Official 251 point to point on Acrylic build and support thread

251 circuit























9 pattern 251 schematic





The Build




Ok now lets Tin all of the turrets! 



Ghetto blue tack board holder!



Tin tin tin! But keep the cooling buds handy Use distilled water to cool and clean solder joints(NO IPA!)





Lets populate!



Keep it clean use distilled water to cool and clean solder joints(NO IPA!) 



Egg cups are great for keeping the I.P.A handy!








































Keep those polystyrene's cool!

















Some high tack double sided tape to hold the tranny to the board





Thread the wire through to the opening and attach as per the visual schematic. Keep it cool!! you don't want to expose the transformer to too much heat!!











Time to attach the ground wires. I'm using green but the stock mic uses white.



































This is the stock wire which can be used if you like. I'm using 22awg Teflon insulated silver coated hookup wire and 28awg for capsule and audio out from the transformers secondaries.











22awg Teflon insulated silver coated hookup wire and 28awg for capsule and audio out from the transformers secondaries.





Make sure the tube socket is installed before marking up the board for drilling!!(Not pictured here but is better for getting the holes in the right position)















We need to offset the tube to make way for the acrylic board and also make space for the wires, so we will we need to trim half off the tube fastening points in order to do so.







were not drilling through the metal just yet, we just wan to mark it with the bit.







Ok drilling time!! I've found it best to go very slow with medium pressure so as not to overheat the bit and cause it to go blunt .









































And don't forget to get yourself a lovely 251 style head-basket from Chunger!!!







Attach the output cap



And finally the polystyrene 100pf RF  cap from plate to ground. some also raise the value of this cap to tame an overly bright capsule.

Looking good! Just wondering about the 250M and 30M resistors. In every ELA M schematic I've seen they're 30M and 8M which seem really low but haven't found info abut them being incorrect.

I've tried the 30M grid leak resistor but it acts as a high-pass and rolls of far to much low end! Perhaps it's to do with the different polarization scheme? I haven't tried it so I cant say...I choose to use  the 250 and 30M because it sounds good in the C12 9 pattern polarization scheme which is similar to what is used in this C12 circuit.

The values for R7 and R4 don't matter anywhere near as much as R6.  Any value of about 10M or greater for R7 or R4 puts the low-pass filter below a few Hz.

The "best" value for R6:  depends.  If you consider a "modern" 50pF to 80pF capacitance for the capsule, then if you want bass response to be "unimpeded" you need the zero in the filter between the capsule and R6 to be somewhere around 40Hz.  If we consider 50pF to be "worst case", then R6 needs to be 80M or larger.  Larger R's give more noise, so you want to balance noise with frequency response.  60M to 100M appears to be typical in these designs, however that may be due to components that were commonly available at the time.  Many modern transistor mikes use 1G in these positions.

Thanks for Chiming in Matador! So are you saying that the 30M grid leak works in the 251 because the capacitance of the CK12 is high enough not to cut off too much low end? I have only tried the 30M with the Chinese "CK12 types" and not an accurate clone like Tims. It wouldn't be inconceivable that the 30M was put there as bit a bass cut considering the huge proximity effect of the the CK12.

Tims capsules range from 75-85 pf ...So with a 30M grid leak and a best case scenario 85pf the Cut-off frequency would be Cut-off frequency  fc = 62.4137031733[Hz] and at 75 fc = 70.7355302631[Hz] I'm thinking it must be design choice to combat the huge proximity effect of the CK12..So by all means if your luck enough to have on of Tims capsules then by all means try the 30M in R6 but not so good for China "Ck12"

I can confirm that the bass and the proximity effect on Tim's capsules are huge. For close miking 30M makes sense. For maximum bottom end one could also change the cathode bypass cap for bigger value.

It all makes seance now 8) In this case it's not so much an issue of the capacitance of the capsule but the frequency response and and proximity effect of the Ck12 capsule as the 30M resistor sounds shocking with the China capsules although they share similar capacitance range. The 251 was put forth as a U47 replacement for broadcast applications where the proximity effect may have been a bit much without additional treatment.

This is beyond cool. I will be building one very shortly

So the original 251 used 30M for R7 and 250M for R6?

R6 was 30M and the 30M at R7 is a borrowed from the C-12 Circuit so we can use PSU supplied 9 patterns instead of the three in the original.

sooo, basically this is just a plex board 251 base on changer's pcb?


why no changes to make small improvements like a very short grid wire/capsule wires etc?

No actually it's not based on Chungers board at all...It's based on an Ela M 251 E

And to confuse matters even more, Chunger's board is in fact Matador's board. 

BTW, I want them all.
But hey, I'm an addict.

OPR said:

No actually it's not based on Chungers board at all...It's based on an Ela M 251 E

Click to expand...

no, I meant the style of layout.

No... Like the original 251 on Plastic 

ok, never mind...


don't get me wrong, I like the plex board build. I think it's good, I just feel that there could be some points where it could be improved (like some other pcb's around here), but let's leave it at that.


I like the build and don't want to take that away from you or others so... cheers!

???Interesting assumptions you make Eemo1.. ..considering that the original mic that is being emulated was point to point on acrylic and not a pcb....  maybe you should educate and grace us all with your own brilliant design... ;D
Besides the sheer fact  that we're going point to point leaves loads of flexability in circuit layout and design which you dont get with a Pcb. Just use your imagination.

EEMO1 said:

ok, never mind...


don't get me wrong, I like the plex board build. I think it's good, I just feel that there could be some points where it could be improved (like some other pcb's around here), but let's leave it at that.


I like the build and don't want to take that away from you or others so... cheers!

Click to expand...

Please do share your thoughts about this. Why would you take anything away? I'm sure everybody here wants to improve things where possible. There's no reason at all to stop at the 1.0 version (and that holds for all boards etc.; not just this one). If discussed in a respectful manner, no one will get hurt and everyone will benefit. 

opr, you understood me wrong, nothing wrong with the plexiglass here or in the 251.


well, I personally would not mount the tube in the middle of the mic just because the mounts are there. that's what I meant referring to the c12 pcb layout style and the layout of this point-to-point plex board.


just because the apex 460 style has the tube mount there, doesn't mean that it's the best possible place mount the tube.