Official C12 Clone - Build and Support Thread

[Matador]

Are you saying it cannot be trimmed up to 120V?  As in 113V is as close as you can get?

 

[beatcynic]

Yes. The B+ trimmer is at its limit when using the 12ay7. I've tried a few different ay7s and  results are the same. 

 

[MicDaddy]

Hello again, I'm curious has anyone built one of these who also might have an MK47?

My issue of noise I'm thinking is more related to SNR.

My MK47 has plenty of output, the c12 is not as hot.  If I bring the c12 preamp gain up to what I feel is an acceptable level for my voice, then unplug the c12 and plug in the MK47 it's howling feedback. 

I'm beginning to think I may have a low output problem and I'm jacking the noise floor up with the preamp.  Brian Fox has hinted that my t14 could have primary/secondary swip swapped, which I will investigate tonight.  I haven't had time to measure sensitivity on either c12 or MK47 but I will as soon as time allows which might be nice to see how these mics plot for anyone interested.

If that is not the issue I admit defeat and will shoot this off to Brian as I'm buried up to my eye balls in dynamic Mic repairs.

Just wanted to update where I am at with this gal.  Thanks everyone

 

[gary o]

Hi MicDaddy I have a Mk47 & have built many circuits similar to the C12 ..... Elam 250 being closest M49C similar & others too, I have a original hauf T14/1 that I tried in each build & tried my best capsules in all trying to find best combination, tim cambel & my favorite Violet vin67 & in my cases the MK47 does have a little more level but not a lot.... & the noise floor is not as good as Elam & M49C.

All the best

 

[Category 5]

I'll preface this with a thank you to Chunger and Matador. Awesome hardware and technical support. Great pcb !
The issue I'm having is not being able to get B+ to 120v loaded with a 12ay7. With a 12ax7 there is no problem.
12ay7 - B+ = 113.2v    Plate 51v    PSU 213 unloaded (maxed)
12ax7 - B+ = 120v      Plate 88v    PSU 195 unloaded

I must say the mic sounds great either way. I understand the voltages of the two different
gain tubes will be relatively different , but I should be able to get the 12ay7 B+ up to 120,
right?
Thanks, B

Did you try with a dummy load resistor in place of the mic?  If you can only get 113 then I think you ,ay have one of the lower output (labeled 175v) transformers that some PSUs shipped with.  You need different resistors in place of r1 and r2 in such case.

 

[teacat]

Hello again, I'm curious has anyone built one of these who also might have an MK47?

My issue of noise I'm thinking is more related to SNR.

My MK47 has plenty of output, the c12 is not as hot.  If I bring the c12 preamp gain up to what I feel is an acceptable level for my voice, then unplug the c12 and plug in the MK47 it's howling feedback. 

I'm beginning to think I may have a low output problem and I'm jacking the noise floor up with the preamp.  Brian Fox has hinted that my t14 could have primary/secondary swip swapped, which I will investigate tonight.  I haven't had time to measure sensitivity on either c12 or MK47 but I will as soon as time allows which might be nice to see how these mics plot for anyone interested.

If that is not the issue I admit defeat and will shoot this off to Brian as I'm buried up to my eye balls in dynamic Mic repairs.

Just wanted to update where I am at with this gal.  Thanks everyone

I built both. The c12 is still burning in, but I would say that so far it is a bit lower in output. Not a huge amount though. The general noise floor of (my) c12 is quite a bit higher than the mk47 but still useable. The mk47 is dead quiet. I would be interested to see your sensitivity measurements.

 

[Matador]

Yes. The B+ trimmer is at its limit when using the 12ay7. I've tried a few different ay7s and  results are the same.

The beauty of a passive PSU design:  I tried to place the middle of the trimmer range in the middle of the typical tube datasheet range.  Sounds like you got some higher-current ones.

You can lower the values of R1 and R2 slightly:  the next standard 5% value down is 82K.  That should put the pot back in the middle of its resistance travel to get you some room to adjust.

 

[Matador]

One simple isolation test to determine tube vs. capsule noise:  make a small wire jump, and hard ground the grid connection of the tube side you are using.  I have a small wire with two small alligator clips on each end, and I clip one on a grounded turret and the other where the grid wire attaches.

You can then button everything back up, fire it up, and then do a test recording.  Obviously it no longer amplifies the capsule signal:  however any noise the tube is making comes through.  This test also completely removes any noise that may be caused by anything prior to the tube.  If the mike is dead silent with the grid shorted, the noise is comping from the capsule and/or it's connections.  If there is a lot of "wind" or "blow" noise then another tube can be substituted (leaving the jumper in) until a quiet setup is found.

It's a simple and fast binary search for noise in this (and any other) tube design.  I used this all the time with guitar amps:  I have another version that attaches to a wooden stick and has one end with no alligator clip:  I can probe around (carefully!) with it at various ground and other circuit points and listen live to the changes.

 

[beatcynic]

Did you try with a dummy load resistor in place of the mic?

Just tried that. Looks like things are allright with the PT. If I leave the B+ trimmer maxed(213v unloaded) I now get 125v with the 180Ω load. 
I'm going to do some recording the way it is. In my next order I'll get some 82k s and try that out. Might as well use these

higher-current ones.

Thanks!

 

[teacat]

How long do you have to burn in the tube before the noise typically quiets down? I've been burning one in off and on for several days and it is still quite noisy. I just did the test as Matador mentioned, grounding the grid, and the noise was exactly the same. I tried a different cable as well with the same noise. It is a general white noise like when you turn up a stereo all the way. The tube is a EH 6072 "tested for low noise and microphonics" from tube depot. I'd rather run down any other possible issues before springing for another tube that may or may not have the same problem. The only other tube I have on hand is a 50's 5 star 6021, would it be ok to put that one in just to check the noise?
Edit: I went ahead and swapped in the 6021 anyway, and indeed it was much quieter, on par now with my mk47 and sounding quite nice. I guess I should wait and see if the 6072 quiets down, or maybe consider modding to use the 6021.

 

[teacat]

Regarding 6021 mods: Since I have a 175v trafo, what value would I need for R1&2? I installed 56k's to get b+ to 120v, would the specified 33k for R1&2 still work with the 175v? and the 47k in R17?
Edit: (Ok, I keep asking ?'s, then going ahead and trying stuff.... 'cause it's sunday) I left the 56k's in R1&2 and dropped in a 56k at R17. Easily dialed in 120v, -1v and the heater stayed the same. Mic is now less noisy and sounds less harsh than with the EH.

 

[dmnieto]

Hi,

I am preparing everything to build two C12 clones...

I was wondering if this transformer would be enough...

http://www.edcorusa.com/p/882/xpwr191

it provides up to 1.5A on the 8.5V and 40mA on the 200V... would that be good?

 

[micaddict]

Question for Matador.

We've already discussed the 12AT7/6201 tube substitution for the 12AY7/6072 and you were kind enough to come up with the necessary adjustments in the circuit. Thanks again for that.

I don't really want to stretch this thing ad infinitum, but there's another, similar tube that some rave about. I'd like to ask you if it would be usuable in a C12 (and/or Ela M251e) circuit.
It is the E180CC or 7062 tube. It's pretty much an 12AT7, but with a gain closer to the 12AY7.
-This could arguably be seen as an advantage.
-A truly objective pro is its long life (10,000 hours).
-Another pro (over the 6072) is that it's -still- very affordable.
-It is said to sound really smooth and musical.
-And finally a very subjective pro; many of those were made in Holland.  8)

Possible cons:
-Its size. It is a half inch longer.
-Higher current draw.

Bad idea or not?

 

[gyraf]

One important parameter is how the tube reacts to very-high grid resistance. Can't read this out of published data sheets (other than for electrometer tubes) because it's so far out of range from usual usage, hundreds of MegaOhms instead of 100K-1M usually specified.

Rated/recommended grid resistance for E180CC is something like 500KOhms - but this spec dosen't tell the entire story.

Microphonics and behavior when underheated is important as well..

Try it. And let us know.

Jakob E.

 

[micaddict]

Thanks, Jakob!
Yes, others may chime in, too, of course. 



Henk

 

[Matador]

Question for Matador.

We've already discussed the 12AT7/6201 tube substitution for the 12AY7/6072 and you were kind enough to come up with the necessary adjustments in the circuit. Thanks again for that.

I don't really want to stretch this thing ad infinitum, but there's another, similar tube that some rave about. I'd like to ask you if it would be usuable in a C12 (and/or Ela M251e) circuit.
It is the E180CC or 7062 tube. It's pretty much an 12AT7, but with a gain closer to the 12AY7.
-This could arguably be seen as an advantage.
-A truly objective pro is its long life (10,000 hours).
-Another pro (over the 6072) is that it's -still- very affordable.
-It is said to sound really smooth and musical.
-And finally a very subjective pro; many of those were made in Holland.  8)

Possible cons:
-Its size. It is a half inch longer.
-Higher current draw.

Bad idea or not?

My eyeball of the curves shows it should work.  It wants to idle at a much higher plate current (and a much lower bias point), but in theory it should work if you follow the 12AT7 conversion.  The curves get real bendy down at 1mA plate current so THD may be a bit too high in this application:  but who knows, it might sound great!

Microphonics is important, and as Jakob says so is the reaction when driven from high-impedance sources.  But there's noting preventing you from trying it out!

 

[micaddict]

Thanks a lot! 

 

[Dennyp]

Hey Guys.  Im building the power supply and I have 188v @ b+ ,-1 @bias but I cant trim the heater which is @ 13.64v is there something im missing here?
I rechecked the solder points to the trimmer and everything appears to be ok.  what could be wrong?

thanks in advance
Denny

 

[etiefenthaler]

Hi MicDaddy

I have both - mk 47 and c12. They are dead quiet, but c12 needs approx +8 dB Gain in comparison to mk47.

Erich

 

[Melodeath00]

I just installed a Russian K42y-2 PIO in my C12. K40y was way too big unfortunately. The verdict? The mic still sounds great . I'm sure there's SOME difference, but it is certainly not obvious if it's there.

Has anyone found their C12 to be darker than expected? I recently heard a very nice shootout with involving a vintage AKG C12, and the mic seems much brighter than mine is.