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 11 
 on: May 18, 2022, 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.


 12 
 on: May 12, 2022, 05:13:59 AM 
Started by pidegreeinc - Last post by pidegreeinc
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 13 
 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.

 14 
 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.

 15 
 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.

 16 
 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.

 17 
 on: May 08, 2022, 12:29:51 AM 
Started by NigelT - Last post by Kai
NigelT,
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.

 18 
 on: May 07, 2022, 06:36:15 PM 
Started by NigelT - Last post by mikezietsman
B+ = 217
Heater = 6.52
Close enough?

NigelT,
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. In practice I haven't noticed any raised noise-floor running mine at something closer to 6.1-6.25, as was recommended to me by more than one very nerdy tube-head.

When testing out tubes on one of my u67s my tech and I noticed that different ef86 seemed to have different enough current draw that heater voltage needed to be adjusted individually, per tube.

When I had a 67 done by Klaus, the power supply came back labelled "for use with serial number x only", which I suspect is because the heater voltage had been adjusted specifically for the tube in that mic. I'm sure Klaus will correct me if I am wrong.

 19 
 on: May 07, 2022, 01:50:39 PM 
Started by NigelT - Last post by NigelT
B+ = 217
Heater = 6.52
Close enough?

 20 
 on: May 07, 2022, 12:57:28 PM 
Started by NigelT - Last post by klaus
Change the input voltage selector to 127VAC and measure again. You then will be in sight of the ballpark, if not already in it.

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