C12 circuit experiments & questions ? also how to wire cariod only?

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gary o

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Hi Im interesed to hear the differeces between some mic circuits C12, M49 (5840) and Ela M251, Im using same components for each a Tim Cambel capsule a Hauf T14/1 from the 60s a RCA 6072A for two of the circuits.....I have had the M49 circuit running perfectly sounding great..... I have now soldered together the C12 circuit its all very temperary crock clips & such .....

Im not sure about the bias voltage for the C12 I havent buil original PSU my 120V 6.3 LT PSU doesnt have Bias voltage ..... But the mic works with NO bias voltage ..... should it ?..... I read somewhere I could power the bias voltage with a AA battery.....I tried it , doesnt seem to make much difference......So do I need to make a power supply like the original C12 supply , does the -1volt have to come from the power supply like the original...... can I use battery ? or LM317 PSU or something ?

As I say its very experimental, I just wanna see if its worth doing a better job with it or is it too similar sounding to my other DIY mics....

Im also wondering if I can wire capsule similar to Ela M251 just for cardiod use ?

Thanks for reading....
 
If by "works" you mean "has sound", then yes it will "work".  However the tube will have quite a bit less headroom as opposed to biasing it slightly off.  If you measure the plate voltage at idle, I'm betting it will be down in the 30-40V range which is about 20V less swing "downwards" towards ground.  You may have also just lucked out with the tube as well.  :D

For cardioid only mode, just disconnect the "rear diaphragm" connection and leave it floating.  It's how the U47 does it.
 
Thanks Matador ...... yes I mean I have sound not sure how good..... but when I add external battery voltage to simulate the bias nothing seems to change sound wise ......
 
I have 70volts on tubes plate ......I seem to also have too much polar voltage ....... hope i have this right Im using 400k step ladder connected across ground & 120V B+ 1 Meg res 2U cap feed 30 meg to capsule .....
 
Think I got polar voltages better 58volts at 30 meg resistor when ladder is set to middle cardiod...... tube plate is 69 V no bias 70.5 V with Bais battery ....I guess tube current is around 0.5 Ma....
 
I've A/B tested RCA vs. GE 6072A five star tubes in a C12 style mic. I found the RCA to have a darker, more bassy sound than the GE 6072A 5 star - noticeably. I found the GE to sound more clear and defined in the track. YMMV.
 
Well I build this circuit to see if I liked the circuit with the parts I have sadly never used a real C12 but I have some quality parts a1960s original Hauf T14/1 tims capsule that recieves rave reviews as to being like the original ( good value too) I do prefer my Violet Vin67 capsule it was £500 years ago not good value tho

The result only using in cardiod mode the circuit does seem kinda brighter sounding than other circuits I have built which I have read here too tho I dont think I had tube running optimal due to the bias but I think I heard heard the general sound.

I re soldered the components to make it into the Ela M251 cardiod again, to my ears a better sound for my vocals just messing with grid resistor now 1 gig & C4 bypass cap also C2 the high fequency taming cap very tunable circuit very low background noise too sounds very similar to the M49 circuit, I like this one.

AusTex64 thanks, I think I have some other makes of tubes here I may try a few.
 
Matador said:
What size did you use for the plate cap (the one leading from plate to ground)?

100pf sounded open with or without a switch would be good.
 
Just thought I would update Ela M251 circuit works lovely but I thought I would have another go at C12 circuit with some spare parts I have just about to solder it together point tp point I think I had the 2uf capC6 in the power supply missing as I was try to power the bias via AA battery.
 
Got circuit together Im confused about connecting capsule ..... for now I only need cardiod, Mattador say disconnect rear diaphram as per U47 so will I still need the pattern switch set to centre 60V via R20 1Meg res & R16 30meg ??? and do I need R12 R13 to divid B+ for R14... ??? confusing my small brain here I have only ever made cardiod circuits so like the Ela M251....

Thanks
 
For a cardioid only C12 just polarize the backplate with 60V and connect the front diaphragm to the ground and leave the rear diaphragm unconnected. It could be done an other (better?) way by connecting the pickup diaphragm straight to the grid and providing an AC path from the backplate to the ground with a small cap. But then it wouldn't t sound like a C12  :)
 
Hi Paparski so which back plate to I feed 60V to the front back plate ? ....so I do need the patern switch but set to middle position 60V ?

The other (Better way .....As Ela M25X.........how about a hybrid capsule connected as per Ela with C12 bias ?.......

A C12 output is slightly lower than Ela M25X .... am I right ?

Soooo the main differences between Ela & C12 are grid resistor value, high cut plate to ground 100pf cap & 1000pf cap on the grid ?

Sorry for lots ??? Im want to hear for myself the difference.

Cheers
 
gary o said:
Hi Paparski so which back plate to I feed 60V to the front back plate ? ....so I do need the patern switch but set to middle position 60V ?

The other (Better way .....As Ela M25X.........how about a hybrid capsule connected as per Ela with C12 bias ?.......

A C12 output is slightly lower than Ela M25X .... am I right ?

Soooo the main differences between Ela & C12 are grid resistor value, high cut plate to ground 100pf cap & 1000pf cap on the grid ?

Sorry for lots ??? Im want to hear for myself the difference.

Cheers

There are several ways:

1) C12 way - ground FC.  Leave BC floating.  Put +60V on the backplate, then tap off of the backplate with a cap to the tube grid.
2) ELAM251/U47 way - tie FC to grid which is grounded with a high value resistor.  Tie +60V to backplate.  Leave RC floating.
 
Thanks Matador, so is it only possible to have C12s the grounded cathode where bias voltage come from your supply and connect capsule the Ela & 47 way .....

I read somewhere the grouded cathode C12 is best way to bias ......... also read capsule to grid is a better way to connect grid ? ........

I guess audio goes thru C12s 1000pf cap so the cap can make a big difference to the sound good or bad ........does audio gop thru say the Elam251s C1 4800pf cap ?

I read the C12 is a brighter mic than some ..... is this down to one thing more than the others..... I get the bit about the Elam251s C2 100pf roll off and that can be removed from Elam251.

Thanks
 
The capsule polarization scheme can be switched without regard to the tube biasing method.  "The best" way is always what sounds best to you.  I would try all of them and then post your results!

For C12, the audio goes through the 1000pF gate-coupling cap to the tube.  For ELAM251, there is no such extra cap (but remember, the capsule itself is a cap!), but audio certainly goes through other caps.
 
Thanks Matador yeah want to try a C12 Ela hybrid just not sure how to connect the tube bias if I use the Ela no capacitor gate wireing, sadly I dont knoe enough basic electronics but willing to solder try listen & post results, thanks for pointers.
 

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