Another mystery big old tube mic

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zebra50

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Hi!

Does anyone recognise this one? It is beautifully machined from aluminium, and stands 32 cm tall. There is no make or model number anywhere visible, so I have called him 'Big Al'.

DSC08018.jpg
DSC08023.jpg


Capsule & tube...

DSC08011.jpg

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The capsule looks to be nickel, or metalised film, but not gold.

The tube is a Telefunken DAF11, which looks similar to EF12 etc, but is a diode-pentode with 1.5V heaters, designed to be battery powered.

The circuit looks like this (probably!)

Big_Al_Tube_Mic.gif


The design and quality look very close to the big Neumann and RFT mics, but it doesn't match any of their models that I can find. It seems a bit better quality than the Reissmann, Telwa and Teladi mics that I have seen before.

Cheers!

Stewart
 

MagnetoSound

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Hi Stewart, interesting mic - no idea what it is though. That's a pretty small output cap. Perhaps designed to be connected unbalanced into a high impedance PA input, do you think?

Could also be powered from the PA, if that were the case.

(Just a bit of conjecture on a cold Sunday ...  :) )
 

volker

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The same mic (this one?) was just recently on Ebay. But the seller has no clue either what it really is. He only says "maybe Neumann prototype or clone".
 

zebra50

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Hi Volker,

It is indeed the same mic - it arrived yesterday. The seller knew little about it either. I thought it went at a reasonable price even for an unknown thing, and the collector in me is really pleased with it. The engineer in me now wants to get it working.  ;)

Dan, I think you are spot on with the PA idea. I have an old Reissman PA head which drives a pair of tube mics - that supplies 105V and 6V through 5 pin connectors for the EF12 tubes. It is quite lethal!

The slightly odd thing about this design is that the heater is supplied by an internal battery, but it still needs an external supply of around 100V or a big battery stack.

With an external supply I could add a big output tranny in the battery space.

DSC08006.jpg


Any thoughts on the purpose of the little coil across the indicator thingy - protection perhaps? I don't really know how those things work.
 

MagnetoSound

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zebra50 said:
Any thoughts on the purpose of the little coil across the indicator thingy - protection perhaps? I don't really know how those things work.


A choke? The filament is also the cathode isn't it? If it is one of those mechanical indicators like the old BBC PSUs have, it will probably generate some noise when vibrated. That wouldn't be a problem on the heater supply, but it might be there to bypass the indicator in order to keep the cathode clean.

Also, what is the diode connected to? It seems to be going nowhere.

 

volker

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Stewart, nice score then :D. Really beautiful body.


Regarding the circuit: there is no grid resistor at all? Also the 25nF/5k combination on the output seems dubious to me. The roll-off would be way high and the 5k looks like an unreasonably big load.
 

zebra50

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The diode is not connection - it doesn't appear to be used in the circuit.

... it might be there to bypass the indicator in order to keep the cathode clean.

That makes sense. It is one of those mechanical indicators, looks like a little motor inside.

I don't have any real experience with these heater-cathode tubes, but I'm guessing that the tube was selected as a pentode with its ability to work off a 1.5V, 50mA battery supply. There can't have been many other options.

http://frank.pocnet.net/sheets/076/d/DAF11.pdf

There is no grid resister visible - the grid lug on the tube base is connected to the capsule wire only.
It could possibly be hidden inside the bullet-head, but that doesn't really make any sense. So, probably relies on grid leak, like the Royer-mod microphones.

... the 25nF/5k combination on the output seems dubious to me. The roll-off would be way high and the 5k looks like an unreasonably big load.

Good point. So I have double checked and that is indeed what is there - the output cap reads 25,000 pF, the output resistor 5Kohms. I make that 1.2 KHz...  :eek:

 

zebra50

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Possible that either of those components may not be original, then.

I think that is quite likely - there is evidence of someone having poked around with their iron at least - some poor soldering and a couple of places where the wire insulation is melted.
 

zebra50

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Yes! There is no grid resistor - I have check this over again and the grid lug on the tube base is connected to the capsule wire only. It probably relies on grid leak, like the Royer-mod microphones.
 

PRR

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> pretty small output cap. Perhaps designed to be connected unbalanced into a high impedance PA input

But he finds a "5K" resistor at the output.

Neither 25n nor 5K seems right. Plate resistor 50K suggests >50K load. OK, 0.025uFd would be enough and that's 25nFd, so cap may be as noted.

> purpose of the little coil across the indicator thingy

It is a low resistance shunt. The filament may be 50mA, the meter may have been a standard 1mA, the shunt is selected to divert 98% of current around the meter so 50mA total puts the 1mA needle on its mark.

 

zebra50

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It is a low resistance shunt. The filament may be 50mA, the meter may have been a standard 1mA, the shunt is selected to divert 98% of current around the meter so 50mA total puts the 1mA needle on its mark.

Makes sense - Thanks!

Neither 25n nor 5K seems right. Plate resistor 50K suggests >50K load. OK, 0.025uFd would be enough and that's 25nFd, so cap may be as noted.

DSC08025.jpg


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I took the 5K out for a better look, and measured it - has drifted up to 9.6K, but is definitely labelled as 5K.

All the metal can caps measured 1.00 ±0.02 uF - they haven't drifted at all over the years!
 

MagnetoSound

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PRR said:
Full section: http://i.imgur.com/boDkV.gif


Thanks PRR, what a fascinating piece!

Stewart, gleaning from the points made towards the end of that article - and the state of your mic in the photos - it mightn't be a bad idea to completely disassemble the wiring and give the tube socket area a thorough cleaning before you do anything else, eh?

 

zebra50

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Thanks! That's a great read!

MagnetoSound said:
...it mightn't be a bad idea to completely disassemble the wiring and give the tube socket area a thorough cleaning before you do anything else, eh?

I think that is a very fine plan! It may even be a job for the ultrasonic bath!

At the moment there are a few unknowns with this - tube, capsule, circuit could all be faulty. I am tempted to pop in a simple circuit with an EF12 and output transformer, run from an existing supply, and at least see if the capsule is working.
 

PRR

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Ah, lazy analysis. The amplifier output impedance is not negligible. It is a pentode plate shunted by 50K. We may estimate this as 45K. This is further shunted by 5K. This is a 10:1 divider. No problem, the pentode's gain is well over 10. The circuit impedance is 45K+5K= 50K. A 25nFd cap in the loop opens-up below 130Hz. This is the bass response.

The mike is often seen as a voice mike and sometimes in outdoor events. Male voice bass extends below 130Hz but this adds nothing to intelligibility. Wind-noise can be very large in deep bass. Domestic radios and outdoor PA speakers did not extend far below 150Hz. Transmitting or amplifying the lower octaves is a waste of power. Strong wind noise may overload the second stage. It is good to dump it between first and second stage (at the mike output).

The output impedance for cable-loading is 4.5K. The cost of multi-core cable suggests the box is not far from the mike. Assume 100 feet 30m of cable, 3,000pFd. Response extends to 12KHz which is far beyond the radio, PA, or phonodisks of the day. 50m of cable would be acceptable.

Balanced 150 ohm cables can be run long distances without interference. My work with un-balanced 470 ohm lines says they can be very robust. The stages of the day would not generally have SCR dimmers, 500 watt monitors, fog-machines, or other electrical crap. The unbal 4,500 ohm line would work in most cases.

So it is 130Hz-12KHz; more treble with shorter line.

In modern studio work the power supply can often be within 10 feet of the mike, so there would be penty of treble and little exposure to interference.

For modern studio you might want switchable 25nFd and 0.1uFd for "authentic" and "full" bass.

The heater battery could be a C-cell, preferably pre-used; or a NiCad would be excellent.

I'd think it would "work" with 45V (five 9V) B+ power, though it is probably intended for 100V-130V supply.
 
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