C12 Clone in T.Bone SCT800 Donor Body

[mihi_fuchs]

Hi,

I extended my microphones by a pair of C12 clones. The PCBs are fitting into the T.Bone SCT800 donor microphone, a relatively cheap mic which is easy to get in Europe for a very decent price (used between 65-120 €) and it comes with a PSU, which can be modified.

As noted in other posts, tube microphones run on high and letal voltage. Even after switching them off they may hold these voltages in the capacitors for quite some time. Therefore only work on these projects if you know what you are doing and bear in mind that you should handle these things with the care and attention that they deserve!

In the link below you can find a building guide, the BOM for the PSU board and the microphone PCBs as well as a sound sample of female vocals (thanks a lot to Julia Wohlfahrt) and a drumset (thanks to Michael Golautschnig).

https://www.dropbox.com/sh/i8lscny4iviod54/AABCSr61SJeYCGhqDt1QfZkIa?dl=0
Here are some pictures of the built:



For the capsule I chose a Beijing 797 Audio Co., Ltd. Cy039 and the transformer is a 3u audio GZT-12 transfomer. The tube is a TAD 12AY7/6072A Highgrade(provided by Banzai music, order# SKU51827). The sound sample was recorded in cardioid pattern. 9 different patterns are available (Omni > 3 intermediate omni/Card > Card. > 3 intermediate Card./Fig. 8 > Fig. 8). They are picked via the switch on the PSU (see pictures below).



Last but not least the project has been highly influenced and some solutions were simply copied from members of this forum (e.g. matador, khron, poctop, chunger, and others) - thanks for all of their posts, ideas and solutions that they have shared over the times.

As usual the remnant PCBs I got left from the order will go the black market if you're interested,
Have a nice time DIYing!
all the best
mike


P.S.: Here's the link for the leftover PCBs if anyone is interested:
https://groupdiy.com/threads/leftov...t800-donor-body-pcbs-only.81199/#post-1045760_______________________________________

Addition: PCBs with UTM transformer and comparison.

Out of curiosity I changed the transformer for the UTM0512 (larger in size). I had to adopt the PCBs as you can see below. The sound is very similar. However I observed a tiny bit more output with the UTM. Guess winding ratio is not 100% the same. I upload the sound files to the Dropbox link above. Also building guide and BOM have been updated and can be used for both versions. Schematics stay the same. PSU is also unchanged.


Cheers
Mike

 

[kingkorg]

Awesome! I've been having some issues with tube mic pattern selection lately. Is there any way you could record pink noise through a speaker in omni and figure of eight of both sides of the capsule. It doesnt matter what speaker you use as long as the distance/position from the speaker is the same for all sides. Don't hurry, whenever and if you make it it's welcome.

 

[mihi_fuchs]

Awesome! I've been having some issues with tube mic pattern selection lately. Is there any way you could record pink noise through a speaker in omni and figure of eight of both sides of the capsule. It doesnt matter what speaker you use as long as the distance/position from the speaker is the same for all sides. Don't hurry, whenever and if you make it it's welcome.

Sure thing ... I'll set it up and then record. Having it played from my studio monitors should be fine, I guess?

 

[mihi_fuchs]

Awesome! I've been having some issues with tube mic pattern selection lately. Is there any way you could record pink noise through a speaker in omni and figure of eight of both sides of the capsule. It doesnt matter what speaker you use as long as the distance/position from the speaker is the same for all sides. Don't hurry, whenever and if you make it it's welcome.

BTW ... I got the same PSU running with the G7 microphone - you are also interested in the two mics comparison?

 

[mihi_fuchs]

Here's the link to the pattern samples for all others who may be interested too:

https://www.dropbox.com/sh/ds3ee5f5b7p7axq/AACYvKBk1xMXEMU1rpnrkABta?dl=0
Best wishes
Michael

 

[mihi_fuchs]

As pointed out there was a mistake in connecting the J1 connector on the Mic PCB. The building guide has been updated. Please make sure you are using building guide V2.

Thanks to micscharf for pointing out the mistake.

Best wishes and happy holidays,
Michael

 

[mihi_fuchs]

I clarified a few further point and updated the building guide also by the Version 2.0 of the PCBs.
Please use the Version 2.1 of the building guide - it applies to all versions of the PCB.

Best regards and merry Christmas,
Michael

 

[soulitude]

The pattern rotary switch for suggested in the BOM is out of stock everywhere (at least in the EU). Does anyone know of a good quality alternative to the C&K switch?

 

[Khron]

The pattern rotary switch for suggested in the BOM is out of stock everywhere (at least in the EU). Does anyone know of a good quality alternative to the C&K switch?

See if you can find something suitable here, perhaps?

https://www.tme.eu/fi/en/katalog/ro...prog&s_order=asc&limit=20&currency=EUR&page=1

 

[mihi_fuchs]

Here's the UTM version of the thing. Comparison will be uploaded soon to the dropbox link in the initial post.

 

[LevinGuitar]

The original pcb of the SCT800 can be used for the C12 circuit with few mods, and also for a multipattern mic, if I'm not mistaken, there are already installed a51M and a cap to ground for the rear diaphragm polarization connection

 

[mihi_fuchs]

The original pcb of the SCT800 can be used for the C12 circuit with few mods, and also for a multipattern mic, if I'm not mistaken, there are already installed a51M and a cap to ground for the rear diaphragm polarization connection

Yes, that's absolutely true. Few things bothered me though, and therefore I went for the full build:

- For Multipattern usage you have to modify the PSU in any case.
- If I remember right, the initial PSU had no trim pots for adjusting the voltages. As you probably change circuit and tube - at least I would have done, if I modded - you may want to readjust voltages.
- The original design used a cathode bypass filtering - it indicated to me that the PSU was not delivering very clean supply. As you remove this filtering, when modding, the PSU may be of an issue.
- Last and least - as I think that this is not the most important point - I could not exclude that the noise issue leading to the filtering system in the original design may be found in the HiZ part being on the same PCB. Therefore I went for a separated HiZ board.
- Last and very, very least: except resistors I would have replaced all components and some footprints were giving me troubles in terms of dimensions.

All in all, I decided to change the mic PCB too - it is basically a few components and I can arrange and sort everything the way I want. Additionally - I could add fixation for the transformer in the last build, which is the current version I'm using.

Nevertheless, if you alter the PSU and exchange half of the components you will be for sure in the same ballpark with the sound quality. Of course Capsule and transformer matter a lot too ...
If the PCB itself may be of a noise issue I cannot say, as I simply never tried it.

I got very happy with the mics - actually became the go to mics for our lead singer.

BR
Mike

 

[LevinGuitar]

I hope you find a decent headbasket for that big work on the mic. I found some U87 shape donors from TBone fet mics that fit, easy to give a try. Have no idea about a better C12 shape one.

When turning this chinese mics into a C12/U67 circuits I cut the cathode trace and use the cathode parts placement as part of the negative bias filtering (also, for a cardioid mic, you even have the rear connection path and parts to use for that purpose too). Not much more to do in the mic itself, as I suggested, of course the PSU requires more significate modifications.

 

[Migs 31]

Yes you can!
After trying the original design & changing to an ECC81/12AT7 Mullard (I needed to change R7).
Still not super impressed with the sound, I then tried changing it to a C12'ish type circuit.
I can be done quite easily, with a bit of thought and planning/cut tracing.
It improved the mic sonics quite a bit. I have another mic stock to compare to.

But the PSU definitely needs to be modded, just to get the Negative Bias voltage & Polar Patterns.
And I agree with Mihi, might as well change the whole PCB, cleaner/easier to work with.
I might do that next, when I have some time.
M

 

[mihi_fuchs]

I hope you find a decent headbasket for that big work on the mic. I found some U87 shape donors from TBone fet mics that fit, easy to give a try. Have no idea about a better C12 shape one.

When turning this chinese mics into a C12/U67 circuits I cut the cathode trace and use the cathode parts placement as part of the negative bias filtering (also, for a cardioid mic, you even have the rear connection path and parts to use for that purpose too). Not much more to do in the mic itself, as I suggested, of course the PSU requires more significate modifications.

cool - thansks for the hint with the head basket. sure worth a try. so far i only used the original one. it gave satisfying results with the right capsule in my ears. but for sure this and maybe checking for unwanted resonances in the body may add up quality …

if i got you right, you managed to put the u67 circuit on that pcb? or did you mean the capsule? i recently modded it to a u67 design - but i found it extremely demanding to get all parts inside. so on the original circuit board i would have never managed. additionally the psu needs a completely different design. but i may have misunderstood, sorry if i did so …

did you also change the capsule - as i think that is the weakest point in that design?
do you know if there‘s soundsamples somewhere for the mod?

thanks a lot in advance and for the hint with the head basket. ;-)
Best regards,
Michael

 

[LevinGuitar]

Not all the original U67 circuit of course, but the negative biasing way u67 use and a EF86 tube with kind of km54 deemphasis way. Sorry, no nice internal pics to show yet, have to organize a final version of the mod. But I'm using the "kk12" electret capsule and a headbasket from Alibaba (not finally installed yet, the holes are not alligned). I have ordered 10 pieces (don't know if they ship less, I didn't ask) but with all customs and shipping included, the price went to near 8-9€/piece. Fast and safe (but expensive) shipping. The shape and mesh are kind of a mix between U67 and C12. Wonder if there is a good way to get rid of the upper body ring, or just paint it as it is.
I remember I had some noise issue with the original capsule so I went electret because of that. Not sure where it came from

The headbasket are built by
Anping County Huihao Hardware Mesh Product Co.
https://spanish.alibaba.com/p-detail/colored-1600751746653.html

 

[LevinGuitar]

Easy way to get rid of the upper body ring out of the visual: flip the body upside down and carve the respective hole with a lime. Paint will be next step