now working on has the HT grounds elevated off chassis, but some of the preamp shunted to chassis. Should I go to the trouble of isolating the input jacks and pots? In both cases, the Dano and the Fender if there is a reversed outlet, the breaker will blow, right?

Just install the ISO transformer the proper way, which is:

* primary to live and neutral and nowhere else.

* simplest connection: secondary winding has 2 ends: one goes to chassis, other goes to tube rectifier.

* Series filaments keep one end grounded and other end to 110/120V SECONDARY.

Amp is happy working exactly as before, it has no clue that 120V it's receiving come from a floating secondary and not straight from mains.

[Randall]

Thank you Juan, this is what I thought I knew before reading this thread, which I admit has confused me. This is how I would have wired this amp, as I have before in other situations. But for one last time, is it correct to wire the green chassis ground according to your above instructions?

[nickb]

NO, neutral is not connected to ANYTHING inside, period.
It only goes to one end of the PRIMARY winding which is floating respect to chassis earth.

Enzo said it clearly:



NO


It does not happen because neutral is NOT connected to chassis.

Fortunately you understood it wrong.


A non solution to a non problem, mains neutral does not connect to anything inside and no PT secondary is called "neutral" , why would it?

If properly wired, none.

Both have a floating primary connected to mains and nothing else, and secondary(es) with at least one winding end grounded ... or it wouldn't provide voltage referred to chassis/circuit ground.

The Fender usually grounds the secondary center tap; the converted widowmaker one end of Iso transformer secondary ... in fact it could be modded a little more (cost difference would be nil) and get a proper PT with full wave rectification, plus a proper filament winding (yes, it can be 100/120VAC .... what's the problem?)


Just install the ISO transformer the proper way, which is:

* primary to live and neutral and nowhere else.

* simplest connection: secondary winding has 2 ends: one goes to chassis, other goes to tube rectifier.

* Series filaments keep one end grounded and other end to 110/120V SECONDARY.

Amp is happy working exactly as before, it has no clue that 120V it's receiving come from a floating secondary and not straight from mains.

Juan - except that the problem is in this post http://music-electronics-forum.com/t...18/#post421818 it was said " is have neutral go to the negative end of the audio & power supply circuits" and I had to highlight this was WRONG just is case anyone thought otherwise. Maybe he did mean otherwise but that is not what is written.

[Leo_Gnardo]

Yes, the misunderstanding arose from referring to one end of the iso transformer secondary as "neutral".

I agree with the following, thanks nickb for pointing out the error and thanks Juan for straightening us out:

A non solution to a non problem, mains neutral does not connect to anything inside and no PT secondary is called "neutral" , why would it?

---

Just install the ISO transformer the proper way, which is:

* primary to live and neutral and nowhere else.

* simplest connection: secondary winding has 2 ends: one goes to chassis, other goes to tube rectifier.

* Series filaments keep one end grounded and other end to 110/120V SECONDARY.

Amp is happy working exactly as before, it has no clue that 120V it's receiving come from a floating secondary and not straight from mains.

[nickb]

Yes, the misunderstanding arose from referring to one end of the iso transformer secondary as "neutral".

I agree with the following, thanks nickb for pointing out the error and thanks Juan for straightening us out:

I'm sooooo glad to hear it! I was worried for you.

[J M Fahey]

Cool, hate losing Friends to deadly Electrons

Yes, once amplifier side is fed from a floating, isolated from mains Iso transformer secondary, now you can connect mains green wire to chassis ground.

It will both make the amp safe and lower hum and buzz substantially .... win-win situation.

And what if the sloppy electrician wired hot/live to outlet ground socket?

Not impossible, might be what happened here, which I already posted several times in different Forums:

[Leo_Gnardo]

I'm sooooo glad to hear it! I was worried for you.

Points also to Randall for persistent questioning!

Juan, in the case of a reverse wired outlet, I expect the hot-ground short should knock down a breaker tout-suite.

OTOH I have run across club wiring which involved no breakers or fuses.

[nickb]

Yes, the misunderstanding arose from referring to one end of the iso transformer secondary as "neutral".

I agree with the following, thanks nickb for pointing out the error and thanks Juan for straightening us out:

Well, I'm glad we got that straightened out.

Putting on my Mr Negative hat again, I still think there is a safety issue with the floating ground.

There is a certain amount of stray capacitance between the windings - just measured it at ~500pF on a 300VA isolating transformer I have here. For 120VAC 60Hz counties this is a leakage of just about 10uA. Won't do you any harm for sure, you might not even feel it. But what happens should the transformer (or other) insulation breakdown? Then the only path to earth is though you. For that reason, it's quite common to put a network between the local floating ground and the safety earth (chassis). I seen simply 100 ohm resistor, sometimes there is a couple of back-back 1A diodes across the resistor and finally you can provide a path for unwanted HF by adding a small capacitor say 10nF too.

Nowadays, with much improved requirements for insulation in transformers this is an unlikely event. And hopefully the RCD will kick in and stop your bacon from frying. On the other hand the extra for the insurance is real cheap...