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Author Topic: Something seems to be contaminating the grids of tubes in a mic I made  (Read 9388 times)

soapfoot

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  • brad allen williams

I received some helpful e-mails from Scott, whom Klaus suggested I contact.

He brought up a good point, which made me realize I don't have a soft start or delayed warm-up for either my heater or high voltage supplies.

Could the 'sudden on' nature of my power supply be causing the tube(s) to prematurely fail?  Would anyone have any advice for creating a soft-start circuit in my supply?  NTC thermistor perhaps, or is it not that simple?

I will try subbing a small cap in place of the capsule.  However, I've had some tubes that have 'behaved' for weeks before failing, so it leads me to believe the capsule is OK.  Replacing with a fresh tube always solves the problem, for a short time-- sometimes 'relatively' short and sometimes 'very' short.

Your ideas about further mods are noted.  However, I'd like to get the mic working in this arrangement first, since it sounds very good.  In conversations with Oliver, Joe, etc. it seems that I should be able to get it working with the schematic above.

My grid leak and polarization resistors right now are 200M, not 1G as listed on the schematic.
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Kai

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..I don't have a soft start or delayed warm-up for either my heater or high voltage supplies..
No vintage device ever had a soft start.
Tube life isn't much affected by power cycling.

Changing the tube mostly cured my mic's if the capsule was failing, for a short time, like you experience it.
I don't know why, might be that the noisy stray current needs to take some time to build up after power cycling.
Try the 50-100pF capacitor as capsule dummy just to be sure.
Use an old, known "bad" tube for the 1st test, maybe it's no longer bad then!


2nd: if it's posible to "refresh" the tube like described above, it would be a strong evidence that underheating isn't suitable with the EF800.


One more idea: have you checked for high voltage peaks from the PSU - those can break a tube.
They could be generated by switching off the PSU or from an outside source (machines, fridges, air conditions etc.).
You can use ceramic C's at the PSU out to block those to gound.

Regards
Kai
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soapfoot

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Ah!  Now this is all helpful info.  Thanks.  I will certainly try the cap thing. I assume something in the neighborhood of 60-100pF would be ideal?

I have tried 'refreshing' the tube by overheating for a period.  Mostly I've had no luck with this, although with the first one that failed, that did seem to work, for a time.  After that, though, I haven't been able to refresh any of them in this manner.

The high voltage peaks are definitely something I'll want to eliminate.  While the power in the studio (where it has been failing) is mostly clean as far as I know, and on a dedicated supply that we had run when we did the buildout (the studio even has its own true earth ground) that wouldn't help with turn-off surges you allude to. If it's just a matter of a few ceramic caps to ground I will definitely do that.

Please forgive me for the following simplistic question-- I have no formal electronics training and am just a hobbyist.

If I attach my power supply schematic, could you instruct me where I would install these ceramic caps?

Thanks!

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Kai

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Did you try to replace the tube socket?

I have tried 'refreshing' the tube by overheating for a period.  Mostly I've had no luck with this, although with the first one that failed, that did seem to work, for a time.  After that, though, I haven't been able to refresh any of them in this manner.
This points to, that underheating is not the problem, as you would have successfully refreshed at least 50% of the tubes then - if you had overheated them strong enough (there's information on the internet how to do it).

It seems more likely something bad happens to the tube grids.

could you instruct me where I would install these ceramic caps?
The caps would go in parallel to the 50uF/350V and 10000uF/25V cap next to the 105V / 6V output.
Use 1nF/500V and 1nF/100V ceramic.

Seeing your PSU architecture (classic LC/RC filtering) it's not too likely that high voltage spikes come through, but I still consider the little effort worth trying.
Another source of high voltage spikes is static electricity from carpets on the studio floor (specially if they are quite new). You would notice the discharge sparks when touching something grounded.
Usually those wold not enter inside the mic, but who knows?

- Did you keep cable shield and 0V wires separate?
This means cable shield goes from PSU housing and XLR1 to mic housing only, and PSU 0V's go to tube circuitry only.
- Is your studio supply line voltage properly grounded to earth?

Regards
Kai
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soapfoot

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Did you try to replace the tube socket?

I have not tried that yet.

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The caps would go in parallel to the 50uF/350V and 10000uF/25V cap next to the 105V / 6V output.
Use 1nF/500V and 1nF/100V ceramic. Seeing your PSU architecture (classic LC/RC filtering) it's not too likely that high voltage spikes come through, but I still consider the little effort worth trying.

Absolutely will do, thanks so much.

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Another source of high voltage spikes is static electricity from carpets on the studio floor (specially if they are quite new). You would notice the discharge sparks when touching something grounded. Usually those wold not enter inside the mic, but who knows?

Our studio's floor is a 4 foot thick concrete slab that's original to the ~125 year-old original outer structure (an old Brooklyn factory) that we did our buildout in. We have a few area rugs but no carpet, and have not noticed any static electricity. I feel like we can look elsewhere for now.

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- Did you keep cable shield and 0V wires separate?cThis means cable shield goes from PSU housing and XLR1 to mic housing only, and PSU 0V's go to tube circuitry only.

Ah!  No, I don't think I did this.  I will have to open my cable and double-check.  I do notice occasionally that we have to use a ground lift adapter on the mic's power supply, and wondered why.  This must be the reason why. I will definitely check this, and remedy it ASAP. Could this be causing the mass slaughter of tubes?

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- Is your studio supply line voltage properly grounded to earth?

According to our electrician, who did the install when we did the buildout several months ago, yes it is. 

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Regards
Kai

Thank you so much for helping me work through this and giving me things to check.  I really appreciate it greatly. Same for everyone here.

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klaus

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....
- Did you keep cable shield and 0V wires separate?
This means cable shield goes from PSU housing and XLR1 to mic housing only, and PSU 0V's go to tube circuitry only.


I advise against this. Most microphone systems do not do this separation of grounds. aside of the fact that ground separation cannot cause discharges in tubes or destroy them, it just invites more problems than it solves (noise, hum, RF...) I always connect all grounds centrally in tube mic systems. This guarantees lowest noise floors without any ill effect. Please also read the sticky about why it is impossible to generate a ground loop at the beginning of the recording chain (microphones).
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Klaus Heyne
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Kai

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I always connect all grounds centrally in tube mic systems.
Where is your ground center - mic or PSU?
...Please also read the sticky about why it is impossible to generate a ground loop at the beginning of the recording chain (microphones).
A tube mic with external PSU is no single ended device, as the PSU is grounded for safety.
So it depends how how the mic pre handles ground. The proper way would be XLR pin 1 goes to case and nowhere else.
This is not the fact in most designs, partly because P48 is referenced to GND, and as it's a bit harder to achieve proper RF shielding.
I have to ground-lift commercial tube mics quite often to remove hum (sometimes on the expense of RF stray effects), so I would say with a tube mic it's well possible to build a ground loop via safety earth.

I prefer to connect audio / supply ground to case/safety earth via a 10nf capacitor in parallel with a 10 Ohm resistor in the PSU.
This reduces ground loops to almost zero, gives good RF shielding and a defined potential for the PSU/Audio 0V.

... aside of the fact that ground separation cannot cause discharges in tubes or destroy them
The problem is, that there is no logical reason at all why the tubes break.
So removing all possible or visible problems might hopefully catch the killer.

Regards
Kai
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Kai

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Looking at the circuit of both your mic and PSU I can see the following problem:

The 0V rails of both PSU 6V (pin 5) and 105V (pin 4) are not connected except via the R2/R3 voltage divider in the mic.
This could mean any potential differences caused by the PSU mains transformer's stray effects will discharge across those, causing adverse effects.

I don't even see how the circuit can work correctly without that bridge, as R2/R3 don't even see the 5,05V they are made to work with.

Maybe the bridge already exists (check pin 4/5 on both mic and PSU with an Ohm-meter, without anything plugged in).

If not linked you could try a bridge inside the PSU.
This would keep the GND current flow in the cable untouched, while unifying the PSU GND potential.

Regards
Kai
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soapfoot

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Thanks, Kai and Klaus.  I'll check on all this.  It might be next week before I get to it.  Aside from killing tubes the mic has been performing well, and is quiet, so I suspect I (inadvertently?) put something in place to unify the ground potential that may not be reflected on the schematic.  I'll have to go back and see what I did.

I do recall "where do the grounds come together?" being a question I had some confusion with when building the PSU and making my 7 pin cable. 

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klaus

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Where is your ground center - mic or PSU? A tube mic with external PSU is no single ended device, as the PSU is grounded for safety.

Theoretically you are right. but in practice, I am sorry to disagree: Rarely will you find a safety ground connection (middle pin of the AC connector) on tube mic powere supplies. But even in cases where the AC-safety ground is connected (some modern replacement p.s.) , I have in 25 years of experience never found a single case where an AC ground in a p.s. caused a ground loop.


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The proper way would be XLR pin 1 goes to case and nowhere else.
This is not the fact in most designs, partly because P48 is referenced to GND, and as it's a bit harder to achieve proper RF shielding.


 To connect the XLR cable ground would be an invitation for RF disaster. Neumann and Gotham (cable maker) suggest to never separate grounds and shields in mic cables, phantom of tube, or to disconnect them from pin 1 or any other ground/shield termination points on connectors. As a matter of fact, Neumann (and I) recommend to connect ground, shield and connector housing to Pin 1 on BOTH ends of an XLR mic cable.

Aside of the (theoretical) AC chassis ground issue there is no chance for a ground loop to occur in microphones: the mic, being the beginning of the chain, has no electrical or physical contact with any component; hence no possibility for a ground loop. So the usual ground-loop rules of interconnects between audio components that are individually powered differ here.

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I prefer to connect audio / supply ground to case/safety earth via a 10nf capacitor in parallel with a 10 Ohm resistor in the PSU.
Neumann did the 10 Ohm resistor solution internally in KM84 mics, but only there, and God knows why in no other model.


But back to the thread starter's complaint:
Quote
The problem is, that there is no logical reason at all why the tubes break.
I totally agree. However, chassis ground routing is not part of the solution for the prevention of failing tubes in his case.

You hinted in your response to something else which needs to be tested by the poster: installing the seemingly defective tubes in another mic, and see whether they work there, or whether they had been permanently damaged.
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Klaus Heyne
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soapfoot

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I finally got around to working on this mic some more.

I added the capacitors Kai suggested-- I figured that, while unlikely to solve the problem, it couldn't hurt.

While inspecting everything, I found one very interesting/promising lead-- I had an intermittent ground connection on my plate supply.  Experimentally, when prototyping my power supply I found that having separate star grounds for 5.1v and 105v supplies seemed to be the quietest arrangement.  The star ground for the 105v supply was connected to chassis at the multipin Binder connector, using a solder lug under the nut that bolts the connector to the chassis.

That screw had worked itself a bit loose (likely was not tightened properly after some experimentation with the ground scheme).  In hindsight, the issues with tube failure would often happen after moving the mic for some purpose (though I never made that correlation before now).  Basically, I feel like I was losing ground reference on my B+ supply intermittently.

I'm hopeful that this intermittent ground connection is what was causing the problems with tube failure.  It's been corrected now.

I have replaced the tube once more, and currently have the mic burning in with a 68pF capacitor in place of the capsule. While I feel somewhat optimistic that I might have found my problem with the loose ground, I still feel it's probably a good idea to rule out the capsule once-and-for-all.

A word on how I ran my grounds, and feel free to comment.  After much experimentation, this seemed to be the quietest arrangement for me.

All 5.1v supply filter capacitors and bridge rectifier grounds, and any other ground in the 5.1v supply, connect together and to chassis at the first filter cap of the 5.1V supply (and connect nowhere else). 

All 105v supply grounds (similar to above) connect together, and their star ground point is right at the multipin connector.  XLR pin 1 connects to the 105v supply ground.  5.1v and 105v supply grounds connect only through the chassis itself, where they're quite some distance apart, physically.  The 3rd prong safety ground connects to the chassis right at the IEC connector, physically very far from both star ground points.

I arrived at this configuration through much experimentation (I tried lots of others, including a single star ground for 5.1v and 105v supplies), and it really did seem to be the quietest.  I'd always be open to suggestions from experts such as yourselves, if you see anything wrong with my scheme. 

Thanks for your help so far, and fingers crossed that I've found and corrected the problem.

--b
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Kai

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If the ground sceme works best, leave it as it is.
I've seen all types of grounding scemes when looking on audio devices.
There is no universal rule how to do it, as ground/earth/shield/safety ground serve different purposes with different, often contrary needs.

In theory, on a transformer balanced interconnection e.g., you wouldn't need ground at all.
Practically this doesn't work with mic's, as the levels are quite low, and even a transformer balance is far from perfect.

The only thing I've learned so far: avoid screwed contacting with GND connections, always use soldered joints.
Resistance is too high and undefined with screw contacts, even if you use toothed washers.
E.g. the housing contact of an XLR connector is catrastrophicly bad on connecting GND to case.


Keep us informed if your fix works, we all had agreed that we could not see an obvious reason for your problem.

Regards
Kai
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soapfoot

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The only thing I've learned so far: avoid screwed contacting with GND connections, always use soldered joints.
Resistance is too high and undefined with screw contacts, even if you use toothed washers.

Duly noted, thanks!

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Keep us informed if your fix works

Will do, thank you!
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ryanstreber

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Any updates?  I'm hopefully going to be finishing up my AMI 47 kit in the next few days, and it would great to hear how things have been working for you...
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soapfoot

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Any updates?  I'm hopefully going to be finishing up my AMI 47 kit in the next few days, and it would great to hear how things have been working for you...

I sent my mic off to have someone look at it.  When I hear what the diagnosis is, I'll let you know.  :)
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