Category: mic repairs

diy audio, mic modification, mic repairs, microphones, Vacuum Tubes

Modifying the Rode K2 mic Part 1: Tube Change

I originally intended to replace both tube and capsule on this mic, but after experimenting and comparing I ended retaining the original capsule but with a replacement tube. In this post I’ll describe the simple tube change and give some audio examples. In the next post I’ll go over the capsule change. To be clear, the capsule upgrade made the mic sound different, rather than better, and based on what I already had in my mic locker I chose to return to the original capsule.This highlighted the importance of following your ears rather than what you read on the internet!

Okay, so here’s what you need:

A Rode K2 valve microphone ( I got mine second hand off EBay for £300 which is a fair price). These are decent mics and are very useable straight out the box. They are multi-pattern, very well made with a good quality power supply, cable and shockmount (unlike the Chinese valve mics often used for mods such as the Apex 460). The case is a bit tacky but you can’t have it all.

A NOS (New Old Stock) Telefunken E88CC (6922) valve or equivalent

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I learned a lot about the vintage tube market whilst researching these. Firstly, they are not cheap! You have to be careful as there are a lot of rip-offs out there, people trying to pass off newer tubes as vintage ones but fortunately there is a wealth of info out there to help as well as several reputable dealers that get recommended by many different people. I paid approx £125 for one from NOS Tube Store , their service was great and the tube was as described. This particular one is a sought-after Telefunken made in West Germany in the 1950’s. There are many other fine vintage tube options but I didn’t have the time and money to experiment, and this tube was highly recommended by a lot of people. If you want to know more and have time to burn, just check out the forums. Different tubes will impart a different sonic character to the mic so the ultimate test is to use your ears!

How to do it

This is very easy and I would recommend you try this first and see if you like the way it sounds. I recommend you record some different sources with the stock tube before you proceed, so you can compare afterwards and understand for yourself how the mics character has changed.

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Firstly, undo the metal retainer ring at the base of the mic and remove it. Then undo the metal sleeve that forms the main body of the mic (hold the mic by the grill end and unscrew). It should then slide off to reveal the tube and circuitry of the mic as shown in the picture.

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The tube is secured in position by a plastic clip which sits on top of the tube (pointy end) and the clip is held in place by 2 screws. Using a Phillips screwdriver, loosen both these screws and fully undo 1 of them to flip the plastic clip up and out of the way.

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The tube can now be pulled carefully from its socket, a slight amount of wiggling (of the tube, not you) may help!

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Carefully insert your new tube into the socket, it only fits in one orientation, make sure it is fully engaged. Replace the plastic retaining clip over the top of the tube and tighten the screws.

To my ears the NOS Telefunken tube makes the mic more mid-focused, the low frequencies are still clear, warm and big, the highs are a little less bright and sharp but the mid presence makes the mic cut through the mix more. I also find it more pleasant to listen to than with the stock tube, but it may not suit every application. I’ve presented some comparisons here to give you an idea of the difference. All clips were recorded via the preamps on my UA Apollo interface, I’ve endeavoured to maintain the same conditions and distances between mic and source for each comparison, but there may be small variations.

Firstly, male voice (don’t worry, I’m not singing)

Stock Tube

NOS Telefunken E88CC

Female voice

Stock Tube

NOS Telefunken E88CC

Double Bass

Stock Tube

NOS Telefunken E88CC

Judge for yourself and decide if this mod is worth the money, for me it’s a yes, but I will qualify it by saying I have other flavours of mic tonality in my locker, this adds something different, and it does it rather well.

In the next post I’ll describe changing the capsule and post some examples of the difference that makes.

 

 

 

 

diy audio, mic modification, mic repairs, microphones

AKG C414B Hum problems

I thought I’d write a quick post on this in case some of you experience a similar problem, it may save you hassle and expense.

I have a pair of AKG C414B mics, they are real workhorses and as the cliche goes, the studio Swiss army knife. On top of that they are solidly built and very reliable. But sometimes things go wrong. I recently did a bit of drum recording for myself and was listening back when I noticed a faint but distinct mains frequency hum on the overheads (my 414s). I had them running through my home made Neve preamps and then into a pair of line-level inputs on my UA Apollo interface. I automatically assumed the hum was coming from my home-made preamps rather than the solid, Austrian-engineered AKG mics. I duly set out to test, swapping cables, microphones and trying various configs of phantom power on / off on each channel, until it was clear that the problem lie with one of the 414s.

Faced with an expensive repair, I thought, well, let’s open it up and see if anything obvious was up with it.

Hmmm, lots of surface mount components and no obvious signs of trouble. A quick Google search led me to this post. Not exactly the same problem but similar, so I read on. Turns out that the grill / mesh of the mic connected to ground via pin 1 of the XLR forms a Faraday Cage, which helps shield the high-impedance capsule from EMI noise in the room. The mesh makes contact mechanically and if this connection is a little dodgy, it won’t work and there will be noise. I tested the resistance between pin 1 on the XLR and the mesh, it was a variable which immediately suggests a problem. I went about gently squeezing the base of the mesh in an attempt to improve the mechanical connection. Checking again, the resistance was now consistently minimal, so time to test. I put the body back on and plugged it in, powered it up and hey presto, no noise. Phew!…. So if you’re having this kind of issue, try this first, the mic is easy enough to open , just remove the 2 screws on the base (star driver or a flat head screw driver will do) and the smaller cross head screw in the XLR connector base and then slide the body off.