Threecircles Recording Studio - 12/251 Mic Mod Kit

C12/251 Tube Mic Build for Client

Soldering iron at the ready as I make two beautiful C12-inspired tube mics for a client from Cardiff.

I recently received a phone call from Cardiff. Huw had seen my blog post on modifying an Alctron MC001 microphone and wanted me to build him some mics. He was after a couple of valve/tube microphones based on the AKG C12/Telefunken ELA m251. “Sure,” I said, “Why not?”

The 12/251 mic mod kit

Threecircles Recording Studio - 12/251 Mic Mod Kit
The “12/251 Mic Mod Kit” as supplied by Microphone-parts.co.uk

Huw had chosen the silver-finish mics and had opted for a pair. Nothing else is needed as part of the build. It has everything you need except a UK mains plug as the units ship from the states.

The Mic Kits

The mics are almost identical to the Apex 460 kit I bought. A difference is that there is no branding on the PSU. Also, the mic body has received a rather fetching silver paint finish. The mic kits took a couple of weeks to arrive as they ship from the USA. There was also a £200-odd tax bill to pay this side of the pond which held up proceedings still further. It’s good that the mic boxes hold everything you need for the build. All components are in bags ready to go and a booklet contains clear, precise instructions.

Threecircles Recording Studio - 12/251 Mic Kit Components
The 12/251 tube mic kit comes with all these components to create the mic. You don’t need anything else except solder, time and patience.

 

First Things First

The first thing I did was to change the power supply unit. To do this you swap out a couple of low-quality diodes – simple enough. This also comes set for 120V mains current and needs changing.

– see pictures below:

The next main thing to do was to connect the capsule and protect it with the headbasket while I did the other work. There was no point damaging the capsule if I didn’t have to! The capsule has 2 diaphragms to give the different switchable polar patterns. There were 3 wires coming off the capsule. One each for the front and rear elements and one for the backplate. I threaded these through the plate at the bottom of the capsule saddle so they could connect to the 2 PCBs in the body.

Soldering

After I tucked the capsule away it was time to get the soldering iron good and hot. I stuffed the 2 PCBs with the supplied components. Several high-grade resistors, electrolytic capacitors and WIMA caps went first. I followed this with the larger Sonicap and input polystyrene capacitor.
At this point, there is also an optional capacitor used to tame the high-end response of the mic. I chose to include this in the circuit as My Apex 460 modified mic has this cap in and I like what it does to the mic’s response.

Threecircles Recording Studio - 12/251 PCB
Completed PCBs

 

Here you can see the completed PCBs with space for the optional 150pF capacitor on the right. I later chose to include this to tailor the mic’s high-frequency response.

Once I completed the PCBs the next major task was to wire up the mic transformer and XLR connector. This was awkward as all 9 wires need to pass both ways through the metal ring at the bottom of the mic chassis.

Sockets & Transformer

I soldered the long wires supplied onto the XLR socket along with 3 of the wires from the transformer. I soldered a small jumper several pins as well. Then I isolated the joints with the supplied heat-shrink tubing. After I did all this I stuffed the transformer into a metal dome. All the wires pass through small holes and up the inside of the mic chassis to connect to the tube socket and PCBs.

Threecircles Recording Studio - 12/251 mic transformer and XLR
Custom microphone transformer

 

The custom microphone transformer (top) and the 7-pin XLR socket that I have wired up. The long wires from the XLR go up the side of the Electro-Harmonix valve. They then connect to the tube socket and PCBs.

Wiring up

Wiring up comes next and this is awkward. Mic parts provide several short wires as well as some solid silver-core wire. These short wires connect both PCBs together as well as the 9-pin tube socket. Connections are also made from the transformer and XLR socket. These wires make their way up the mic chassis and have retaining clips to secure them away from the tube.

 

Threecircles Recording Studio - 12/251 Tube wiring
Here I have started to wire up the tube socket. There are several coloured wires to connect between the PCB, tube socket and the PCB.

 

Finishing up

Once I wired up the tube socket the wires connect to the PCBs and only a few tasks remain. Wiring the capsule up to the PCB and connecting the capsule backplate to the input capacitor. Heat can damage this so the kit comes with a copper clip to act as a heatsink when making this crucial connection. After I made a few continuity checks I fastened the tube and retaining mount and checked that the mic works. See picture below.

Threecircles Recording Studio - Finished 12/251 Tube Mic
Here you can see the finished mic with the microphone body reattached.

The mic has a lovely paint finish and looks and sounds the part!

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