Miniature Tube DI Idea

[Bo Hansen]

Hi guys,

There are a subminiature tube in Gefell U900 microphone that draw very low filament current, so they can drive the tube circuit from the phantom power. (there are some sort of internal switching supply, but the tube must still have very little filment consumption)
I have ask Gefell about the tube, but they will not answer this question.
(they only say that this tube was made for hearing aid devices)

Are there somebody that know something about this tube ??

--Bo

 

[jrmintz]

Hi Bo,

I posted this a few pages back:

I have a UM900. I was told by the former importer that the tube was not a hearing aid tube, but something the former East German secret police designed for miniature eavesdropping devices. I don't know how to open the mic, but if I ever figure it out I'll take some pics of the tube.

I can't confirm that, but I see no reason the guy would BS me. Nice to see them put to some constructive use. It really is a great mic.

:thumb:

 

[Bo Hansen]

Hi Seth,

Sorry, I have miss your early posting regarding the UM-900.

BTW, here is a nice link to a big list of sub miniature tubes:
http://pw2.netcom.com/~wa2ise/radios/penciltubes.html

--Bo

 

[Guest]

Do you have the UM900 schematic?

 

[jrmintz]

I don't have a schematic.

 

[Guest]

I finally finished the phantom powered miniature tube DI. I got a MAX5033 DC-DC converter to work for the heater supply after a little experimentation. Here is the final schematic: http://users.adelphia.net/~thomasholley/Guitar/Mini Tube DI.gif

As I tend to do, I got carried away with "features" and added switches for triode/pentode operation, a switch for transformer ratio adjustment to match these modes best, and a ground lift. They work pretty well. I also added an unballanced monitor out.

The MAX5033 DC-DC converter is set to supply 1.4VDC at 20mA output, drawing about a miliamp on the input side. This is the lowest draw among several devices I looked at. The output has a small residual switching noise using the component values derived from data sheet calculations. I added another small choke and larger capacitor to the output which got rid of this residual noise successfully.

 

[Gus]

Very Cool! What did the DC to DC converter voltage end up at? I ask because I see you have a resistor in the cathode leg.

 

[Guest]

Gus, I put that resistor there to decouple the converter a bit from the plate supply. Since the draw of this chip under load is only about 1mA I just dropped a volt. In this case it is about 46VDC at the input of the DC-DC converter.

I tried a couple of other chips that worked, most notably the LM2594 which only took a few external parts and was very stable under light load, but all the others drew at least 5mA.

Thanks for the suggestion to use a converter. I had never considered their use before and didn't really understand how they worked. Now I have learned something new and I have a nice DYI project as well.

 

[Gus]

Is a condenser microphone in the works? 46V to the backplate via a 15meg 22meg decupled with a .1uf cap, grid to the skin maybe no grid to ground, triode mode 5:1 transformer. Cardiod only like this but cool.

I think a PCB could be made to install this in china microphones. I would try a studio projects B1, MXL2003 or a MXL V67. To see if this would work out

 

[Guest]

It certainly could be a candidate for for a condenser mike. One of my friends, a "junkyard genius" and I are wirking to make a condenser. He did a Royer mod to one of his mikes and got hooked.

We move slowly, but in a month or so I should post some results of our experiments.

 

[Whoops]

Hi,
I know that you did this project a long time ago, but hod did it sound in the end?

do you have any documents on the project?