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C12/251 Tube Mic Build for Client

by Mar 11, 2016

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 KitThe “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 ComponentsThe 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.


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 PCBHere 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.
Mic Transformer & XLR connector

Threecircles Recording Studio - 12/251 mic transformer and XLRThe 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.

Tube Socket Wiring

Threecircles Recording Studio - 12/251 Tube wiringHere 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.
Finished mic

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

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

Threecircles Recording Studio
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Richard Simmonds
Richard Simmonds
Adrian, thank you for a great day and a half of recording finishing last Wednesday. Having been together as a group... back in the 60s and now getting together 50 years on to record was a bit nerve racking but you made the thing so much easier with your professional advice and guidance. You've got a great set up and since we had Rog playing flute and drums [for some reason not at the same time!] the layering down of the instruments went really well. We had a great time and ended up with a CD and lots of happy memories. Martin living in France doesn't make it easy but we have all vowed to be back with something completely different for you to cut to disc. Thanks again and see you soon. Sam, Andy, Roger and Martin of Old Lemons fame.read more
Richard Ellis
Richard Ellis
This is a relaxed studio in a lovely location near the beautiful town of Saffron Walden. Adrian is a great sound guy,... he listens and responds and adds enthusiasm to the day. Top recordings with the best equipment for a great price ... highly recommended ��read more
Ashley 'mAsh' Garbett
Ashley 'mAsh' Garbett
Having recorded at Threecircles several times, there's good reason why I keep going back... with such high end... equipment, Ade's expertise and competitive pricing there's no need to go anywhere else!!read more
Dan Mailer
Dan Mailer
Three Circles is excellent. Great studio that I have recorded at various times over the year. Ade is a hard working... producer who gets great results.read more
Victoria Mills
Victoria Mills
I've both recorded and rehearsed at Threecircles and I can't recommend them enough! Nothing was too much trouble, the... facilities are excellent and our demo sounds amazing! Thank-you!read more
Steve Watts
Steve Watts
Loved working with the chaps at Three Cirlces, they have all the gear and every idea! Super talented guys who get the... very best out of you! 5/5read more
Rob Clark
Rob Clark
Very professional sound, genuinely listens to what you want and the sound you wish to create, and makes it better in... every way! Highly recommend!read more
Stephen Ellis
Stephen Ellis
Recorded the last Kaine single at this studio; very professional set up, Ade is a great guy and the coffee is also good.
Gemma Marner
Gemma Marner
I've always loved recording at Three Circles - lovely, comfortable space and surroundings and Adrian is so easy to work... with. Would recommend it in a heartbeat!read more
Kennedy Clark Herring
Kennedy Clark Herring
Truly professional attitude and highly capable. Recommending these guys for studio work as well as live sound... Cheers... guys, appreciate the extra effort you put in,making us sound truly amazing. 5 stars : )read more
Mac Kinnon
Mac Kinnon
Ade at 3C is a true hero. Aside from having professional equipment, incredible understanding and knowledge of music... production and a brilliant studio - he is a real genuine man. If I could I'd give this studio 6*, Ade recently recorded a single for my band completely for free because he knew we were doing it for charity.read more
Matt Milligan
Matt Milligan
Excellent recording studio, highly recommended!