D-EF47 Tribute To Oliver Archut U47 Build Thread.

[rock soderstrom]

This is actually a Stam SA-47 MkI that I bought before I got into DIY mics and I'm fairly certain it's just a D-EF47.
If I crack it open, where should I look to determine the grid resistor value?

I'm just trying to collect some data on the frequency of sudden crackling and noise from microphone tubes.

My suspicion is that the higher the Rg, the more frequently this occurs. Extremely high 1G impedance on the grid is not good for the tubes and can cause noise problems.

The mic was SILENT yesterday...been tracking all week!

Your EF800 is a German special tube designed for longevity, they don't really break that quickly. Of course, nothing lasts forever, but the connection between high Rg and such problems should be taken into consideration.

The sister tube EF804 for example runs under normal circumstances for decades in V72/V76 mic amps, just for comparison.

If there is a 1G resistor in your mic, you can consider replacing it with smaller one.

 

[egroys]

I'm just trying to collect some data on the frequency of sudden crackling and noise from microphone tubes.

My suspicion is that the higher the Rg, the more frequently this occurs. Extremely high 1G impedance on the grid is not good for the tubes and can cause noise problems.

Your EF800 is a German special tube designed for longevity, they don't really break that quickly. Of course, nothing lasts forever, but the connection between high Rg and such problems should be taken into consideration.

The sister tube EF804 for example runs under normal circumstances for decades in V72/V76 mic amps, just for comparison.

Interesting. Yes, this tube was replaced by a very experienced tech, Toby Foster, after I connected the mic to a hot PSU in 2019.
It was apparently hand selected by himself and I trust his expertise that it was a quality tube. I will open the mic up and check shortly...

 

[egroys]

As suspected, it's a D-EF47
If I'm not mistaken, that's Brown Black Grey Silver? So exactly, 1G

 

[egroys]

Here is the noise from the morning. First it was crackling, and 5 min later, I noticed the 'fly buzz'. The mic had been on for maybe 2 hours at that point.

I powered down after hearing it.

Powered up and warming up again now but so far, not hearing any noise.
I'll report back after another hour or so.

 

[rock soderstrom]

As suspected, it's a D-EF47
If I'm not mistaken, that's Brown Black Grey Silver? So exactly, 1G

Yep, 1G with 10% tolerance.

Here is the noise from the morning. First it was crackling, and 5 min later, I noticed the 'fly buzz'. The mic had been on for maybe 2 hours at that point.

This could actually be the tube or moisture in the high impedance range. I have also experienced both types of noise, change the tube and you will probably be able to solve the problem. If not, use 99.X% alcohol and clean the high impedance area. However, I would first replace the tube with one that is known to work.

The tube is a SIEMENS labeled Telefunken EF800. Most of the EF800s I've seen so far have been from Telefunken. The AEGs are also from Telefunken. There are also some EF800 from the former GDR that are sometimes called EF860 as well from RFT.

 

[rock soderstrom]

Powered up and warming up again now but so far, not hearing any noise.
I'll report back after another hour or so.

I guess it will come back, preferably during an important recording!

 

[rock soderstrom]

Powered up and warming up again now but so far, not hearing any noise.
I'll report back after another hour or so.

This gives me no peace now, can you please see if the tube has a diamond in the base? (or a serial number on the side?)

 

[egroys]

This gives me no peace now, can you please see if the tube has a diamond in the base? (or a serial number on the side?)

Edit: Nothing on the other side

But this is the base: Looks like a 58 or 53. And an N on top?

 

[JeffKall]

Interesting. Yes, this tube was replaced by a very experienced tech, Toby Foster, after I connected the mic to a hot PSU in 2019.
It was apparently hand selected by himself and I trust his expertise that it was a quality tube. I will open the mic up and check shortly...

Was the THE music engineer Toby Foster from Los Angeles ?

Haven’t heard Toby’s name in years … but, if so, you can absolutely trust his expertise !

 

[rock soderstrom]

But this is the base:

It is a Telefunken EF800, as the diamond in the base indicates!

For comparison here one with Telefunken print + serial number that shows that it was produced in the german city Ulm. The last additional print shows that this tube was produced for the german army.

 

[HerbertR]

As suspected, it's a D-EF47
If I'm not mistaken, that's Brown Black Grey Silver? So exactly, 1G...

Hello, are you sure that these artifacts are not caused by the missing insulating Teflon turret pin in the high impedance part? It is the main connection between 1G Ohm R2, the front diaphragm, tube grid and the reed relais switch through the rear diaphragm for omni mode, which is supposed to go through the board and act as a spacer...when the resistor or its feet... touches the board, or the reed relay metal housing, or what else... then you have built some kind of antenna or something, from oscillation, to white noise and crackling... Not a good solution imo.

 

[egroys]

Was the THE music engineer Toby Foster from Los Angeles ?

Haven’t heard Toby’s name in years … but, if so, you can absolutely trust his expertise !

Indeed, it was! Good to know

 

[egroys]

Hello, are you sure that these artifacts are not caused by the missing insulating Teflon turret pin in the high impedance part? It is the main connection between 1G Ohm R2, the front diaphragm tube grid and the reed switch through the rear diaphragm for omni mode, which is supposed to go through the board and act as a spacer...when the resistor or its feet... touches the board, or the reed relay metal housing, or what else... then you have built some kind of antenna or something, from oscillation, to white noise and crackling... Not a good solution imo.

I certainly can't be sure since I did not build this mic, but I have been using it for almost 6 years without any noise like this. It's currently no longer happening but I ordered some EF800s to have as backups!

 

[AndyM]

Hello all,
this is my first post on this forum after reading a good share on this subject. I'm building a U47/U48-like microphone based on a EF800 (860 or 802). Still in the process of gathering all the parts and as much info as possible. The schematic of the mike would look like this:

Will this work (cardoid and figure 8)?

Furthermore, the PSU based on https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a0714e.pdf gives some headaches.
Could I use this tranny https://www.tube-town.net/ttstore/de/ringkern-netztrafo-30va-8808.html (with the 6.3V secondary for the 5.05V)?

For eample, doesn't the branch for the heater

boil down to a voltage divider like this?:


What values can I assume for the dummy loads? 20ohms for the heater (6.3V/275mA) and 190K for the mike (B+)?

For some reason simulating the PSU in Duncan's PSU Designer II gives me higher results than expected.

Hope someone can point me in the right direction. Thank you very much in advance!

Cheers,
Andy

 

[rock soderstrom]

Furthermore, the PSU based on https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a0714e.pdf gives some headaches.
Could I use this tranny https://www.tube-town.net/ttstore/de/ringkern-netztrafo-30va-8808.html (with the 6.3V secondary for the 5.05V)?

No, the secondary AC voltage is way to low for passive filtering, but you could get away with a LD voltage regulator or a voltage doubler, like this project of mine.

Thread 'DIY project: A handy power supply PCB for small tube amps like preamps, buffers or microphones.' https://groupdiy.com/threads/diy-pr...ps-like-preamps-buffers-or-microphones.84324/

 

[rock soderstrom]

For eample, doesn't the branch for the heater

boil down to a voltage divider like this?:

No, the series resistors a part of RC filters stages.

What values can I assume for the dummy loads? 20ohms for the heater (6.3V/275mA) and 190K for the mike (B+)?

for a 5.05V heater it would be 18.36R, for 6.3V it is 22.91R, so your 20R is pretty close.
P is 1.73W for 6 3V heater, so a 3W resistor would be good.

For B+ it would be a 105K resistor with 105V/1mA.

 

[AndyM]

Hi Rock,
thanks for your help. I think I've even come across your PSU thread a week or two ago and found it very interesting. Will get back to doing my homework for now.
BTW: Would you happen to have such a pcb "left over" for me? ;-)
Greetings from Chemnitz
Andy

 

[rock soderstrom]

BTW: Would you happen to have such a pcb "left over" for me? ;-)
Greetings from Chemnitz

Hi Andy, you're in luck, I'll be baking a few PCBs on Tuesday or Wednesday! Home brewed , now with silkscreen ;-). I need to charge you for the base material and shipping, maybe we can swap cool stuff?

Cheers

PS: I will send you a PM

 

[AndyM]

Now that sounds good! Maybe I can lend a helping hand in metal work?