Teladi tube bottle mic

Hi,

I'm in the process of tracing out a 1930's Teladi bottle mic. I'll take some photos tomorrow, but for now here's most of the circuit.



A couple of features here which are worth noting:

The PSU is standard thing , heaters are 6.3V AC twisted pairs.

It has a little 150V voltage regulator tube inside the mic. I've never seen this before in a mic. The manufacturer is "Stabiliovolt"

Tubes are 2 x ECB3.

Although it has an output transformer, it runs unbalanced.

There are grotty electro output caps in the PSU. This seems unnecessary to me unless it is to protect the output xformer from accidental connection to DC? Maybe it is to stop the switch grounding the input of whatever comes next?

There is a long wax-filled tube that runs the entire length of the body, which has 7 wires coming out of it. From the circuit I believe this is 4 capacitors (two with a common ground - yellow box marked 3 4 5). I'll desolder and get some measurements to confirm this.

I do wonder why they are in a tube. There were a lot of small mic companies in Germany at the time so maybe it is some kind of attempt to keep the values secret?

[Finally, the 1uF cap between the 40K and 50K resistors looks odd to me. Is it some kind of filter for the power supply? I often see stabilisation caps to ground, but this seems strange.] EDIT: THIS IS WRONG! NOTE ADDED BELOW

Let me know your thoughts!

Hi Zebra,

I don't see the schemo. Pasting the link brings me to "Page cannot be displayed".

Hi Mark,

That's strange! Can your browser cope with PNGs? I'll put up a jpeg and see if that works.

EDIT: Should be OK! You may need to refresh your browser!

Stewart

Hi!

I've investigated further and updated the schematic, correcting some mistakes and filling in the unknowns.
Here are a couple of pics of the insides:





You can see it's quite a big mic. The Reslo is there for scale. A couple of the components were confusing me. You can see a big metal can, which I thought was a cap, and a long card tube filled with wax.

The can is actually two caps in a single unit, with the can itself being one terminal. It was once a nice capacitor, I think, with metallised & waxed paper inside. Sadly damp had got in and the whole thing had swollen up and become conductive. I pulled out the original material and hid two modern metal film caps inside.



The tube contains 4 caps, although two were old electros that had long since leaked and failed. A third cap was unreadable. These three will be replaced of course. The 4th was paper-in-oil type, still in good order. I'm always happy to see this type of cap because they seem to sound good and measure perfectly even after 60 or 70 years!

I'm slowly going over all the juctions and wires. This one is essentially complete so I hope to get it working in the next few days.

Stay tuned!

Oh man, that's some scary stuff. Be brave, be brave.

I'm currently sitting with a pile of broken ribbon mics wondering where to start... I should post photos.

I always start by posting photos. By the time I've gotten myself into trouble, usually someone has posted the solution!

Oh man, that's some scary stuff. Be brave, be brave.

Click to expand...

Hell yes! I'm going through everything very carefully before I plug it in.

Always problems with the heaters on these old European mics - the supply here is usually above 240V, but they were built in a time when 220V was probably a bit less than that! I'll run it first on a modern regulated supply and take it from there.

Been staring at this for nearly two days now. There's nothing 'micro' about this microphone. 2 full size valves and a massive transformer. The funny looking third tube in the second photo is the regulator. Don't yet know if this or either of the ECB3s are good - the latter are cheap and easy to get hold of, but I haven't yet tracked down a supplier for the regulator.

All good! All fun!

Same AC kind of voltage problem in the USA. I have measured 124VAC at my house.

I often use a Variac set to about 110VAC to start, to a 1:2 transformer for microphones that run at 220VAC. I then adjust the variac with the supply on for 220VAC. Some have RC type heater voltage circuits and one can over voltage the heaters if not careful.

Check geofex.com look for the vintage voltage circuit. It is a Buck transformer circuit that lowers the input voltage to older guitar power supply transformer.

Thanks Gus for the Geofex link. It's simple and useful

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

Hi!

I've mostly got this mic cleaned, restored and back together. I had to make a few mods - the circuit is below. The major difference is removal of the stabiliovolt regulator - I couldn't find a good one, and am using a regulator in the PSU instead.

Other parts are all original, except for failed capacitors and a couple of resistors that had drifted by more than a few% (or were never that close in the first place!)



[edit: approx. voltages & approximate currents added to schematic]

But I'm having some problems firing up the tubes.

Using a 6V regulated DC supply (7806), the heaters first come on and the supply stays at 6V for a couple of minutes. But as the tubes start to heat up the supply drops down to around 2V.

Data sheet says 200mA per tube, and I think my supply should be good for that. Obviously i need to run some tests to make sure. I was just going to wire in a 16R, >4W load and measure V.

Should I be looking for a bad EBC3 tube? Or could the regulator be shutting down?

Tube data is at Frank's:

http://www.mif.pg.gda.pl/homepages/frank/sheets/046/e/EBC3.pdf

Data says they are OK on DC heaters.

I'll run some more tests tonight. As always, I am grateful for your thoughts and comments.

z50 :thumb:

It could be the 7806 shutting down.
Did you use thermal compound and an electrical insulator(if needed) or a thermal pad on a good size heatsink to mount the 7806?

Hi Gus,

Just got back from looking at it - it was indeed the 7806. It should be bolted to the PSU chassis, but the screw had worked loose. F+ is dead steady now.

I reassembled the mic and had my first listen. It is at least working.
There are some noise issues - the noise builds up over a minute or so, and then there is a squeak, then the noise vanishes again. I need to have another look at the capsule and surrounding circuitry, and to check the grounding.

Probably these tubes have not been used in decades. I have four to play with, but at least one is definitely bad.

The high voltage may not be quite right yet - I have 170V on the 47uF cap - where the 150V regulator would have been. I'll pop up the rest of the measured voltages when I find my notes!

I don't normally post sound clips as I think they are next to meaningless, but this mic is so unusual that I thought I'd share. Here's a clip of acoustic guitar (Levin 1947 archtop). The mic pre is a littlelabs LMNOpre with the o/p transformer bypassed. Then into protools 003 at 88.2KHz, 24 bit, bounced to mono .wav at 44.1/16. Mic is about 2ft back from the middle of the guitar. Ignore my sloppy playing.

http://www.xaudia.com/omnip/Sounds/Teladi_Levin.wav

Clearly there are the noise issues and a lack of bottom end, but I don't think it is actually 'bad' sounding.

I like that soundclip!
Of course it is noisy but it somehow has style, warmth and character!
I would like on of these;-) Maybe as an effect...

Thanks for the great thread and the sound clip !!!

Thanks for the support, guys! More to come soon - I took a few more photos today but left the camera at the studio. Tomorrow I'll rebuild the whole Hi-Z section to see if I can get rid of the fizz.

:guinness:

Don´t forget to triple clean the Hi Z section of the mike with alcohol. You don´t want 50MOhm fingerprints in parallel to your resistors or shorting to nirvana.

Thanks! Thankfully it is not the capsule. I'm getting the same noise without the capsule (small cap wired in its place). I'm beginning to suspect the oil in film cap at the Hi-Z end. I'll try replacing it tomorrow.

Here's a picture showing just how massive this mic is. The SM57 is there for comparison.

Looking pretty bad-ass, Stewart.

I'm impressed with your documentation and ability to trace things like these. I'm interested to see how this one goes in the end.

[quote author="zebra50"]The 4th was paper-in-oil type, still in good order. I'm always happy to see this type of cap because they seem to sound good and measure perfectly even after 60 or 70 years![/quote]
Be careful! Some paper-in-oil caps are terrible, and resemble resistors more than caps. The Japanese Suzuki caps from the 60's are often bad for instance...

I think the "Stabilovolt" device is just a gas-filled tube - perhaps the 150C2/0A2 would work?

Best regards,

Mikkel C. Simonsen

Be careful! Some paper-in-oil caps are terrible, and resemble resistors more than caps. The Japanese Suzuki caps from the 60's are often bad for instance...

Click to expand...

I've generally had good experiences with the old German ones, but thanks for the warning! I'll double check (again!)

:thumb:

I joked with an associate once that some very expensive new paper-copper foil-oil caps were the first coupling caps I'd used that might need a d.c. servo.

(that's a recycled joke from an old thread iirc but I still like it)