I need a microphone

[Tubetec]

My take on the subject of insulation resistance is that the weakest link in the chain is always going to be the limiting factor.
In the case of the Vf14 it used a low loss base material and underwent special testing for suitability  in high grid resistance situations such as tube mics . The Ef14,which is a very similar valve wont always perform as well in place of the Vf14,and careful selection will be nesessary to find a usable specimen. Insulation resistance between the pins of the tube being the limiting factor no amount of special precautions/or exotic materials is likely to improve matters a great deal.In models like the Neumann M49 and M50,which used an all glass sub mini wire ended tube ,plexiglass is used to good effect.

I did notice that the original post decrying my work on the B-2 tube mic conversion was edited later, criticisms constructive or deconstructive  I have no issue with ,being accused of lying is another matter completely.

Anyway I hope this post finally puts a lid on it ,and hope we can move on to a more constructive approach

 

[KrIVIUM2323]

Insulation resistance between the pins of the tube being the limiting factor no amount of special precautions/or exotic materials is likely to improve matters a great deal.

I do agree from an electrical point of view. But as Frederic pointed from a mechanical point of view this can have an effect (microphony). I like PTFE for electrical properties and mechanical properties: very low friction factor and used with some damping material it has nice effect i think.

I've used silicon glue (for aquarium) to make more or less same thing that Frederic showed in pictures of 47 to limit vibration transmission to the socket for an ef14. Not really scientific  and a real mess to do the 'pads' but it work well.

I'll probably couple that with ptfe plate for next build.

 

[Tubetec]

I was lucky enough to pick up a bunch of  ptfe valve bases from a man who breaks old military gear down ,some used some nos ,but with a bit of a clean and a polish as good as new . I havent used ptfe much for structural work though. I was thinking that some kitchen heat resistant cooking tools etc might be a good place to find it on the cheap , I must check into it more.

 

[KrIVIUM2323]

I would be careful about kitchen ptfe: there is different kinf of ptfe, some with additive making it much less interesting for electrical insulation properties. Don t know if that is the case for the kind used for food tools though.

 

[Tubetec]

Yeah you probably right about that ,the real stuff  costs alot more , and tends to be pure white in colour . must be other stuff added to the standard kitchen variety . Just as a guide 1200x1200x2mm  310 pounds sterling but thats an awefull lot, must try and source some in smaller quantities locally .

 

[Tubetec]

Hmmm forgot to add the vat ,so all up 372 quid

 

[KrIVIUM2323]

You can find small quantity at ebay.
If i remember correctly in eu there is some offers from germany.
2mm is a bit thin from my experience, 3mm is stiffer and more useful for structural duty imho.

 

[cyrano]

PFTE is abundant in the terabrik caps...

Easy to melt down and press into slabs. See Dave Hakken's channel:

https://www.youtube.com/watch?v=76AFNIxYjUE

 

[Tubetec]

Thanks Cy,
Interesting video ,makes alot of sense to re-cycle/upcycle wherever possible.
Im gonna try a few local engineering supply places and see if they have any off cuts of sheet ptfe,Im sure they will be willing to let it go cheap .

 

[KrIVIUM2323]

Im gonna try a few local engineering supply places and see if they have any off cuts of sheet ptfe

Makes sense. Just be sure they are not loaded with bronze or other fiber to strengh them up. 

 

[granger.frederic]

My take on the subject of insulation resistance is that the weakest link in the chain is always going to be the limiting factor.
In the case of the Vf14 it used a low loss base material and underwent special testing for suitability  in high grid resistance situations such as tube mics . The Ef14,which is a very similar valve wont always perform as well in place of the Vf14,and careful selection will be nesessary to find a usable specimen. Insulation resistance between the pins of the tube being the limiting factor no amount of special precautions/or exotic materials is likely to improve matters a great deal.In models like the Neumann M49 and M50,which used an all glass sub mini wire ended tube ,plexiglass is used to good effect.

I did notice that the original post decrying my work on the B-2 tube mic conversion was edited later, criticisms constructive or deconstructive  I have no issue with ,being accused of lying is another matter completely.

Anyway I hope this post finally puts a lid on it ,and hope we can move on to a more constructive approach

no problem tubetec...
concerning the grid leaks , the tubes (EF14 or 12 or everything else)  must be cleaned also , the socket as well , especially after soldering...
my experience is that 1 tube of 5 usual  mics tubes is unsuitable if you have a good tube dealer ...
if the tube in its datasheet has enough inter-electrodes insulation (let's say a bit more than the grid resistor), chances that it will work are pretty good.
high insulation  sockets are preferable of course
but at the end, i notice that the mechanical insulation (ALL the components of the highZ section) affects the sound more than the electrical insulation (mainly on low end)

 

[KrIVIUM2323]

I just watched the video Cyrano linked.
Re use of plastic material is really something good and we should do everytime possible ( i can t stand all sorts of plastic items i encounter each time i go bodyboarding in Atlantic ocean... and it s a relatively protected ocean).

Nevertheless, i won t do that with ptfe without being supervised by someone who know is stuff: melting point is relatively high and i repeat if it release smoke it can kill you.
Trying to machining some using a friend s lathe i ve experienced it.  Won t do that again.

 

[cyrano]

Thanks Cy,
Interesting video ,makes alot of sense to re-cycle/upcycle wherever possible.
Im gonna try a few local engineering supply places and see if they have any off cuts of sheet ptfe,Im sure they will be willing to let it go cheap .

I get leftovers for free. I buy perspex and stuff like that from one local supplier. Asked them if I could do some dumpster diving. Sure, OK.

Had to prove that to police, once. Doing it at around 1 pm on a sunday night wasn't one of my brightest moments.

And tetrabrik caps are clean. Food safe. Unlike some of the mil spec stuff...

 

[Bo Deadly]

I got the MXL 440 donor body. It's SMD and the capsule has visible corrosion on it. So I'm going to just gut it. Sounds pretty good though for such a cheap mic (as does the 441 that came with it). Zero hum anyway.

So now I'm thinking of doing a KM84 pcb with a Peluso P-K47 capsule and an AMI T13 transformer (basically same as T84 from micparts).

Does anyone see a problem with this?

The T13 is 9.5:1 so with a 1.2K / 300R mic pre input thats going to make a load of around  108K / 27K on the FET.

Will a 34mm capsule fit in the 440 body? Will the Peluso mount work with the 440 body? The 440 body seems a little small.

Anyone have any experience with zenproaudio.com?

ZenPro claims both the capsule and transformer are "Stock Item, Usually Ships Same or Next Business Day", free shipping and no sales tax.

 

[RuudNL]

A 34 mm. capsule will fit the body. I did it before.
(The mounting post should not be too high.)

 

[G-Sun]

My suggestion:
- Fet 847 (diy)
- Aston Origin (new)
- Miktek MK300 (new)
- any diy U87 kit

Provided that you have a good tracking-room.
If it's just need a mic, then I'd buy new (or used)
if you like the diy-challenge then diy

btw: I have a C1 jj mod for sale

 

[micaddict]

Yes, the 847 was the first one that came to my mind, too.
https://groupdiy.com/index.php?topic=61057.0

 

[Gus]

Why don't you adjust the SMD circuit using the information available on the web for the Schoeps like circuit.
A properly adjusted circuit like that can sound fine.

 

[Bo Deadly]

Yes, the 847 was the first one that came to my mind, too.
https://groupdiy.com/index.php?topic=61057.0

This is interesting.

But why did they DC couple the capsule to the JFET?

 

[RuudNL]

But why did they DC couple the capsule to the JFET?

To avoid the effect of the capsule coupling capacitor.
(Many people think a coupling capacitor has an audible effect on the sound quality.)