So C3 was changed to 22uf wet tantalum and he mic took a different character. Mids got pushed back a little and it sounds smoother ( less grit) overall...not so in your face.  Both sounds are great and this is a great place to experiment and fine tune the mic to your liking.



Need a sub for the relay - would this be ok?

https://au.mouser.com/ProductDetail/Omron-Electronics/G6A-274P-ST-US-DC6?qs=%2Fha2pyFadugsPlTFOHXH%252BX51zV%2F%252BTHhGwDLyOo3fryU%3D

I see the relays from the main BOM are back in stock but these worked fine in mine. The mic is spectacular! So rich and buttery on vocals but just a bit more weight and swagger than the C12. I used Tim Campbell capsule and got an electro harmonix tube in. Can't wait to try some NOS tubes and play with the capacitor values a bit. But not for a while I just want to put it to work it's a great addition.

I have a 3.6uf wet tantalum that actually measures 3.7uf. Would it be ok to use it as a coupling cap? Are there any dangers exceeding the specification of the coupling cap? Thanks.

kpearsall

So i got the build a long time ago, built it, has some issues but due to life, im only finally getting around to getting it fixed and working. I've built the mic and psu, have a b+ at 117V and a heater at 6.29V. I'm having an issue that I can hear any tapping on the frame that its mounted in, but the level of it if you try to speak/get audio from the capsule, its almost 0 output. (maxed out preamp and only get a low signal). I'm testing it with the chinese tube the Apex 460 came with, but i have a NOS 6072 for it once everything works. Its a TC CT12 capsule and AMI T14. Any assistance would be fantastic!

edit: I noticed that my heater voltage drops to 0.03 when its set to omni, but on card and f8 its 6.29V.

Hey man. If you can tap on the mic and hear it loud and clear, but can't speak into it, then check the signal from the capsule to the tube grid. This is almost always the issue. Just a break in the connection. I've fixed this problem a lot lately in u67's, C37a's, C61's, and many others. Also happened to my 49 once. Seems to be a universal truth, so you can start there.

kpearsall

So after working on and recording with so many of these mics over the past two years, I thought I'd look into building one. What I can tell you all with dead certainty, is those schematics are wrong! absolutely wrong. I've never ever seen a 3.2uF output cap in these, or the 1.6uF output cap for the non-export version. I've worked on at least 40 of these. I've seen some weird things, like an 8 MEG  grid resistor (very low bass response), and the most obvious fake AC701k tube ever (painted over AC761) but  almost every mic i've seen that had the original wet tantalum output cap, it was 1uF. You'll get better results adjusting low end with a grid resistor than an output cap. If you're going for the original sound, stay away from those 3uF output caps. They aren't doing you any favors, and they are simply the result of a bad schematic. I find anything 100 Meg or higher for the grid is just fine.

So after working on and recording with so many of these mics over the past two years, I thought I'd look into building one. What I can tell you all with dead certainty, is those schematics are wrong! absolutely wrong. I've never ever seen a 3.2uF output cap in these, or the 1.6uF output cap for the non-export version. I've worked on at least 40 of these. I've seen some weird things, like an 8 MEG  grid resistor (very low bass response), and the most obvious fake AC701k tube ever (painted over AC761) but  almost every mic i've seen that had the original wet tantalum output cap, it was 1uF. You'll get better results adjusting low end with a grid resistor than an output cap. If you're going for the original sound, stay away from those 3uF output caps. They aren't doing you any favors, and they are simply the result of a bad schematic. I find anything 100 Meg or higher for the grid is just fine.
Thanks, I have seen schematics that display different values but I don’t know if they are all the original Tele/akg. In my first 251 I have a grid resistor of 250M and it felt 3uf for the coupling cap was a little too much so now I have it down to 2uf and I will possibly go to 1uf ( I have a few wet tantalums that I can use in parallel).
On the one I am building now I have a 30M grid res to keep it closer to the original and an AMI T14...since the word is that the Ami is maybe a little shy on the bass I thought the ~3.2uf could be an option.
Since you have seen so many 251’s could you please tell me if possible what are the types of the rest of the caps? I think the 4700 and 100pf are ceramics and the 1000pf is styrene. How about the 22uf 6.3v one? Thanks again

kpearsall

I can only go based on the ones i've seen, and almost all have had that 22uf replaced. There was at least one though, that I have pictures of, that had what looks to be a wet tantalum in that position. I didn't have reason to pull it out to test it though, so I have no idea the value. I've seen everything from 22uF to 800 uF in that spot. A common value these days is a 47uF. and usually with a .47uF output cap. as for your mic, I'd go down to 1uF output, and up the grid resistor to 100M and use that as a starting point.

Those small value caps are the odd looking tubular things. some other techs tell me they are ceramic, others say they are polystyrene. I've never had a bad one that I could cut open to find out. But I will say I've never seen anyone ever replace them with a ceramic cap. Always styrene.

I can only go based on the ones i've seen, and almost all have had that 22uf replaced. There was at least one though, that I have pictures of, that had what looks to be a wet tantalum in that position. I didn't have reason to pull it out to test it though, so I have no idea the value. I've seen everything from 22uF to 800 uF in that spot. A common value these days is a 47uF. and usually with a .47uF output cap. as for your mic, I'd go down to 1uF output, and up the grid resistor to 100M and use that as a starting point.

Those small value caps are the odd looking tubular things. some other techs tell me they are ceramic, others say they are polystyrene. I've never had a bad one that I could cut open to find out. But I will say I've never seen anyone ever replace them with a ceramic cap. Always styrene.
Great info, thank you so much for taking the time. Actually I changed the value of the coupling cap today down to 1uf wet tantalum. The strange thing is that I have noticed very little difference in the low end but for some reason the high end became smoother...totally unexpected. The type of the 22uf plays a big role too. The tantalum is grittier, more compressed and mid forward. A wet tantalum in that position is really smooth and...easy/liquid sounding.
Thank you again for the info, cheers.

So second 251 finished and calibrated. This one with a Tim Campbell ct12. Here are some personal/subjective observations for those interested.
The Tim Campbell sounds very different than the BeesNeez. Both sound great and I can see people favoring one over the other. The BeesNeez is mellower/softer...more forgiving. The Tim Campbell has that detail and definition with a smooth exciter like high end I hear on SOME originals. It also feels a little scooped compared to the BeesNeez...the BeesNeez sounds denser. I have also found that I prefer the Tim Campbell with 110v h+ and the BeesNeez at 117/120v h+. My personal favorite is the Tim Campbell...tracks sound mix ready FOR MY TASTE.
Both mics have Ami t14, 1uf 150v wet tantalum coupling cap, nos ge 6072. The differences are the type of 22uf cap ( I kept the value) and grid resistor value. Beesneez sounds better to me with an electrolytic 22uf and tim Campbell with a wet tantalum at that position.
As far as the grid resistor goes the beesneez is 250M and the ct12 30M.
I prefer the mic with stock/original values. It might not sound as impressive in the low end but I don’t need to touch the low end during mix. Plus the proximity effect is much more usable. The 250M to me sounds a little bloated and I always have to take some lows out during mix.


Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #710 on: August 14, 2020, 12:50:09 PM »
Hey man. If you can tap on the mic and hear it loud and clear, but can't speak into it, then check the signal from the capsule to the tube grid. This is almost always the issue. Just a break in the connection. I've fixed this problem a lot lately in u67's, C37a's, C61's, and many others. Also happened to my 49 once. Seems to be a universal truth, so you can start there.

Thanks! I have actually since fixed the issue with polar pattern heater voltage, but still have not been able to get things rolling. I've gone through about 2 dozen full run throughs of all checks and everything seems okay. I'm  just about ready to rip out the entire PTP section and re-order all the parts and rebuild it from scratch. On the bright side all my other DIY projects I've done in the last couple months have turned out perfect.

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #711 on: September 05, 2020, 02:51:34 PM »
Hey guys, can anyone recommend an equivalent part for the pattern selector?  It’s backordered at mouser since...forever haha. Thanks.

Pusch3l

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #712 on: October 19, 2020, 09:59:50 AM »
I have just a short question. Today I soldered the last parts on the PSU and it powers up fine without smoke. Without mic connected I cannot regulate B+ lower than 165vDC.
I get 219v out on the secondary winding from the Transformer.
Can anyone tell me if it's safe to connect the Mic because I cannot imagine that the B+ drop will be over 40v...

Greets

rockinrob86

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #713 on: October 20, 2020, 09:52:40 AM »
The voltage drop can be alot, and anyways, it's a tube, it won't blow up.  It's not like you have 365 coming out of the power supply!

just set it up with your multimeter ready to measure, the drop should happen pretty much right away, and then you'll see it's probably fine and you can breathe a sigh of relief!

Pusch3l

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #714 on: October 20, 2020, 01:55:33 PM »
You're right, the voltage drop is actually pretty big. After some trial and error on R1 a put back the 75k. I was relieved that B+ is now set to 120vDC with mic plugged in and I measured around 35v on the front capsule in cardioid... tomorrow I will do some tests with it.

OH and I have seen tubes glowing like a 60w bulb on steroids. Not funny at all  ;)

Anyway it was a fun built.

Greets

Re: Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)
« Reply #715 on: October 26, 2020, 02:51:23 PM »
It seems like C&K have stopped manufacturing the 251 rotary switch. Any way to go around it? :)