Official C12 Clone - Build and Support Thread
- Thread starter Matador
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Help Support GroupDIY Audio Forum:
Hmm thanks. I was looking at the schematic for the Matador boards. I’ll try the original connection and see what happens.
I am still learning about schematics but from what I can tell, there are 4 leads from the capsule all going to different places.
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shabtek
Well-known member
top and bottom wire exiting schematic capsule are connected to the same node (backplate(s))
I’m seeing it now. Thank you. Just connected the original capsule in this way and going to give it a test with the original tube and capsule.
Alright, Im learning. With this tube pin 9 is the CT for the heaters. Which is grounded.
Im not getting heater voltage to the tube through the cable although I have heater voltage at the power supply when the cable is not plugged in.
I've double checked may cable and everything is correct. Looking for other errors.
Im not getting heater voltage to the tube through the cable although I have heater voltage at the power supply when the cable is not plugged in.
I've double checked may cable and everything is correct. Looking for other errors.
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With power supply loaded with 180k resistor, im getting reasonable values and the power supply appears to be working correctly.
With the mic connected, im getting no heater voltage at the test point in the power supply or on the mic. And the voltage on the 9.5 rails of the secondary drops to about hafl of what it was without the mic connected. With no heater voltage, tube isnt providing load so the B+ so its up at 195.
Strangely, with only the cable plugged in (no mic), the heater voltage goes down to half (~6V) of what it was without the cable...
Seems like my heater voltage is being messed with somewhere.
Any ideas?
With the mic connected, im getting no heater voltage at the test point in the power supply or on the mic. And the voltage on the 9.5 rails of the secondary drops to about hafl of what it was without the mic connected. With no heater voltage, tube isnt providing load so the B+ so its up at 195.
Strangely, with only the cable plugged in (no mic), the heater voltage goes down to half (~6V) of what it was without the cable...
Seems like my heater voltage is being messed with somewhere.
Any ideas?
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This is from you in the other thread.
I didnt understand this but when you jumper the -heater to ground, the + heater becomes DCV where it was AC before... I didnt realize this was meant to provide DC heater voltage!
Once jumpered, the tub is heating heater voltage can be easily dialed. The B+ settles in proper 120V and the bias voltage looks good.
WOW, missed that jumper!
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First time building one of these. I had no idea! Coming from amp land I assumed ac heaters.
I was confused as well. Didn't really pay attention to the heater- terminal as I was looking at the build pictures of the versions prior to my board. And they didn't have this connection. When I finished building the PSU I was measuring 0V DC but like 40V AC or so which was weird as there is a bridge rectifier on the PSU for the heater voltage. So I did a little research and found out that you need to jumper to enable the circuit to work correctly
OK, everything is working and the values are dialed (120V B+ / -1.00V bias), but the most the heater voltage can go with the heater adjust pot maxed out is 8.06V. According to the data sheet, 12AT7 needs 12.6 for series. As I'm typing this, Im realizing that this mic only uses one side of the triode! SO if I'm only heating one side does it only need 6.3? Since it has a center tap, the voltage from one side is draining to ground through the CT!!! I figured it out! I guess typing things out helps sometimes... Dialing it back to 6.3 before I fry my lovely Bowie tube.
Matador
Well-known member
The AC secondary is 9.5VAC, which is only about 12.5V after rectification, which leaves nothing for the regulator to operate on. So there isn't any way to do series heaters with the stock transformer....
However as you figured out, you only need one half of the tube, so only 6.3V is needed.
However as you figured out, you only need one half of the tube, so only 6.3V is needed.
Yep, makes sense. Thank you.
I went to the studio and did some tests with the mic today. Good news is it works great and is super quiet! No hum or other noise related issues.
Sound wise, I think there is room for improvement. Obviously with the original cheap capsule its kindof worthless doing any comments and adjustments until I get the new Heiserman capsule. That being said, there was quite a bit of low end resonance. Like everything below 200Hz was quite boomy. I did end up going with a 1uf output cap due to limited availability of the .5 so I might look to adjust that when I get the new capsule. The high end was nice and bright but not too crispy. When compressed it did become a bit harsh on some sources.
Also used the double lining head basket from Chunger due to not being able to grab the single layer version. Not sure how they differ in performance.
Will do some dialing in when I get the new capsule.
You can always rip out the mesh with a pair of pliers. I have done so multiple times and once I got the hang of it, it became a piece of cake. But honestly the mesh being there is probably best for the health of the capsule--unless spittle and particulate contributes to its sound (which is possible).
Matador
Well-known member
TBH I haven't found all too much actual difference between 0.5uF and 1uF for the coupling cap, however the 0.5uF was usually cheaper and more available. I tend to stick with Cinemag CM-2480's in my builds, so results might be different with a different transformer. I did try a 0.1uF and thought it was fine too (and much easier to obtain because that's a common coupling cap size in guitar amps). I almost always high-pass recordings on the way in between 60 and 80Hz so manybe that's why it never became objectionable too me.
Good point. I did this to an Octava 319 like 10 years ago. Forgot about that. Im not sure how much of a difference the basket would make to the resonance issues.
I used the AMI T14 in this build, but I have a CM13114 as well... According to Chunger's site, the AMI might be a bit less bright. Thanks for the wisdom on the coupling cap. I will look to other changes first. Chunger mentioned something similar over email when asking if I was ok with the substitution.
It took me about 7 seconds with a 550B to dial in the tone I wanted (high pass at 60, one shelved notch cut at 100). But without eq it felt a bit wild. Like on acoustic guitar at 2ft from 12th fret the low freq info was still huge. Sounded pretty killer on a tenor vocal though...
I put it about head height over a little parlor gretsch kit and the kick and snare lows were HUGE. Like unusable without much processing. But again the highs were smooth and lovely! haha.
What would be the biggest factor for this low end? Funny enough, the original cheap mic was quite balanced and sounded lovely. This has better top end and way more 3D detail in the midrange, but the low end...
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Matador
Well-known member
I think it would be transformer inductance: both the AMI and Cinemag are spec'd to be down only 0.2dB at 20Hz with a 10K source impedance. This is a LOT of bandwidth. The stock circuit is CCDA topology, with a cheap, low-ratio transformer. So it's naturally high-pass filtered.
I've found exactly what you are describing: this mike can act much like a "wire-with-gain" and it pretty broad-band across a lot of different instruments. This is both a blessing and a curse: meaning you can use it in a lot of situations, but it also means more EQ'ing will be necessary. If you don't want to EQ going in, and find you are always reaching for a HPF when using the mike then don't be squeamish about lowering the output cap until it's voiced the way you want.
Great points. Thank you!
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