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 on: Today at 06:49:42 AM 
Started by jackreynolds - Last post by jackreynolds
It looks as thought the press fit may have failed some while ago and been glued in place with what looks like cyano or similar.

I'm guessing the thin, what I'm assuming is teflon, ring is essentially a spacer to electrically isolate the front and back diaphragms?

I did buy some adhesive with a primer designed for teflon but am wary of the glue contaminating the diaphragm.

I like the idea of a few drops of E6000 as that's a lot more controllable.

I'll give that a go and will report back.

Thanks for your knowledge!


 on: Today at 04:53:29 AM 
Started by jackreynolds - Last post by Kai
What do you have to loose?

Obsolete now as Klaus anwered:
(Only that some cyanoacrylate glue might vapor and contaminate.
Maybe Klaus has an idea what glue works best.

I found “ 3M™ Scotch Weld™ DP 8010 “ is said to be suited for PTFE, if a primer like  “LOCTITE® 770 Primer “ is used.
An Amazon search for Teflon or PTFE glue yields results too.
A primer is mandatory.)


If you put in something permanently sticky, e.g. where the white cable runs, chances are good that it will collect and hold possible dust over the time.

 on: Today at 04:37:38 AM 
Started by jackreynolds - Last post by klaus
I am all for experimenting.

What I would do in your situation: after removing any contaminants from the inner diaphragm surface and backplate surface with a fine brush, press-fit the diaphragm ring onto the backplate assembly, as it originally was constructed, but do not under any circumstance use  adhesive, including cyanoacrylate! Just press-fit for now, to see whether the capacitance you dialed in will yield good frequency balance.

If the sound is too boomy, increase the space between backplate and diaphragm ring by using less force on the parts you pressed together, and if there is too little bass, do the opposite.

Once the sound is ok, do not change or touch anything, but apply a couple of drops of E6000 to three or four spots all around the circumference at the gap where backplate and diaphragm ring meet, to permanently arrest the current setting (E6000 can be removed without residue, if you need to readjust later on).

Please report back with your results!

 on: Yesterday at 03:31:50 PM 
Started by jackreynolds - Last post by jackreynolds
I’m looking for advice whether a Nylon C414EB capsule can be repaired, if it has separated into two halves as shown in these pictures.

There appears to be a thin ring of Teflon or Nylon with glue residue between diaphragm ring backplate assembly and the backplate electrical contact.

I suppose I’m really wondering whether I should attempt to glue the pieces back together with cyanoacrylate or is it likely beyond repair?

The diaphragms look ok but im guessing some dust will have got in through the backplate holes while it’s been rattling around inside the head basket.

 on: May 12, 2022, 05:13:59 AM 
Started by pidegreeinc - Last post by pidegreeinc
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 on: May 10, 2022, 10:47:49 AM 
Started by NigelT - Last post by Kai
Your mains voltage seems to be on the high side, even a bit out of standard.

Maybe your supplier can do something about that, switch to another transformer tap for your individual household supply, if exists.

For the tube’s heater supply:
In general little undervoltage is more preferable than overvoltage, if you can’t get it stable.

Anode voltage (the high one) is uncritical.

 on: May 10, 2022, 04:35:28 AM 
Started by NigelT - Last post by NigelT
Thanks Kai.
An update. All ten supplies are now working and in spec, sort of...my biggest problem now seems to be fluctuating line voltage. Today I am at 121vac. I have found that some of my units have a 110v tap and others at 117v. My procedure is switching between the high (127) and low (110 or 117) tap and swapping mics between all ten PSU's and by doing that, I am getting all of them within spec. I have also found that these need at least 30 minutes to warm up and stabilize. Thanks for all your help, I have fallen in love with these mics! Although I am blessed with all of the great ones,  (251x2, U47x2, M49, C12x2, C24) and they all sound amazing, something keeps pulling me back to the U67. Out of ten, there are four that are just outstanding but only when compared side by side and by very minute differences. Any of them alone are amazing.

 on: May 08, 2022, 05:01:34 PM 
Started by NigelT - Last post by Kai
Here’s an example of technical date of a high quality tube microphone’s cable, Mogami 3172.
Based on these values one could calculate the heater voltage drop using Ohms Law:

Cable Length x Resistance per Lenght x factor 2 [forward+return] x Current = Voltage

Exxample from above (AC701 has 0.1 A heater current):
7 m x 0.046 Ohm/m x 2 x 0.1 A = 0.0644 V

If the shield is used for return current “factor 2“ is not used in the equation.

 on: May 08, 2022, 06:12:03 AM 
Started by NigelT - Last post by mikezietsman
Hey! You learn something every day. Last time my tech did measurements at both sides the difference was about 0.06v over a 7m cable, but that was with an ac701.

 on: May 08, 2022, 12:29:51 AM 
Started by NigelT - Last post by Kai
I don't know a tonne about tube mics, but I do know that it's best for tube life to be conservative with heater voltages. 6.52 is 0.22v above the recommended 6.3v for an ef86.
6.5 V measured at the PSU is OK.
The cable‘s resistance drops some voltage, measured at the mic you will end up close to or even below 6.3 V.

Each 1 Ohm cable resistance drops 0.2 V (heater current 0.2 A).
Most cables have more than 1 Ohm.

This is why the schematic says 6.5 V instead of the tube‘s nominal 6.3 V, and why it makes sense to adjust the voltage for a certain PSU / cable / mic combination, while measuring at the mic.

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