zebra50

Antique DIY tube mic & capsule rebuild
« on: July 22, 2010, 05:25:24 AM »
I picked up this microphone on Ebay recently...

Photo

Another photo

inside photo

It's a bit of an oddball mic. At first I thought it was some kind of prototype by Reissmann, Teladi or some other mic manufacturer, but the more I look at it, the more I think it's an old DIY microphone, including the capsule. It may well be inspired by some of the old tube microphone articles by Williamson, Debenham and o on.

The metalwork is nicely done - by someone with some skill on the lathe, and the wiring is done by someone who undertood working with high impedance circuits. However, the designer made it impossible to change the tubes without dismantling the entire mic. ::)

I've traced out the circuit, which is based around an EF40 and ECC81. The original power supply is not present.



The capsule itself is interesting as the tension, or gap to the backplate, may be adjusted using springs and screws. It arrived with no diaphragm, but I'll see if I can get it working with some aluminium coated mylar. I'll take some better photos of that later.

S
« Last Edit: August 04, 2010, 04:42:34 AM by zebra50 »
Ribbon microphone services
http://www.xaudia.com
Microphone blog


mad.ax

Re: Antique DIY tube mic...
« Reply #1 on: July 22, 2010, 06:17:08 AM »
You always come up with the most unusual stuff! ;)

The capsule polarization voltage divider (2M/400K) suggest either a low polarization voltage, either a HIGH plate voltage...

Axel

mrclunk

Re: Antique DIY tube mic...
« Reply #2 on: July 22, 2010, 07:08:10 AM »
mmm, that looks nice.
thanks for posting the Reissmann info btw. I've a mr51 that i'll be sorting out at some point. You can expect some annoying emails from me at some point :P

zebra50

Re: Antique DIY tube mic...
« Reply #3 on: July 22, 2010, 09:57:48 AM »
The capsule polarization voltage divider (2M/400K) suggest either a low polarization voltage, either a HIGH plate voltage...

I spotted that too.  Something like 240V to give 40V on the capsule? If I were building this, I'd choose the parts to make the maths easy!

Quote
You always come up with the most unusual stuff!
It's more fun that way - who needs normal?!

Quote
I've a mr51 that i'll be sorting out at some point. You can expect some annoying emails from me at some point
No worries, I'll help if I can.

Anyway, here's a photo of the capsule.



As far as i can tell, the membrane would be clamped between the aluminium ring shown, and a second ring which is part of the mic head. Tension is set from the rear with four screws and springs! Front of the membrane connects to ground, and the backplate connects to the head amp via a screw through the centre.

Incidentally, the EF40 pentode was cracked. Shall I take some photos inside that?  :)
Ribbon microphone services
http://www.xaudia.com
Microphone blog

Rossi

Re: Antique DIY tube mic...
« Reply #4 on: July 22, 2010, 06:44:14 PM »
It could be loosely based on the DIY sugestions in Fritz Kühne's micropone book. That book went through many editions, and an early one had the circuit based on two EF40 tubes, and some of the values are identical (30 meg grid resistor, for instance). That edition must be floating on the internet somewhere; I downloaded a scan sometime. The paper version that I have is a later edition with EF86 tubes.  I think Kühne's DIY mic was based on some Telradi mic.
"I am not a number, I'm a free man!"
"Hahahahaaaaaa!!!!!"

rodabod

Re: Antique DIY tube mic...
« Reply #5 on: July 26, 2010, 07:10:43 PM »
Nice work. That's a cool looking mic.

I've been away for a while, so I think I may have missed a few interesting threads here.
Quote from: tv
punchy fat bastard chip

zebra50

Re: Antique DIY tube mic...
« Reply #6 on: July 27, 2010, 07:29:26 PM »
Hey Roddy, good to see you back here. We thought the beeb had sent you to Afghanistan as an undercover dude. Or maybe just vuvuzela microphone consultant to the world cup.

Anyway, I just found me some al-coated mylar, so hopefully I can get the capsule working again. Will keep y'all posted.
Ribbon microphone services
http://www.xaudia.com
Microphone blog

zebra50

Re: Antique DIY tube mic & capsule rebuild
« Reply #7 on: August 04, 2010, 05:12:34 AM »
It could be loosely based on the DIY sugestions in Fritz Kühne's microphone book.


Thanks Rossi. The front end does indeed look a bit like the Teladi mic. I haven's seen Kühne's book. Do you know if there is a copy on line anywhere?

Some progress. I got the mic up and running with one of Dales C37 style capsules. It sounded OK, and even better once I'd ditched the second tube and put in a balancing transformer (7:1 Sowter) instead. One of the NOS Siemens EF40s I bought was very noisy, but the second one was quiet, if very slightly microphonic.

With few exceptions, I rarely seem to like two or three tube stages in a microphone. And the first stage gives plenty of output level so it's really not needed.


Yesterday I had a go at putting a new skin on the capsule. I thought you might like to see progress so far.



I superglued some aluminium coated mylar to a heavy metal ring a bit bigger than the capsule.



 I then used the weight of the ring to tension the diaphragm, and glued that in place over the relevant support ring. I saw this trick in one of the Neumann videos that was posted here some time ago.



I then cut off the residual film and screwed the capsule back together.



This is my first time attempting a reskin, and I was pleased to find that the capsule works. However it doesn't sound very good - really no high end whatsoever.

With this capsule you can adjust the distance between the backplate and the membrane with four screws at the rear. Fiddling with these did not make a great deal of difference

Also, you'll have noticed that I didn't make any effort to tune the tension of the diaphragm. I'm going to take a good look at some of the tuning rigs posted here and see if I can make something. (in particular I recall Tim Cambell posted some good instructions)

Or it may just be that the DIY capsule design or build is not great! Either way, i'm treating this as a learning experience, and it's fun so far!

z50
Ribbon microphone services
http://www.xaudia.com
Microphone blog

zebra50

Re: Antique DIY tube mic & capsule rebuild
« Reply #8 on: August 05, 2010, 06:34:25 PM »
A quick update in case anyone is still watching.

I don't think the capsule has ever worked in the past.

Having sat and stared at that capsule for a bit, I realised that the plastic rim (top right in the post above) is supposed to seal against the membrane. To make this work I had mill out the plastic part so that the backplate sits below the membrane. I also polished the backplate and rim, and put it all back together and it sounds much better - suddenly it has a top end and it sounds pretty good for a DIY capsule

Here's a shot of the back of the mic, showing the tension adjustment screws & springs. These push the plastic part against the mylar. Actually it's a nice way to experiment with membrane tension - not even Neumann give you that!  :P



Still some noise issues with the mic, but wiring in a pF cap tells me that they're not capsule related. Might be the EF40 but I have an idea that they're related to the circuit design. I'll keep you posted.

(edited for stupidity...)
« Last Edit: August 12, 2010, 01:00:25 PM by zebra50 »
Ribbon microphone services
http://www.xaudia.com
Microphone blog

rodabod

Re: Antique DIY tube mic & capsule rebuild
« Reply #9 on: August 05, 2010, 09:05:43 PM »
This guy's hardcore. Thumbs-up to you Stewart, nice work.
Quote from: tv
punchy fat bastard chip


zebra50

Re: Antique DIY tube mic & capsule rebuild
« Reply #10 on: August 06, 2010, 05:06:18 AM »
 ;)

...hardcore.
Tube porn... here's the smashed EF40. It's funny seeing those concentric grids exposed - at least I assume that's what they are!




Anyway, Rossi was kind enough to send over this circuit, from Kühne's book



Interestingly it has the first pentode wired NOT as a triode. Also G3 is wired to the cathode - my mic has it wired to the anode. Time for some experiments!
Ribbon microphone services
http://www.xaudia.com
Microphone blog

strangeandbouncy

Re: Antique DIY tube mic & capsule rebuild
« Reply #11 on: August 06, 2010, 05:12:51 AM »
Hi Stewart,


    Looks like fun! Maybe it never really worked . . .



       Kindest regards,]



     ANdyP
. . . . RUH ROH . . . . .

Gus

Re: Antique DIY tube mic & capsule rebuild
« Reply #12 on: August 06, 2010, 05:54:03 AM »
I see only blind holes in the backplate.
How is the omni pattern?

zebra50

Re: Antique DIY tube mic & capsule rebuild
« Reply #13 on: August 06, 2010, 12:20:43 PM »
I see only blind holes in the backplate.
How is the omni pattern?

That's right. It is actually quiet directional, although some that may be from the physical arrangement of the microphone 'head' being open at one side only.
Ribbon microphone services
http://www.xaudia.com
Microphone blog

mad.ax

Re: Antique DIY tube mic & capsule rebuild
« Reply #14 on: August 12, 2010, 04:29:38 AM »
Quote from Zebra50 (in another thread: http://www.groupdiy.com/index.php?topic=40406.0)
"Axel, you might just have accidentally solved a problem for me! I have very poor signal from an old DIY capsule - i may have it polarised way too low."

Actually I feel rather guilty, cause I suggested a low polarization voltage at the beginning of this thread!

Since you don't think the capsule ever worked in the past, we cannot assume that the voltage divider values are right...
Being that membrane tension, and distance from backplate are both variable in this design, optimal polarization voltage is variable too.

That's why I think it's good to have a variable voltage source (nothing fancy, just a multi turn 1M trimmer will do) at hand to find out the best polarization voltage when experimenting with mics.

Axel

zebra50

Re: Antique DIY tube mic & capsule rebuild
« Reply #15 on: August 12, 2010, 11:35:00 AM »
After a few experiments it is better up around 80V, but still doesn't put out very high level.

I need a really quiet circuit for this.

 :-\
Ribbon microphone services
http://www.xaudia.com
Microphone blog

3nity

Re: Antique DIY tube mic & capsule rebuild
« Reply #16 on: August 12, 2010, 11:36:49 AM »
So its actually possible to rebuild a capsule?
even a U67 original one?

Thanks i havent noticed this thread before good lecture...

Jorge.
Life is a path, death the destination.

rodabod

Re: Antique DIY tube mic & capsule rebuild
« Reply #17 on: August 12, 2010, 11:46:03 AM »
I need a really quiet circuit for this.

[fishes out AC701k out of the back of the drawer, kisses box]

"I never thought I'd have to use you like this, darling.
Quote from: tv
punchy fat bastard chip

zebra50

Re: Antique DIY tube mic & capsule rebuild
« Reply #18 on: August 12, 2010, 01:08:38 PM »
So its actually possible to rebuild a capsule?
even a U67 original one?

It's certainly possible. But I'm not sure it's easy!

[fishes out AC701k out of the back of the drawer, kisses box]

:) What? Are those cheap little valves any good? If I'd known that I wouldn't have put a box in the dumpster last week... :D

Really not sure that this is worth chucking money at it. It's a good job that time ≠money, at least not in my world.
I think I just need to build a circuit over from scratch, simple and clean, and then see where we are.
Ribbon microphone services
http://www.xaudia.com
Microphone blog

mad.ax

Re: Antique DIY tube mic & capsule rebuild
« Reply #19 on: August 12, 2010, 06:02:08 PM »
There are sure a lot of interesting project one could envision with AC701K!

I believe you could get better performances from an EF40. This tube was made by Phillips Valvo as an improvement of the EF12. According to this page:
http://www.tab-funkenwerk.com/id25.html
"this tube achieved a modern measurement of -75 dB of self noise, which was unheard of in professional audio equipment at that time."

So, since you got good result with an EF12, you should be able to find  quiet EF40. Maybe use the space freed by the second tube to build some kind of supension in order to tame the microphonics...

Axel


 

Related Topics

  Subject / Started by Replies Last post
Antique wire

Started by PRR The Lab

1 Replies
1499 Views
Last post December 23, 2005, 08:28:47 PM
by Jeremy H
12 Replies
2283 Views
Last post December 29, 2005, 05:25:56 PM
by yodermr
3 Replies
1360 Views
Last post February 08, 2006, 07:41:27 AM
by ziller
18 Replies
5131 Views
Last post May 22, 2011, 11:39:11 PM
by Silvas