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[mikezietsman]

Through some very good luck I have recently acquired a neumann m269 to go along with my pair of u67s.

I got a good enough deal that I didn't mind particularly that the capsule had been swapped for what looks to be a reskin. I am also lucky that in a past life I happened on a large batch of spare k67 capsules.

I am however looking forward to finally hearing, in person, the difference between a reskinned capsule and a legitimate neumann capsule.

Would it be safe to use one of the heads of my u67s to test the difference between capsules without the delay that comes with desoldering etc etc etc?

I also wanted to ask about the longevity of the ac701k in this circuit...

I know that tubes can go at anytime but, assuming that the power supply is properly calibrated... what is the predicted lifespan of the tube in these mics?

And are there any major no-nos with regard to tube life?

(I am in South Africa, we suffer from power blackouts and I suspect the occasional voltage dip.)

[klaus]

Keep the M269 head in cardioid when testing on the U67 body. That's the only pattern that will work in this combination.

The AC701 is a very fragile tube that does not like voltage spikes: one hot plug (full, unloaded voltage from the power supply jamming into tube without gradual ramp-up time) may be deadly; so the best you can do to preserve the tube is to always connect the mic to the supply before turning it on.A gentle start-up with a well designed power supply that's well calibrated is your friend.

Read also my primer on the AC701 tube: https://repforums.prosoundweb.com/index.php/topic,1150.0.html

[mikezietsman]

So either head will work on either mic as long as they are in Cardiod?

Will voltage dips in the mains supply pose any risk to the ac701?

We’re currently in a phase of scheduled blackouts and I suspect the supply is not great.

The power supply is an nn48h

[Kai]

Will voltage dips in the mains supply pose any risk to the ac701?

We’re currently in a phase of scheduled blackouts and I suspect the supply is not great.

A power line situation like this is always connected to large overvoltage spikes which are likely to kill all kinds of electronic devices.
Even here in Germany, when I had to put an over-voltage protection into my central breaker box, failure of studio devices immediately was reduced from often to very few.

Here is an example of such a protection:
https://www.hager.de/modulargeraete/ueberspannungsschutz/ueberspannungsableiter-typ-3/spn203n/950999.htm

These protections are simple, cheap ($200 for a professional version), very effective, and pay off soon. The stationary breaker-box versions are much more effective than the ones built into multi-outlet-strips.
Installation must be done by a qualified electrician.

Over-voltage protections don't deal with smaller scale voltage variations, they cut surges of 1250+ Volts only.
For sensitive devices I suggest to ADDITIONALLY use a battery backup computer UPS (uninterrupted power supply - K.H.) with integrated regulation and sine-wave output.
The cheap ones don't regulate and put out waveforms with huge amounts of overtones (called power line voltage distortion in %) which creep into the audio, so get information before you buy.

A good UPS can bridge blackouts for several minutes, and give you the possibility to properly power down your computers without data loss, too.

And while we're at this: I strongly recommend to install a "ground fault circuit interrupter", other names see:
https://en.m.wikipedia.org/wiki/Residual-current_device

This thing saves lives, especially with historical music equipment not built by today's safety standards.
In my studio it has saved at least one person from beeing electrocuted, who had the great idea to solder a directly connected line level audio output to the internals of a TV set he used as monitor for his computer.

[Jim Williams]

One can fit a MOV device (metal oxide varister) across the AC pins on the internals of the gear. Those clamp surge voltages and are effective and cheap.

[klaus]

Jim,
Please expand on your post in a non-technical language everyone can follow.
Thanks,
KH

[Jim Williams]

Bourns makes the MOV devices, available from Mouser and other distributors. The MOV-07DxxxK series is small and can be soldered across the IEC AC power inlet jack. You pick the surge voltage from 18 to 820 volts. Use the 120 volt for the USA, the 250 volt for the EU.
p/n 652-MOV-07D121K for 120 volts, p/n 652-MOV-07D101K for 250 volts. Those are Mouser part numbers. They only cost 10 cents each in single quantities, a rather low cost "insurance" policy.
www.mouser.com

[mikezietsman]

Just a quick follow up...

It would appear that a u67 head does not work on an m269 in cardioid mode. Luckily it also appears to have not done any damage. (there was just no output)

Thank you so much to everyone for their advise on the power variations...

My current strategy until I can get something like Uwe's reccomendation, is to be powered down before the blackouts (we have a nationwide schedule) and to make sure I stay powered down for a good ten minutes after the power comes back on.

[klaus]

I guess you misinterpreted my sentence, clumsily written:
("Keep the M269 head in cardioid when testing on the U67 body. That's the only pattern that will work in this combination."
I was referring to the ONLY combination that will work without modifying mics or heads: M269 head on U67 body, and it will only work in cardioid.

[Kai]

One can fit a MOV device (metal oxide varister) across the AC pins on the internals of the gear. Those clamp surge voltages and are effective and cheap.

Jim, I have long thought about your proposal here and it causes me headaches.

In my opinion it's no good idea to suggest doing modifications in the line voltage internals of any device.
The manufacturer has thought out, tested and got approvals on those parts and any changes here mess up the construction.
There are a too many things that can go wrong.

You buy a minor (and incomplete) improvement in surge protection on the expense of making the whole device unsafe in a lot of other aspects.
This can cause fire, injuries or death of persons.

If someone wants to go this cheap way a socket board with built in protection can be used to achieve the same or even better protective results.
Just don't expect wonders from that.

Professional overvoltage protectors don't rely on varistors only, they include spark gaps and a circuit breaker and are built to safely deal with the high energy amounts that can appear.
They even indicate a failure so you know that they have lost their protective effect after a very strong surge like from a lightning flash.

[Jim Williams]

Many manufacturers use these MOV devices in their construction, it's a proven product. Relying on the bean counters to look out for you is risky as you have determined.

Besides the MOV, a Corcom IEC line filter is another good add on to any AC powered equipment. Those cost a few dollars and provide extra AC line filtering.

[Kai]

Many manufacturers use these MOV devices in their construction, it's a proven product...
...a Corcom IEC line filter is another good add on to any AC powered equipment...

It's not if, but how the mentioned parts are incorporated into a design.
E.g. the MOVs are often covered with extra glass cloth/silicon isolation.
It's important how these are mounted, as they can become extremly hot and can melt plastic and unsolder mains cables if simply soldered across the mains inlet sockets.
There needs to be a fuse in front that can swich off 2000+ Volts without turning into a light arc ...

I could go on an on.

This is not a power electricians DIY forum, people reading this often have minor or no skills in that area.
Telling people to "simply spend few cents and mod your device" leads into catastrophe.

I am skilled and would not mod an existing contruction, as because I am skilled I know about the consequences and know what (destructive!) testing would need to be done before I could call something safe.
 
The only exception if I see a definite faulty construction, mainly those from early US products where powerlines with only single isolation run all across the device in an unearthed cabinet (have you ever looked inside the famous Carver/Phase Linear amp constructions?).

My personal conclusion and last word on this topic:
Leave everything as it is as long as it's factory original safely constructed. There is not much that can be gained from mods in the power line section.
The more you do the more can go wrong which can cost a life at worse.