Official C12 Clone - Build and Support Thread

[Matador]

No.  I will settle with the recommended GE 6072A.  I want to get as close to original as possible.

Keep in mind that since the 12AT7 pulls more idle current, you will dial in less resistance in order to achieve 120V on the top of the plate resistor (B+).  When you sub in the 6072A, the B+ voltage will rise significantly, which means the polarization voltage will also rise, perhaps enough to suck in the diaphragms.

Once everything is up and running with the 12AT7, disconnect the 4 capsule leads and then do the tube swap.  You can then readjust the B+ and bias voltages to the 120V and -1V levels.  After all of that is done, you can then re-attach the capsule leads and everything should be fine.

 

[Gilgamesh]

Gilgamesh do you mean cap between plate & ground on 1961 schematic for ELA251 its a 100pf.......
did I read the ELA was a redesigned C12 for broadcast use kind of nobbling the C12 by rolling off bass b& top ...... 

Im currently experimenting with several tube mics to learn & to decide which to build to use for vocals so far I tried ELA250 with 6072 & 5840, Mk47,M49C (5840) MGM Church mic, & a TB1 (like a ELA C12 crossbread i guess) I hated the sound of the ELA compared to others I guess now it was the low 30M grid resistor which I guess make sense if not meant for recording vocals....

Im gonna have play with this here C12 circuit

Yes that's the cap I mean, maybe I will try a 100pf and see if it's a better result with a more modest effect on the highs. IIRC the 251 was a modification of the C12 designed to compete with the U47 which is why they rolled off the tops through the circuit modifications. I have also read that, although the same capsule was used the CK12 capsule in the 251 was tuned more for its specific purpose. I am building a C12 mic but also want to build a second in the 251 style as I think it shines as a vocal mic more than the C12. I compared my modifed 460 with a Tim Campbell CT12 to a Bock in the 251 style and it almost matched its class (quite impressively considering the relatively modest cost) but it didn't have the same width in the mids to give it that thick honey vocal I'm chasing. I'm hoping a genuine 251 circuit and the double layered head basket Chunger is working on will get me a bit closer. The search continues...

 

[micaddict]

I'm hoping a genuine 251 circuit and the double layered head basket Chunger is working on will get me a bit closer.

Like I've stated before, I'm hoping there'll also be a non-export version, with a subminiature tube (e.g. the 5840) rather than the 6072a. Sort of like Dany does with the M49 and M269. With a little luck they could all use the same (basic) PSU.

 

[Category 5]

I'm hoping a genuine 251 circuit and the double layered head basket Chunger is working on will get me a bit closer.

Like I've stated before, I'm hoping there'll also be a non-export version, with a subminiature tube (e.g. the 5840) rather than the 6072a. Sort of like Dany does with the M49 and M269. With a little luck they could all use the same (basic) PSU.

I sure hope not.  These kits are so nice I will be forced to build every single one they offer.  This could get wicked expensive.

Any idea when the first 251 project is due out?  If it's soon, I'll pick up an extra headbasket and donor mic with the next batch.

 

[micaddict]

I sure hope not.  These kits are so nice I will be forced to build every single one they offer.  This could get wicked expensive.

;D

Hey, but being able to run different tube mics from the same PSU (like in the old broadcast days) could save you time and money. 

 

[Pip]

Getting back to the B+ adjustment question.

Ok now granted empirical means were used throughout this process. But here are my findings such as they are.

Using the same 6072 tube I have from beginning (this does beg the question is the tube to blame but it sounds good and was bought tested from a reputable source. I will have to breakout the Hickock Tube tester down the line) these are my results.

I first changed the R17 resistor to a 400k as per the original C12 schematic. This resulted in the B+ midpoint adjustment on the pot becoming proper. There was a real midpoint. Voltage now 120VDC.

I then replaced the R17 with the original Matador specced 100K and changed resistors 1 and 2 to 100K. This resulted in the same problem returning could not get B+ voltage up higher than 106VDC (measured with Fluke 289)! :

So I then replaced the R17 resistor once again with 400K leaving PSU resistors at the new 100K value. This resulted in a lower midpoint on B+ adjustment pot but it works so I have left it this way for now.

Mic still sounds good with plenty of output. So there you have it.

When Tim Campbells capsule arrives I will go back at this in ernest.

At present:

Measured at PSU with mic connected.
B+ 120VDC
Heater 6.3VDC
Bias -1VDC

 

[chipss36]

So I have ordered parts for the build, after reading about others finished mics ,
Opted for the 1 uf cap, anyone else tryed both? Suppose more than an exact c-12 clone , just looking to make a great sounding mic.

 

[Category 5]

Contacted Christian about getting a tube for mic 1 and got schooled on the variants.  What a great guy to deal with.  He mentioned he's got some really fantastic 1970's tubes right now so if you're planning on contacting him for this build I'd say do it soon.  He seems really impressed with his current stock.

Also, anyone know the best way to get added to the waiting list for a CT12?  I sent an email to Tim but haven't heard back.  I imagine he's sick of hearing from group DIY'ers and I don't want to badger him but I'd like to get on the list soon so that completing the mic will coincide with my capsule arrival.

 

[KMB-AUDIO]

Getting back to the B+ adjustment question.

Ok now granted empirical means were used throughout this process. But here are my findings such as they are.

Using the same 6072 tube I have from beginning (this does beg the question is the tube to blame but it sounds good and was bought tested from a reputable source. I will have to breakout the Hickock Tube tester down the line) these are my results.

I first changed the R17 resistor to a 400k as per the original C12 schematic. This resulted in the bias midpoint adjustment on the pot becoming proper. There was a real midpoint. Voltage now 120VDC.

I then replaced the R17 with the original Matador specced 100K and changed resistors 1 and 2 to 100K. This resulted in the same problem returning could not get B+ voltage up higher than 106VDC (measured with Fluke 289)! :

So I then replaced the R17 resistor once again with 400K leaving PSU resistors at the new 100K value. This resulted in a lower midpoint on B+ adjustment pot but it works so I have left it this way for now.

Mic still sounds good with plenty of output. So there you have it.

When Tim Campbells capsule arrives I will go back at this in ernest.

At present:

Measured at PSU with mic connected.
B+ 120VDC
Heater 6.3VDC
Bias -1VDC

I'm hoping Matador can chime in on this one.  I too am having this issue.  I've built two mics so far, one of them I was able to dial in all the voltages as per Chunger's tutorial.  The other I was not.  I've eliminated the tube (6072a from Christian Whitmore) and capsule (RK12) as possible causes.  When I went back to troubleshoot, I discovered that on the mic that I was able to set the voltages, I made a mistake and put a 500k resistor at R17 instead of the 100k as specified.

So I guess what I would like to know is: what is the consequence of putting a 400k resistor, as per the original design, at R17?

 

[Matador]

Using 400K is fine.

There is obviously much higher tube tolerance than the original datasheets suggest.  Getting more current through the bleeder is one way to get more current flowing through the bias network.  In reality, the original C12 schematic shows a 300K resistor (R9) as the B+ bleeder, but that design was also only half-wave rectified, so it's not apple to apples in any case.  This had to be done in order to make use of the stock transformer in this circuit.

As a general principle, if the B+ voltage is too low (can't be trimmed high enough), then one has several options:  a) reduce R1 and R2 in value, b) increase the value of the bleeder R6, or c) make the bias voltage more negative.  B and C reduce the current through the network which increases B+, and A reduces the voltage drop while keeping the current roughly the same.

If the B+ voltage is too high (can't be trimmed low enough), then reverse the direction of everything above.

I was on the fence about making the B+ part of the supply regulated (with a MOSFET pass element and a Zener diode bias with feedback), however this is a drastic departure from the original supply and likely has more sonic implications. 

 

[mwrichardson]

I built my mic and supply with the parts in the BOM and had no trouble setting all 3 voltages.

Correct me if I am wrong, but R17 is 100k on schematic, not 400k. Look at how the draftsman writes his numeral 1 with a descending top slash and how he writes numeral 4 elsewhere like at power supply R2 (4W). A 400k in R17 would make the output impedance rise dramatically (and change gain) and thus would not be a good match for the T14 transformer. Also the 251 and C24 schematics all have 100k as Plate resistor.

So I don't think R17 at 400k is a good solution. The problem is probably elsewhere.

 

[KMB-AUDIO]

I built my mic and supply with the parts in the BOM and had no trouble setting all 3 voltages.

Correct me if I am wrong, but R17 is 100k on schematic, not 400k. Look at how the draftsman writes his numeral 1 with a descending top slash and how he writes numeral 4 elsewhere like at power supply R2 (4W). A 400k in R17 would make the output impedance rise dramatically (and change gain) and thus would not be a good match for the T14 transformer. Also the 251 and C24 schematics all have 100k as Plate resistor.

So I don't think R17 at 400k is a good solution. The problem is probably elsewhere.

@mwrichardson, I think you are right about R17 on the original schematic.  Which capsule and tube are you using?

@ Matador, which option do you suggest starting with? I am using a T14 from Oliver. Eventually, these will have CT12's in them, but for now I have RK12.

 

[mwrichardson]

I am using a Peluso PK12 capsule and a 6072 I got from him too. I am ordering tubes from Proaudiotubes and waiting to be put on the list for Tim Campbells capsules (Hoping for an email from Tim!). The Peluso capsule has that "U67 type capsule that is not compensated by eq" over brightness. I believe he adds a cap to ground at a junction of 2 resistors that replace the single grid bypass resistor (R15 on Matadors drawing) )to control HF in the P12.

 

[Pip]

Thank You Matador. You are very generous with your time to this group. That helps very much, much better explaination too!

Also I have corrected my intial post to reflect the term B+ throughout as I at one point incorrectly used the term bias. Ooops!

Just for information purposes this is the schematic I am referring to as the original:

 

[Matador]

R17 is the plate load resistor: don't change it from 100k unless you know what you are doing.

The resistors I am referring to are the filter resistors R1 and R2, and the bleeder resistor R6 (per my schematics not AKG's).

 

[KMB-AUDIO]

R17 is the plate load resistor: don't change it from 100k unless you know what you are doing.

The resistors I am referring to are the filter resistors R1 and R2, and the bleeder resistor R6 (per my schematics not AKG's).

Understood. I will change R1 and R2 tonight and report back my findings.  Thanks for your help!

 

[Pip]

R17 is the plate load resistor: don't change it from 100k unless you know what you are doing.

The resistors I am referring to are the filter resistors R1 and R2, and the bleeder resistor R6 (per my schematics not AKG's).

There is, I think, the clarification I needed to understand. What would you suggest starting with as a value for substituting R6 if plate resistor (R17)is at 100K. And not enough B+ voltage being obtained.

Thanks again.

 

[Gearsix]

Hello, someone can say me if it is possible to replace the SMD 0 ohm resistor with a small jumper?
Thank you!

 

[KMB-AUDIO]

Hello, someone can say me if it is possible to replace the SMD 0 ohm resistor with a small jumper?
Thank you!

Check out post #182: http://www.groupdiy.com/microphones/official-c12-clone-build-and-support-thread/182/

This is the first SMD component I've ever soldered...it wasn't too tough! Quite easy in fact.

 

[Pip]

.

Correct me if I am wrong, but R17 is 100k on schematic, not 400k. Look at how the draftsman writes his numeral 1 with a descending top slash and how he writes numeral 4 elsewhere like at power supply R2 (4W). A 400k in R17 would make the output impedance rise dramatically (and change gain) and thus would not be a good match for the T14 transformer. Also the 251 and C24 schematics all have 100k as Plate resistor.

So I don't think R17 at 400k is a good solution. The problem is probably elsewhere.

R17 is the plate load resistor: don't change it from 100k unless you know what you are doing.

The resistors I am referring to are the filter resistors R1 and R2, and the bleeder resistor R6 (per my schematics not AKG's).

Yes I do believe you are right all the way around now that I take yet another look at schematics.

Again Thanks All.