Adding A Shure CM to Blues Blaster and Roadhouse Microphones

If you’ve got an Astatic JT-30RH Roadhouse, or a Hohner Blues Blaster microphone, you may not be satisfied by the tone. Both of these mics are good, solid shells, but the elements these came with are pretty bad for getting good tone with blues harp. If you want to improve the sound you can install a much better element. Replace the inferior stock element with a Shure Controlled-magnetic, or Controlled-reluctance element, and be amazed at the difference.

See What You’ve Got

These microphones usually come with an inexpensive crystal element installed by the manufacturer. If you’re lucky, you might instead have an Astatic MC-151, which can be quite good, but the small Japanese element is pretty poor and tinny-sounding.

You can see the difference between the two easily. If you’ve got the smaller one, you’re a good candidate for replacement.

Picking a Shure Element

Arguments abound about which Shure element is best, and the prices vary widely between them. The most available are the 99A86 models, and they sound great. Don’t overlook the 99S556 though, they are readily available and usually moderately priced. For even better tone you can consider 1950’s controlled-reluctance elements, which are usually more expensive (but I have found to have better bass response.)

For a complete history and rundown of the differences, be sure to visit Green Bullet Mics on the web. It’s got tremendous information on all the Shure elements.

Gutting The Mic

To start, clip off the wires real close to the element. The Roadhouse and the Blues Blaster are wired slightly different, but inside you’ll find a collection of wires, a ground lug with screw, and a volume control.

You’ll need a small Hex key to remove the control knob. These mics have a unique stepped hole, and proprietary dual-slotted bushing that hold the volume control into the shell. I use a pair of needle-nosed pliers to unscrew the bushing.

Now you’ll see the hole that fits the bushing. Do not lose this bushing! They are rare as hen’s teeth, and you’ll need it later on.

Replacing the Volume Control

Why do we want to replace the potentiometer (volume control)? Well, the stock potentiometer is either a 500K (not so bad), or a 1-5M model (not good at all.) These work pretty poorly with a Shure CM. You’ll have very limited control from low to high volume.  You’re better off using a 100K-250K potentiometer, which will give you the full sweep from off to full volume.

Locating the right resistance volume control, with the correct shaft and thread size took me forever. After much research, and a worldwide hunt, I discovered the part, and convinced Mouser Electronics to stock it online. The Bourns PDB181-GTR04-254A2 is 250K, and to my knowledge, the only one that fits the Blues Blaster and Roadhouse. I stock them too.

Rewiring the Microphone

I’m not going to get into the finer points of soldering, shrink tubing, and variety of layouts for the correct wiring, here’s how I do it with these mics.

I connect two wires to the ground lug (negative), and the hot wire from the connector (positive). One of the negative wires connects to the volume control, and the other goes to the element. The red wire from the connector goes to the middle lug on the volume control. Another positive wire connects the potentiometer to the lug on the element.

Installing the VC and Adding the Gasket

The Bourns volume control shaft is a bit longer than the stock one that comes with the mic. To properly fit it, I leave the washer and nut of the potentiometer on the shaft before replacing it in the shell hole. Then that all-important bushing you’ve saved fits just right.

You’ll need the right gasket to seat the Shure CM in the grill of the Blues Blaster and  Roadhouse. You can fashion your own, but I buy custom ones from Greg Heumann at Blows Me Away Productions. They run $15. You can also purchase them direct from me for $20

Cables and Connectors

The cables that come with the Blues Blaster and Roadhouse are pretty crummy. They’ll work, but eventually they’ll short out and you’ll be bummed. Do yourself a favor and replace them if you can.

Though the Roadhouse has a screw-on connector, it’s got a custom thread that does not fit the standard Switchcraft 5/8″ connector. If you want to fix that, you need the correct re-threading die. Mine comes from Latch Lake Music, and converts the thread just right.

With this modification you can use a Switchcraft adapter, or one of the fine custom harmonica cables available out there, like those Tone Defender ones from Christopher Richards at Harmonica Planet.

Blow Baby, Blow

  • Bill
    Posted at 07:45h, 23 December Reply

    Thanks James,
    Great tutorial that cuts enough of the technical fat off and gets to the point. I only had one question about which terminal was the ground on the newly installed Shure 99B86 to my JT30, but now understand a few more things. Definitely worthy of book a mark and will be referring others to you, Thanks, Bill NZ

    • JamesWaldron
      Posted at 14:56h, 23 December Reply

      Thanks Bill. Glad it worked out for you.

      • Junior Marks
        Posted at 10:53h, 20 January Reply

        Hi James. For the Hohner Blues Blaster, can you please explain how to remove the chunky steel XLR connector from the shell? I can’t see how the XLR connector has been fitted in the factory (screwed in, glued?) and would like to replace it with a Switchcraft connector. Cheers!

        • JamesWaldron
          Posted at 16:00h, 20 January Reply

          It may be hard to believe, but that connector is just glued in. I use a pair of large pliers to grab it and twist left and right until it comes loose. I have found that heating the shell up a bit helps release the glue. I use a heat gun, but a blow dryer will work too. Heat around the connector, and even inside it. That will make your job easier.


    Posted at 02:54h, 29 September Reply

    The potentiometer of the blues blaster is logarithmic, this represents a problem to control the volume.

    Do you have linear potentiometers of this type?


  • orhun keskinbıçak
    Posted at 14:07h, 03 February Reply

    Hi James,

    Firstly apologize for my bad english. this is google translate. 🙂 i dont speak english

    I collected a microphone with the narration on your site. Thank you for your help. I installed magnetic elements on the broken roadhouse (i think jt30). after painting yellow

    I had to use the original old pot of the roadhous because I couldn’t find a 250k pod

    But I could not understand whether I should use the transistor on the pot. Also, do I have to connect 39 K resistors on the Element?

    The microphone works fine now, but the pot suddenly turns on the volume after a certain level.

    Thank you, you are good.

    Orhun from Turkey

    • JamesWaldron
      Posted at 13:44h, 12 February Reply

      Using the resistors is your option. The microphone will work either way. Your volume control will be very limited without a new pot. I do have the correct 25oK pot in stock if you would like one.

      250K pot

  • Marc de la Bastide
    Posted at 18:33h, 24 March Reply

    Hi James,
    Thanks so much for the instructions to replace my cheap crystal element with a Shure CR. My mic sounds great now! One questions about the new 250k pot. I replaced the old pot with the one you suggested from Mouser electronics, but the new one does not turn the sound completely off. Is there something else I need to do? I followed your instructions, but I did not use the resister that was on the old pot.


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