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bockaudio

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Dec 26 15 4:23 PM

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klaus wrote:
bockaudio wrote: I've seen modded 251's where the plate was connected directly to the winding with the cap at what is normally the ground end, achieve open windings. Also failed Triads in Church mics (same direct connection of plate configuration).


I am not familiar with Triad transformer durability, but can attest, from experience, that no transformer primary blows as often as that of the T14/1 regardless how you hook it up. The wire is simply too thin to buffer current spikes, acting like a fast blow fuse.

I've only seen them fail with that direct connection to the plate (which creates undue voltage potential) but interesting to hear you've seen it regardless of configuration. The Triad is not the same tiny wire and would otherwise be fine were it not for the odd hookup. For example, i've never seen a 660 or 670 requiring a new (Triad) output xfmr.

Last Edited By: bockaudio Dec 29 15 11:55 AM. Edited 1 time

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klaus

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Dec 27 15 1:57 AM

I have a box full of T14/1 with blown primaries. Most from C24, the rest from C12 and ELA M. I think the coupling cap (regardless of position before or after the transformer) sometimes blocks too slowly when a DC surge to ground occurs (f. ex. hot-plugging) and momentarily overheats the winding. 


(When I was much younger and dumber I tried to prevent this with a 120V Zener, but clients complained about distortion. D'oh!)

Last Edited By: klaus Dec 27 15 2:03 AM. Edited 1 time.

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bockaudio

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Dec 28 15 12:46 PM

I want to understand this mechanism you suspect. How does sudden connection of a completed circuit, or disconnection, cause DC to flow through a low leakage 1uF capacitor?
Further, the 120v zener which had unintended consequences, where was it located?

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klaus

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Dec 28 15 5:13 PM

The zener was placed at the B+ input from the power supply, inside the mic.

The surge that kills a T14/1 primary must be either an unloaded B+  (before the tube draws down the initial supply voltage to 120V) or a momentary failure of the coupling capacitor blocking a DC surge caused by hot-plugging or spikes in the supply voltage. 

I am not going perform real-life tests of various scenarios to find out what the actual cause for primary failure may be. The T14/1 is too rare and expensive to come by to warranty its systematic destruction. And I am open to alternative theories why it fails, but I see a correlation between the thinnest wire/wire insulation of any condenser mic transformer primary and its unusually high failure rate.

Last Edited By: klaus Dec 28 15 5:16 PM. Edited 1 time.

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bockaudio

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Dec 30 15 4:43 PM

I have not taken a T 14/1 apart but some thoughts
Comments from a friend of the forum Gus:
Maybe the wire is over tensioned during winding and the insulation is damaged.
this could help cause copper corrosion or even a breakdown of the voltage rating of the covering
Is the T14 wound with enamel or polyimide or formvar or ? covered wire
Where does the winding fail?
Has anyone done a leakage test at low to high voltages to find out when the wire insulation fails on a new production or NOS one?

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klaus

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Dec 30 15 8:38 PM

I can address your last question, having unwound several T14/1: the winding always seem to fail at the core, not the surface.

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gustav

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Jan 1 16 4:12 PM

Some more ideas If the wire break close to the core

Maybe the bend angle is tighter that the bend angles farther from the center and the magnet wire coating fractures a little.

Are the transformers varnish coated/impregnated? If so, could the compound used slowly break down the wire covering?
 
Does this happen with newly installed transformers or only after some time? Are the plate to transformer caps leaky enough to cause a heating of the wire in the microphones that the transformer failed in? Check leakage with a meter that applies working voltage to the cap when testing. I have tested film caps that tested good for leakage at lower voltages but then leaked badly at higher voltages

 Was phantom power involved with the failures? What happens when you plug into a input with phantom power on?
I wonder if you could get a step up voltage at the cap to plate?https://en.wikipedia.org/wiki/Ignition_system
Would need to check in you could make and break at pins 2, 3 and 1(phantom power on) at different times so you could charge the coil and then discharge at the primary side. Again need to test I don't think the B+ power up and power down, even hot plugging transits are fast enough to cause issues but testing would be needed to be sure.

Gustav 

Last Edited By: gustav Jan 1 16 4:14 PM. Edited 1 time.

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bockaudio

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Jan 1 16 7:29 PM

mikerivers wrote:

bockaudio wrote:
As with several of the dead xfmrs I have here, of course it's at the core, otherwise we could easily fix them. image


This follows the law that toast always lands on the floor with the buttered side down.image

Precisely what I meant to say, thanks.

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bockaudio

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Jan 1 16 7:50 PM

"Are the transformers varnish coated/impregnated? If so, could the compound used slowly break down the wire covering?"

Working units all fall into a small resistance range so we are not seeing the loss of enamel leading to lower resistance of previously insulated wires now touching, as we see with U47 power transformers now.
"Does this happen with newly installed transformers or only after some time?"
I couldn't comment (no NOS inventory of the part) but perhaps others have installed NOS T14/1's which have failed. I have installed many of the clone T14/1 Oliver made for me about 13 years ago with no failures.
"Was phantom power involved with the failures?"
In my cases yes.
"What happens when you plug into a input with phantom power on?"
There was a VERY long internal AES SC0404 (the standards committe) about this, I don't recall a conclusion. Long story short there are many permutations, but the majority of gear exposed to the event is robust enough for it to be a non-event.
"I wonder if you could get a step up voltage at the cap to plate?"
This part already does not have extraordinary low frequency response so I don't see (on first glance) how it could possibly be sensitive to a step pulse from the secondary to the primary in the first place, that step is NOT getting through. If this were the cause of the failure, would we not see it in other mics with super tiny wire gauges? Example, AKG C28's using the part, not a frequent failure. Other mics using tiny Beyer or Hauffe xfmrs, though not with the same step up, do use that wire ga w/o failure.
I do recall Oliver discussing the ignition like possibilities of phantom power a few years ago, but really for all the potential damage I see little evidence that it is actually occuring (at least in ccondenser mics). I don't spend a lot of time with ribbons or dynamics so that is a different evidence pool than mine.
off topic ish I have seen the ignition phenomena destroy U47 power supply chokes.

 

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bockaudio

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Jan 1 16 7:52 PM

klaus wrote:
(When I was much younger and dumber I tried to prevent this with a 120V Zener, but clients complained about distortion. D'oh!)

 

Why would a zener shunt accross the B+ cause distortion? Was the zener bypassed with a film cap 0.1-0.01uF?

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