Tube mic PSU with no safety ground

[jordan s]

I am working on the PSU for an Altec 175 small cap tube mic and noticed it has no safety ground, just the two prong mains connector. It looks like the power transformer case, mic output transformer and mic output are grounded to the chassis but that's it. I want to install a new cable with safety ground and connect it to the chassis. It is worth modifying the original design? Is there a chance this could introduce noise?

 

[emrr]

The idea here is that it grounds the case through the next piece of equipment, so as to not introduce a ground loop between mic and preamp since they are in many cases not likely on the same circuit.  Tangentially related, you find that many (most? all?) stand-alone phantom power supplies don't have a ground for the same reason, it carries through from the preamp.

 

[gridcurrent]

It is worth modifying the original design?

no.

Is there a chance this could introduce noise?

yes.

Doug's comments are +1.

The psu case is grounded to the following device via the cable shield.
The method has worked well for 70+ years.

Not as effective as a large gauge wire to a safety ground, but "good enough".

 

[ruffrecords]

I have to disagree with the others. People's lives are far to valuable to risk safety on a possible connection to another puce of equipment which may or may not be properly safety grounded. The output is transformer balanced and floating so there is no danger of causing a hum loop - there is no signal flowing is the screen.

My advice is the rewire it with a the3  pin plug with the earth bonded to the chassis  at the point of entry. If you do get a hum loop, which I doubt,  then lift the screen at the far end.

Cheers

Ian

 

[Tubetec]

I agree with Ian ,the output transformer will galavanically isolate and prevent ground loops .
Its better to have a good solid fat wire direct to ground and break the connection in the screen between the two devices ,your removing any possibility of an ungrounded condition .

Guitarists who run stereo amps often end up with a ground loop , many will lift the plug ground on one of the amps to cure this ,but as Ian said the correct way to do it is break the screen connection and leave both units with a proper safety ground . Moulded iec mains cables are manditory nowadays so as its not possible to lift the ground at the plug .

 

[JMFahey]

+1 on what Ian and others said: properly ground the chassis.

"It worked for 70 years"  does not cut much ice, and consider that if that fine piece of equipment were made **today** ,it would have to comply with all current safety measures or else.

Compare a 60's Fender tube amp schematic, any of them, say a Bassman, Deluxe, Champ, *any*  of them side by side with current reissues by the same Company, and check how *all*  of them, not a single exception, have been modded to meet current rules.

 

[emrr]

I have to disagree with the others.

You're disagreeing, yet I gave him no particular advice, simply explained the working concept so the OP could consider it. 

The over-reactionists here are neglecting to consider this is not at all like an ungrounded guitar amp.  There is always a ground through the following equipment, or you have no audio.  These can still create ground loops, even with an audio transformer, even with a side lifted, suggesting they cannot proves one has never worked a job with a transformer mic splitter having multiple varying mains connections. 

You may disagree with that if you like. 

 

[cyrano]

The over-reactionists here are neglecting to consider this is not at all like an ungrounded guitar amp.  There is always a ground through the following equipment, or you have no audio.  These can still create ground loops, even with an audio transformer, even with a side lifted, suggesting they cannot proves one has never worked a job with a transformer mic splitter having multiple varying mains connections.

Unless there's nothing plugged in, or a faulty cable that already had shield ground lifted because there's a hum problem.

I've seen that happen and have been shocked by it.

 

[Matador]

Re-wire it.  There's no way to control what's on the receiving end, there are 0 reasons to chance it.

 

[emrr]

It's been explained why it is the way it is, and what the various fault potentials may be, why it may not like a ground connection through the immediate AC mains.  The OP should be able to make an informed decision based on the way it reacts in the system install. 

Another option is to make sure it's used with a GFCI receptacle.  We don't know the situation in which it will be used.  You can interpret that last sentence two ways. 

 

[Whoops]

The over-reactionists here are neglecting to consider this is not at all like an ungrounded guitar amp.  There is always a ground through the following equipment, or you have no audio.  These can still create ground loops, even with an audio transformer, even with a side lifted, suggesting they cannot proves one has never worked a job with a transformer mic splitter having multiple varying mains connections.

I agree with Ian and everyone else, that grounding scheme is outdated and not safe.

Safety should be put first.

Then EmRR is right that there's still a potencial for ground loops, but that doesnt mean you should not connect Mains Earth to chassis, it just mean that after you connect Mains Earth to chassis you should change the old grounding scheme so it eliminates or reduce any ground loop potencial.

You Should disconnect all the 0V reference connections that are tied to the chassis, and then connect the 0V reference (often called audio ground) to chassis only at one point and through a 100nf and 100r resistor

Output screens , or output XLR Pin 1 I would leave disconected

 

[jordan s]

Thanks for all the responses. On every guitar amp I've worked on, I've chosen to install a safety ground where they were not originally implemented because the power situation at music venues is less predicable and people often get shocked through the mic grill.
Ultimately, my client (the owner of these units) has quite predictably decided it's not worth paying me extra to install a safety ground. If they belonged to me, I probably would. He's been informed of the risks and it's out of my hands. Thanks all.

 

[JohnRoberts]

Thanks for all the responses. On every guitar amp I've worked on, I've chosen to install a safety ground where they were not originally implemented because the power situation at music venues is less predicable and people often get shocked through the mic grill.
Ultimately, my client (the owner of these units) has quite predictably decided it's not worth paying me extra to install a safety ground. If they belonged to me, I probably would. He's been informed of the risks and it's out of my hands. Thanks all.

Tell him he can purchase a portable GFCI outlet strip that will protect against dangerous leakage current (by shutting down)... But musicians usually ignore such warnings... Luckily we are not running out of musicians, but everyone of them hurt is still unnecessary.

JR

 

[JMFahey]

YOU-CAN-NOT-TRUST-VENUE-WIRING/GROUNDING, period.

I paste a Video link of a guy I know (not "something I saw on the Net") , using amplifiers (Alien) made by a guy I know personally, who is an Engineer and works very well, yet ... "shit happens" .... so best is to play it as safe as possible at every link in the chain, it´s unwise to leave some link unprotected "because others will".

In this particular case , either amp or PA ground was inverted, making it HOT.

Straight 220V across each other, watch the sparks fly, then Guitar Player mutes strings with left hand, grabs mike stand with the right one, instant 220V across his chest and "he couldn´t let go".

He´s alive *only*  because of a happy combination:

1) his father in Law had made a CPR course just a few days before .

2) it was a Municipal Convention center and next room was full of "Senior Citizens" who had hired an Ambulance "just in case" so he was given full Ressucitation within 2 minutes ... they told him that waiting 10 minutes or more for a "phoned in"  ambulance might have been fatal.

3) he was about 25 y.o.  and had a strong young heart ... even so NOW he has scar tissue in his heart, exactly as if he had a massive heart attack, (Doctors found a specific enzyme or hormone which is released after a heart attack and which confirms it) and now he´s half crippled for the rest of his life (no heavy Sports, no running, avoid stairs, etc.)

https://www.youtube.com/watch?v=vqMehAA2Fso

PS: some izzits have criticized the "unprofessional video"  :  .... just consider his Wife was recording the show on her phone and dropped it when her Husband was dying before her.

 

[zamproject]

Hi all

There should not be any debate about safety ground and (metallic) chassis wiring in a public forum...

Best
Zam

 

[jordan s]

So there is no confusion, these mics will be used in a controlled studio environment, NOT live music venues. It is the owners decision on whether or not he wants them to be modified or remain original. He has chosen the later. While it may be the riskier choice and I believe he came to that decision for financial reasons, it it his choice and I have informed him of the risks.

 

[JohnRoberts]

Good luck making people do the right thing (in our opinion).

+1 chassis safety ground is useful (ASSuming you have grounded power outlets.)
+2 GFCI power drops will protect against most imaginable fault conditions. (Note: GFCI will protect even with ungrounded outlets).

JR

PS: I just noticed that the carpenter working on my roof last week, had the ground pin broken off his extension cord... no doubt a better fit with my ungrounded outdoor outlets. (something else I need to upgrade).

 

[gridcurrent]

So there is no confusion, these mics will be used in a controlled studio environment, NOT live music venues. It is the owners decision on whether or not he wants them to be modified or remain original. He has chosen the later. While it may be the riskier choice and I believe he came to that decision for financial reasons, it it his choice and I have informed him of the risks.

maybe the owner is reading this thread.  smart to leave the psu un-molested.
you know, life involves taking risks, including plugging in the Altec power supply before it is grounded to the console.
one takes a risk walking across the street or flying in a modern aircraft.
so far 2631 lives have perisished in 747 accidents.
https://www.theguardian.com/news/datablog/2009/jun/30/yemen-plane-crashes
but most of us have no qualms hopping on one of those machines.
a question to all:  how many people have been hurt from 2 wire fitted condensor mic power supplies,
Altec, Neumann, AKG, Schoeps, Sony, RFT, Stephens, did I leave any out?
check police reports, insurance settlements, whatever.  lets see the numbers.
if it has not happened in 70+ years, it probably won't.
and we are NOT talking about mickymouse guitar amplifiers with death capacitors used in damp, beer soaked taverns.
oh, lets add to the 2 wire list:  ALL the Pultecs and LA-2's.

 

[ruffrecords]

maybe the owner is reading this thread.  smart to leave the psu un-molested.
you know, life involves taking risks, including plugging in the Altec power supply before it is grounded to the console.
one takes a risk walking across the street or flying in a modern aircraft.

Indeed one does, but it is not the risks themselves that are important it is whether or not we choose to take them that matters. In other words, there is no point in taking unnecessary risks.

Cheers

Ian

 

[Newmarket]

there is no point in taking unnecessary risks.

+1