Confidence / stupidity - modifying C24 headgrill 😬😬😬

[VanguardAudioLabs]

Long story short - the mk1 can’t fit the newer NOS brass CK12s I’m installing.

At some point AKG had serviced this mk1 C24 and replaced the original capsules with Teflon ones.

Someone else had serviced it and put in wrong value cathode caps (2.2 instead of 20uF), and completely decoupled chassis and signal ground.

The cable connector to the mic also had an intermittent short.

-replaced the wrong-value cathode caps with 22uF Nichicon KA (high temp audio grade).
-replaced the 180K resistor in the PSU with 500K 1W trimmer to adjust plate supply (it was running about 10V higher than it ought to).
-calibrated heater to 12.2V
-repaired cable short
-replaced some brittle tube socket wiring
-re-coupled signal and shield ground in the pattern box, power supply according to schematic.
-repaired bent pin receivers in Tuchel/Cannon connectors
-and now…I’ve modified the headgrill ever so slightly to allow the brass capsule to fit properly inside the headgrill.
-replaced tube

It’s really just the rotation lip that’s an issue. The CK12 is 33.5mm OD, and the rotation lip on the upper headgrill is 33mm ID. I used a rotary tool to carefully remove material in a 15mm wide section on the inside of the lip at 180° tangent. This JUST lets the capsule pass without compromising the structure of the upper headgrill.

Once inside, the headgrill is ≈38mm ID so it has just enough clearance for the capsules to move.

Klaus had pointed out on PSW that the later-version brass CK12s capsules (w/o serial flats) couldn’t fit in the mk1 C24. Technically correct…unless you’re dumb enough to take a dremel to the internal lip of the headgrill and have permission from the owner . Turns out it will fit just fine after that, with just enough internal clearance.

Super excited to see this C24 come back to life. It has been a bit more work than initially expected - lots of hum, poor PSU calibration, and other issues - but it should be a true gem of a mic once it’s cleaned up and the brass capsules are in.

Anyways, it certainly lends credence to the axiom of confidence and stupidity being divided by a very blurry line!

 

[klem]

Nice work! Gotta wonder how the tight headbasket impacts internal reflections. Lately I've been reflecting on internal reflections. (Just trying to tip scale towards stupidity here

 

[VanguardAudioLabs]

On Vanguard mics there's a reason we use only one layer of mesh. You can calculate the diameter mode fairly easily and we did notice a difference once we removed the internal mesh. The downside - the heavier mesh is more prone to resonance without the benefit of the internal tight mesh, and you have to be careful when installing to help prevent that.

My hunch is that's why some of the early Asian imported mics with cheap K67 clones were so "spitty" and "essy" - you have a mode in the 5-7k range on some of those tighter headbaskets with 3 layers of mesh, along with a capsule that's very peaky in those ranges. As a result you get some crazy sibilance.

 

[klem]

I hate that sound! Have you experimented with clear, neutral cure silicone on the headbasket? Unsure how it would work for your professional builds, but in my hobby world, a layer near the base has helped damp some zing and resonances in MXL 2001 / similar head baskets and even 47FET and 414 bodies.

 

[VanguardAudioLabs]

we've done RTV silicone as you've mentioned, as well as trying an internal rubber damping ring.

The RTV is incredibly messy and smells…well, like silicone. After laying all my floors in our house and running a silicone dam around the perimeter, i could be ok never smelling that again!

 

[klem]

RTV is not the same as neutral cure silicone AFAIK, and while it still smells, it's not nearly as noxious as RTV IMHO.

 

[rock soderstrom]

I hate that sound! Have you experimented with clear, neutral cure silicone on the headbasket? Unsure how it would work for your professional builds, but in my hobby world, a layer near the base has helped damp some zing and resonances in MXL 2001 / similar head baskets and even 47FET and 414 bodies.

Do you have a photo of how you did it?

 

[VanguardAudioLabs]

Do you have a photo of how you did it?

I'll see if i can find some photos from what we tried