zebra50

Joachim Wetzel tube mic
« on: August 08, 2008, 01:13:42 PM »
Hi!

I've just been dissecting an old German Wetzel tube mic. Here's my sketch of the schematic.



A couple of unusual features here.
One is the use of light bulbs as part of the heater supply filter network.
Another is that the capsule is referenced to the cathode of the first tube stage, rather than to ground. I don''t recall seeing that before.

One thing that's puzzling me is that the PSU gives exactly 6.3V when run off exactly 220V (variac), but the tube (ECC83) seems to be wired for 12V operation (pins 4 & 5). It doesn't look like the mic has been modified at all (before I got my hands on it). Could be a factory error, or that the PSU is really for another model, although both mic and PSU have the same manufacturer's name and the components look to be a similar vintage.

I'll get some photos later.
« Last Edit: January 02, 2009, 04:14:59 PM by zebra50 »
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pucho812

Another dodgy tube mic
« Reply #1 on: August 08, 2008, 01:32:28 PM »
an ecc83 is equal to a 12ax7 tube.  The heaters can operate at voltages ranging from 6.3V - 12.6V  AC or DC
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

zebra50

Another dodgy tube mic
« Reply #2 on: August 08, 2008, 01:46:12 PM »
Thanks! But the mic seems to be wired for 12V operation, and the PSU for 6V, which is odd.

When i got 6.3 V at the PSU I expected to see the heater filaments in parallel, but they appear to be in series. I've triple checked this, and this one has 6.3V at pin 4, 0V at 5. Pin 9 has no connection.
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rodabod

Another dodgy tube mic
« Reply #3 on: August 08, 2008, 01:49:17 PM »
That's pretty cool, Stewart.

I might measure the AC at the heatre secondary in case there is a hint as to whether the heater voltage could have potentially been 12V (eg. with different lightbulbs).

As you say, low anode voltage, and particularly low capsule voltage. Not sure about the cathode reference instead of ground for the capsule.... I was thinking about noise, but I can't see what benefits this could provide.

A macro shot of the capsule would be nice, or maybe a back-lit one so we can see the drill pattern.

Roddy
Quote from: tv
punchy fat bastard chip

zebra50

Another dodgy tube mic
« Reply #4 on: August 08, 2008, 01:57:35 PM »
Good point - that hadn't occured to me. Heater circuit gives 15-16V DC after rectification, so very possible. I didn't note the AC but must be 12V-ish. I'll look at the bulbs again tomorrow. I didn't even check their values.
I was contemplating popping in a 7812 if things get desperate!

Maybe the tube has been swapped at some point. Are there any pin equivalent twin triodes to ECC8_ / 12A_7 that have a 6V heater across pins 4 and 5?

A couple of photos - capsule has obviously seen better days!



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zebra50

Another dodgy tube mic
« Reply #5 on: August 08, 2008, 02:06:20 PM »
Quote
A macro shot of the capsule would be nice, or maybe a back-lit one so we can see the drill pattern




That's about the best shot I got of the pattern. The capsule is all packed up now to go off for surgery.
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Gus

Another dodgy tube mic
« Reply #6 on: August 08, 2008, 02:33:24 PM »
6DJ8  
  Look for twin triodes with the same base as the 6dj8

Check the following

6aq8
6bq7
6cg7
6fq7
6gm8
6h30

check the heater current

zebra50

Another dodgy tube mic
« Reply #7 on: August 08, 2008, 03:04:03 PM »
Hi Gus,

I couldn't remember off the top of my head. I have some 6DJ8 and 6CG7 in stock.

6DJ8 is ECC88 - the tube inside is ECC83. I wonder if someone made a mistake?

Thanks!

Stewart
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mad.ax

Another dodgy tube mic
« Reply #8 on: August 08, 2008, 03:33:06 PM »
6AX7 is identical to 12AX7/ECC83 except for heater which is suposed to be 6.3V / 300mA when wired in serie and 3.15V / 600mA wired in parallel.

I didn't found a trace of an european equivalence, and your mike being German, it would be odd that they have used an American tube...

Anyways, you could try with a 12V supply and see if it makes an improvement...

Axel

rodabod

Another dodgy tube mic
« Reply #9 on: August 08, 2008, 03:40:39 PM »
Is the valve an RFT? I wouldn't be surprised if it was the original valve.
Quote from: tv
punchy fat bastard chip


zebra50

Another dodgy tube mic
« Reply #10 on: August 09, 2008, 01:54:20 AM »
Quote from: "rodabod"
. I was thinking about noise, but I can't see what benefits this could provide.


At first I thought feedback, but I think the just acting as a 'fixed' bias for the cathode. So the 12Meg/22.5Meg/5K act as voltage divider and give the reference for the capsule polarisation & cathode bias.

As well as getting these old things up and running, the motivation for tracing out and sharing the schematic is to preserve the information and see what we can learn from the designers. Obviously it would be nice to know if it's the PSU or the tube that's been switched.

Quote
Is the valve an RFT? I wouldn't be surprised if it was the original valve.

Possibly - like a lot of the tubes in the old german gear, this is unbranded.
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rodabod

Another dodgy tube mic
« Reply #11 on: August 09, 2008, 03:12:53 PM »
Quote from: "zebra50"

Quote
Is the valve an RFT? I wouldn't be surprised if it was the original valve.

Possibly - like a lot of the tubes in the old german gear, this is unbranded.


You tell RFTs (and their re-labelled counterparts) often by looking at the "seals" around the pins inside the valves - they are often a frosted-looking, slightly opaque glass.
Quote from: tv
punchy fat bastard chip

Sonic

Another dodgy tube mic
« Reply #12 on: August 09, 2008, 03:52:22 PM »
To zebra50
Seems to me, that for ECC83 not enough anode voltage. Under your parameters approaches 6DJ8 more. At you the scheme not present the output transformer and the condenser of small capacitance on output - this your omission, or the transformer there really is not present? If the transformer is not present, too big  impedance turns out.

zebra50

Another dodgy tube mic
« Reply #13 on: August 10, 2008, 04:10:40 AM »
Hi Sonic,

It's transformerless. The output capacitor is in the power supply (marked bottom right). I've seen a few old tube mics like this that run hi-Z and unbalanced. The output connector is marked '20Kohm'. It was probably aimed at home reel-to-reel recording, rather than serious studio work. I could easily add a transformer.

Thanks for the comments on the voltages. I agree that they look low for ECC83. I'll try a 6DJ8 & remeasure.

 :thumb:
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Sonic

Another dodgy tube mic
« Reply #14 on: August 10, 2008, 04:41:33 PM »
Hi zebra50 !

I on Russian lamp 6Í23Ï-analogue 6DJ8 expose an anode voltage within the limits of 80-90 volt. At a level of an anode voltage in 50 volt sensitivity will be small.

rodabod

Another dodgy tube mic
« Reply #15 on: August 10, 2008, 06:22:23 PM »
Maybe this person can help:

http://ds-audioservice.de/mikrofone-fotos/mikro2.html

Also note that the CM7156 on that page has a "Joachim Wetzel" label on the body. Maybe Wetzel was part of the RFT company.
Quote from: tv
punchy fat bastard chip

zebra50

Another dodgy tube mic
« Reply #16 on: August 12, 2008, 02:52:51 PM »
Made a little progress with this one. I think there may be an issue with the big old (selenium?, Germanium?) diodes that make up the rectifiers.

With the tube out of the circuit, I get 20V ac and 28VD before and after the heater rectifier. With the tube back in it drops to 14V after the diodes.  

The HT measures up around 110V but then drops steadily as things warm up. It ends up around 40-50V.

I'll tear them out tomorrow and pop in some Si - see if that a difference. I would have done it today but we had a big electrical storm and the power went down.
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hessenmusik

Re: Another dodgy tube mic
« Reply #17 on: January 02, 2009, 12:56:49 PM »
Are you still working on the Wetzel mic? I have two of them and i like the vintage sound.
I´ve retubed them with selected fast speed tubes ECC- 83 / Fast Rise by Reussenzehn.
bg from Germany

zebra50

Re: Another dodgy tube mic
« Reply #18 on: January 02, 2009, 04:13:50 PM »
Hi!

I'm still waiting for the capsule to come back so I haven't heard this one yet, but no rush - we'll get there sometime. There is also a problem with the old diodes in the PSU - they seem to have become resistive over time, hence the strange voltage readings.

Are you running yours unbalanced into a hi-Z input?
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hessenmusik

Re: Joachim Wetzel tube mic
« Reply #19 on: January 03, 2009, 09:43:48 AM »
I´m lucky to put only new tubes in without probs with the power.
Perhaps i must also check the capacitors and other parts, but at the moment the microphones work well.
I use the two unbalanced hi-Z inputs from my Mackie onyx mixer...the line input does not have the full sound.


 

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