Teladi tube bottle mic

[Jeremy.Starseed]

I am buying a Teladi K43 microphone. Thanks for all your work, here, it will be of great value to me when the mic arrives.

Did you solve the PSU-noise issue?

Thanks again,

Jeremy

 

[Jeremy.Starseed]

I've been acquiring parts for this project for some time, getting the resistors swapped, and building a PSU, that is.

It is my first tube-build, and so I am moving cautiously forward, reading  as much as I can on the way.

I have 90% of the parts I need...but I'd like to get some opinions on the PSU before I get to putting it together...a kind of "measure twice, cut once" approach, I suppose.

The sound file Xaudia posted sounds really yummy to me, I will be very happy if I can get that sound.

Any tips from any of you (who actually have centuries of experience between you) ?

 

[zebra50]

Hi, (and thanks for your email to xaudia - that's the business end of things, so if you want to send your mic in for service then drop us a line).

...but I'd like to get some opinions on the PSU before I get to putting it together...

What specifically do you need to know about the PSU? The schematic posted is traced from the original PSU. I built a new one for the mic I had with a regulated heater at 6V DC, and adjusted the high voltage side to get the required voltage.

If I recall right, the mic was pretty dirty around the high-Z area (and throughout). A thorough clean and replacement of some parts helped, but also the capsule was dirty and fairly prone to moisture.

... I will be very happy if I can get that sound. 

then you'll need one of these too....

http://www.vintage-guitars.se/Levin_1947_Levin_Model_1_De_Luxe_194463.htm

 

[Jeremy.Starseed]

Hi. Thanks for your prompt response. 

I'll ask a few questions here, if you're OK with answering them.

1&2 The required voltage is? What do I place in the circuit to achieve that voltage from my transformer, which leaves 261 V ?

3&4 Did you end up changing the resistors closest to the capsule? What are good values?

5 I managed to find a mic stand to hold the thing, but haven't found a connector. Do you know the name or model name of it? It is like an old tube socket...

That is one fine guitar, sir. A lovely tune too.

Thanks,

Jeremy 

 

[zebra50]

I'll ask a few questions here, if you're OK with answering them.

Happy to help, if I can. Do bare in mind that this is going back a couple of years now.

1&2 The required voltage is? What do I place in the circuit to achieve that voltage from my transformer, which leaves 261 V ?

Look at the voltages on...

http://www.xaudia.com/omnip/Mics/MicSchLib/TeladiK43_mods.gif

You'll see there is a resistor between the high voltage supply and the spot marked 155V (where the stabilovolt was). You'll need to change the value of this resistor to get something like 150 to 155V at that point.

If you have 261V to start with, that resistor will eat up 110 V or so. That'll kick out quite a lot of heat, and you might be better off starting a bit lower. The power supply for the MK7 microphone project might be suitable for this, although I haven't tested it. I used 245V simply because I had that on the bench from another project, nothing magic about this number.

3&4 Did you end up changing the resistors closest to the capsule? What are good values?

I don't recall which ones got changed, if any. But the values are on the schematic. With an antique like this the trick is to replace as little as possible. If your mic hasn't been serviced an the past few decades there is a good chance that you'll need to replace the electrolytic capacitors too.

...but haven't found a connector. Do you know the name or model name of it? It is like an old tube socket...

This is the same connector as on the old RFT CV bottle mics, but without the screw thread. The connector has not been made for a very long time, and they sometimes go for silly money (http://cgi.ebay.de/Vintage-Neumann-CMV3-type-microphone-cable-/160428249554).

Those are not the best of connectors, and in my case I replaced it with a small metal plate with a tuchel-style connector. That wasn't an easy operation as I had to take the baseplate off the mic, drill and tap holes in that very soft crumbly metal!

 

[Jeremy.Starseed]

I was told at the local Radio museum to switch those resistors before applying any power to the capsule...they said that at those values, there is no way that they would have maintained stability for so long. Of course, now I realize that the values are measured after the fact.

Did you compare actual values with the components original intended values, per chance? I haven't started picking it apart yet, before getting a firm grasp of what the parts do, so I haven't been able to see if the values are easily read. Some of them seem cloaked, as I think you hinted at. I can't see why one would cover a capacitor in vaxed cloth, for instance...or hide them in a paper tube, must be a better way of shielding, if that was the purpose.

What sort of rating would the resistor following voltage transformation need to have? Should it be placed before or after a rectification device? The 150 V should be DC, no?

The e'lytics are marked with heavier black on your schematic, I'll make sure to swap them, sure.

Nice of you to answer my newb questions. I'm sure you detect the inevitable anxiety that comes with these stages of development  :

Thank you,

Jeremy

 

[zebra50]

[quote author=Jeremy.Starseed]
Of course, now I realize that the values are measured after the fact. Did you compare actual values with the components original intended values, per chance?[/quote]

The values on the schematic would be as the maker intended - marked on the resistor itself. I would have checked the value and left or replaced it. If I couldn't reads it I would have measured and checked that the value made sense in the circuit. This is two years ago now, and I've seen thousands of resistors since then!

...before getting a firm grasp of what the parts do...

That's a really good idea. Look at other, simpler microphone schematics and build up from there.

...or hide them in a paper tube, must be a better way of shielding, if that was the purpose.

Probably to hide their design, and to give some physical structure to the circuit. Not shielding.

What sort of rating would the resistor following voltage transformation need to have?

Power = IxV =  I2xR

Should it be placed before or after a rectification device? The 150 V should be DC, no?

That question worries me slightly! DC, so yes, it would be after rectification, and you need very good filtering too for a quiet microphone. Remember you can get a fairly heft shock from these voltages, so work carefully. And of course you also need a low voltage, high current supply for the tube heaters.

Nice of you to answer my newb questions. I'm sure you detect the inevitable anxiety that comes with these stages of development  :

We all have to start somewhere - but you've decided to throw yourself in at the deep end with this one. It's a difficult mic to work on, even with some experience. Lots of things could be in bad shape - capsule, tubes, transformer, capacitors, resistors, connector. Also, mechanically the construction is weak and there may be issues like stripped threads and rusty screws.

I'd really suggest starting on something simpler and well documented, like the G7 or MK7 tube microphone projects. That'll give you the learning curve you might want.

 

[btown2009]

I'm curious about that capsule and the 100nf or 170nf capacitor in the schematic. Does one end of the capsule go directly to ground? With the capacitor also going to ground(100nf)??