Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)

[CBarx]

Gonna have to swallow my pride here folks.  I just finished up building two of these mics:  One works in cardioid and omni (but NOT figure 8, mic goes silent in Figure 8 mode) and the other one isn't passing signal at all.  I built them side by side, and I can find no differences in the two builds.  I've spent my day checking voltages and continuity and can't find anything.  Is anyone able to think of something I haven't?  Technically if one is working in cardioid and omni then the other should be too.  This has me all sorts of puzzled.  Any help would be greatly appreciated.

CB

 

[Tim Campbell]

Use a continuity tester to make sure you don't have a short between the backplates or membranes of the capsules.

 

[CBarx]

OK,  So I have the second mic working now, that was due to a small solder issue between the leg of the tube socket and the resistor.  Now this mic is passing signal in all patterns, but cardioid output is very low.  In Omni the Front capsule is louder than the rear, and in Figure 8 the rear capsule is louder than the front.  I'm going to keep digging into it, but I'm guessing some of the solder points in the PTP portion of the build need to be touched up.

 

[Le Roux]

Just finished a 251 with a Tim Campbell capsule, AMI output transformer, and hand picked tube from Christian.
Sounds fantastic!
I have one small issue though. A small amount of crackling around 8khz-16khz.
Tube breakin for 4 days, and even swapped out for the tube out of my C12. Same thing. Any ideas? I'm not sure where to look, other than starting to swap out some caps?
Thanks

 

[Tim Campbell]

When does it make this noise? Just standing? While you're speaking into it? Could it be power supply or computer related?

 

[Le Roux]

It makes the noise all the time.

Power supply is plugged into UPS, and I tried several other mics/psu and no issues.

Its a subtle click, crackling, but can be heard in recordings. Other than that, lots of gain, lots, and sounds fantastic!

With more playing around, I notice the crackling goes away in omni, but still present in the other patterns.

 

[Le Roux]

Alright, problem fixed!

I had omni on pin 3, which was my shield, swapped pin 3 to ground, all quiet!
Sounds awesome!!
Thanks for the project!

 

[poctop]

Alright, problem fixed!

I had omni on pin 3, which was my shield, swapped pin 3 to ground, all quiet!
Sounds awesome!!
Thanks for the project!

Glad you made it ,
Lots of the time in complex problem lies the simplest Solution ,
send a Pic over so we can see this new Toy,
Best,
Dan,

 

[CBarx]

Use a continuity tester to make sure you don't have a short between the backplates or membranes of the capsules.

Tim, turns out that one of these capsules has a short between the backplates.  It's not in the wiring, it's in the capsule itself.  Is there anything I can do to fix this, or do I just have to get a new capsule?

 

[Tim Campbell]

This isn't something you can fix. You must send it back to the manufacturer.

 

[poctop]

I will be receiving a small batch of D-251 PCB next week,  Let me know if you are interested,
Best,
dan,

 

[duantro]

I'd be interested in a pcb set. Just ordered a Beezneez body and ck12. Also have an ac701 tube and an original ( I think)  t14/1 ready to go.  Also have a telefunken elektroakustik psu.  Should be almost set!

 

[poctop]

I'd be interested in a pcb set. Just ordered a Beezneez body and ck12. Also have an ac701 tube and an original ( I think)  t14/1 ready to go.  Also have a telefunken elektroakustik psu.  Should be almost set!

PCB just Arrived but not updated on webstore yet,  they should be available as of tommorow,
Best,
Dan,

 

[poctop]

PCB's Kit Back in Stock,
Best,
Dan,

 

[duantro]

Just sent money. Thanks!

 

[HHH]

Edit:
I was asking if someone could please explain the effect of using a paper in oil [PIO] output cap [as is sometimes used in a C12] versus using polystyrene - but I was since able to find some info.  Apparently they are very close. But some people seem to prefer the PIO more.  However there doesn't seem to any of the preferred K40 series Russian PIO's on ebay in the larger 1uf though 3uf size. There're some K75's, but they are a combo of polythelene film, paper and oil. Nevertheless some people think they sound pretty good too.

 

[HHH]

Thought I'd add that the polyethylene in the K75 series is "polyethylene terephthalate", which is a common polyester. And polyester caps are widely considered the favorite of guitar amp builders for their clear warm and pleasing sound.  Mylar [a product name] is actually polyester too.

 

[LHS]

Has anyone tried successfully to remove the apex 460 logo without painting over it? I'm hoping there's a chemical that I can wipe it off with?

 

[Banzai]

If it's silkscreen: acetone.

Won't damage the nickel plating.

Has anyone tried successfully to remove the apex 460 logo without painting over it? I'm hoping there's a chemical that I can wipe it off with?

 

[duantro]

I'm gathering all of the parts to build two of these mics, 1with a 6072 and 1 with an Ac701k. For the Ac701k in this circuit It would appear that I would need to use a 200K plate resistor (instead of the 100K in the schematics here), a 3k6 cathode resistor (instead of  1K8 ), and 8M for R4 ( instead of the 30M). Also I'd need to set Heaters to 4V and maybe use lower voltage relays? Perhaps a 769-DS2E-S-DC3V (http://www.mouser.com/ProductDetail/Panasonic-Industrial-Devices/DS2E-S-DC3V/?qs=sGAEpiMZZMs3UE%252bXNiFaVG3FKEHiD4jOgP2YhcnFJhc%3d) with some added resistance to drop 4V Heaters to 3V for this relay? or would it make more sense to just use a trimmer after the 6.3 volt relay pad for the heaters? Any thoughts here?
Thanks!