Starting a C12 clone

I'm just starting assembly of my C12 clone & thought it might be interesting to share some pictures.

I was lucky that Laurent (Group member pyjamaman), was kind enough to supply me with a piece of brass tube that was the perfect fit inside the outer tube.  Getting this tube enabled my cousins husband Keith to turn some other case components for me.  Volker also helped me obtain some of the high value resistors from mouser.

I'm going to try using a modified Tbone PSU from a mic I used as a donor for another project.  After changing a couple of zeners it seems to be perfect for the job.  The original safety earthing looked rather dangerous, & I replaced that.  The case earth being a rather thin wire that connected to the regulator PCB & from there to the case !!!!


Capsule is courtesy of Tim Campbell, & o/p transformer from AMI.

Looking forward to seeing this progress!

I've been collecting parts for a build like this as well. Did you build the mic body & grill or is that a T Bone?

dmp said:

I've been collecting parts for a build like this as well. Did you build the mic body & grill or is that a T Bone?

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They are real ones.

Yay! Go Rob ... I like those Russian caps.

MagnetoSound said:

Yay! Go Rob ... I like those Russian caps.

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I thought I'd buy 2 so I had a spare, not realizing that I was buying 2 strips of 10,  Can't believe how cheap they were.

Looks wonderful,

Could you post the complete list BOM,

And where did you order it all, i see you're from the UK, i'm in EU as well so would be nice to know where you ordered from...

Thanks
Manu

They are real ones.

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Cool!

Yay! Go Rob ... I like those Russian caps.

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I did some cap comparisons with a 5840 circuit and I didn't like the K40 PIO as much as Mallory 150s... go figure...

Nice! How are you going to arrange the biasing with the tbone PSU, have you modified it to provide the negative voltage to the grid like in C12?

pasarski said:

Nice! How are you going to arrange the biasing with the tbone PSU, have you modified it to provide the negative voltage to the grid like in C12?

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Not really understanding what you're saying ?  The negative grid supply is obtained by elevating the cathode using the filament supply, unless I'm missing something  ?

Not really understanding what you're saying ?  The negative grid supply is obtained by elevating the cathode using the filament supply, unless I'm missing something  ?

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In the original schematic there is a 4k resistor between the grounds of the B+ psu filtering caps that lowers the grid voltage with respect to the cathode, if I recall correctly... I think there's a copy of the schem in the technical library.

Could you post the dimensions of the original body? Diameter& grill height? What is it made of, brass?

dmp said:

Not really understanding what you're saying ?  The negative grid supply is obtained by elevating the cathode using the filament supply, unless I'm missing something  ?

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In the original schematic there is a 4k resistor between the grounds of the B+ psu filtering caps that lowers the grid voltage with respect to the cathode, if I recall correctly... I think there's a copy of the schem in the technical library.

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Yes what I ment. Here's the schem:  http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315

I don't know if elevating the cathode from the filament supply will fundamentally affect the sound, but for what I've read,  noise will be an issue with the standard tbone three leg regulator heater supply.

pasarski said:

dmp said:

Not really understanding what you're saying ?  The negative grid supply is obtained by elevating the cathode using the filament supply, unless I'm missing something  ?

Click to expand...

In the original schematic there is a 4k resistor between the grounds of the B+ psu filtering caps that lowers the grid voltage with respect to the cathode, if I recall correctly... I think there's a copy of the schem in the technical library.

Click to expand...

Yes what I ment. Here's the schem:  http://www.groupdiy.com/index.php?action=dlattach;topic=44737.0;attach=8315

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Correct - the cathode is at ground, grid is made negative.

It's 1k btw, not 4k. It just looks like a 4 in that italic handwriting they used.

Correct - the cathode is at ground, grid is made negative.
It's 1k btw, not 4k. It just looks like a 4 in that italic handwriting they used.

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Ahh, thanks for clarifying that.

Note that the C24 does away with this biasing method and uses a 2.5k and 20uf cap on the cathode with the grid at ground. This is what I would try first.

OK I understand.  It's a shame these AKG diagrams aren't a bit clearer visually.

I think this Tbone supply is modifiable to the original C12 spec, but I may just try the C24 method purely because it's easier.

Managed a bit more work today.

The skeleton bit I'm making out of the brass tube Laurent supplied, but I'm guessing the real thing is steel like on my C28's

That generic TBone PSU wont work with fixed bias. Too much ripple. You have to mod it into something like the MK7 heater PSU with a choke plus RCRCRC stages. And you still have to place the transformer outside the case....

Telefunk,

Is that why you opted for cathode bias in your clone ?

Rob Flinn said:

Telefunk,

Is that why you opted for cathode bias in your clone ?

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Not really, cathode bias sounds good, too.

telefunk said:

Rob Flinn said:

Telefunk,

Is that why you opted for cathode bias in your clone ?

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Not really, cathode bias sounds good, too.

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I guess, but so does a real C12.............