For those of you wondering, “just what is a salt crystal?”, I included this picture. A crystal is just that, a crystal formed with salt that has a very thin sheet of metal foil going through the middle, or “sandwhiched” between the layers of salt crystals. There are normally two very thin copper ribbons attatched to the foil, (not shown here) that run from the salt crystal over to the elements lead tabs.
The elements diaphram is usually glued to the pin that is shown in the picture that is attatched to the crystal. Salt crystal’s are very fragile, and they are sometimes covered in wax or something else to moisture proof them. The crystal is held in place usually with glue in older elements, or by the cross shaped piece as shown in this picture in the Astatic MC-151 crystals. Just how they produce an electronic signal I’m really not sure. Crystal elements usually have an impedance that is so high that most meters can’t measure the resistance. It’s resistance is almost infinite, which is why crystal mics need to use much higher value volume pots. If you use a crystal with a volume pot, or plan on having one put into your crystal mic, try to use a pot with as high a value that you can find. Usually 2.5 to 5 Meg will work well. I have a 14.5 Meg pot in one of my crystal mics.
Most crystal mics cannot handle the high temperatures of being locked up in a car on a hot sunny day or damp environments so it’s a good idea to keep them in a cool dry place when being stored. They are also very fragile and can break easily with a good hard drop so keep a good grip on your mics when playing.
All of these things that can kill a crystal mic do not have any affect on the Shure CR and CM mics which is one of the reasons why there are still so many around today. About the only thing that will destroy a Shure CR or CM is physical damage although they can get weak if you drop it enough times to weaken the magnet. It takes quit a few drops to hurt one but it can happen. The only thing I would advise against in storing a Shure CR or CM mic is to keep it out of the back of your amp cabinet. The strong magnetic fields produced by the speaker magnets can also cause a Shure transducer magnet to weaken, causing the element to sound weak. Take good care of your Shure mics and they’ll literally last a lifetime.
This is a 1940 crystal element labeled as 99-09 that I believe was used by Shure but I can’t say I know what mic’s it was used in. I’ve never seen another like it. The frontal picture shows the crystal in an a Shure 520 gasket that was customized to fit it. It did work when it was sent to me but I don’t believe it was at full strength although it didn’t sound bad at all.
The 99 172 is another crystal that appears to be a predecessor to the 99A94, better known as the “R7”. Again, I don’t know of any particular bullet mics that used this crystal. I have 2 of them and neither one shows a distinct date on them so I’m not sure when these were used or what they were used in.
Here’s the model 99-131 that was used in the Shure 707A bullet mics from 1940 up to around 1947 or 48 before they starting using the R7 style crystal. The crystal was described as being a moisture-proofed Bimorph Rochelle Salt crystal. These crystals had a great sound with a fat bottom end. Unfortunately, they don’t stand up to the test of time very well and very few are usable these days.
The 99A94 and an early model labeled as 99A47. There was also a 99B94 but all three appear to be identical. These are the crystals better known as the “R7” crystal although the R7 is just the Shure replacement model # for these elements. These crystals are the most popular of the vintage crystals with the exception of the Astatic MC-151. These Shure crystals were very likely the best crystals ever made for use in a harp mic. The crystal is the same Bimorph Rochelle Salt crystal as used in the 99-131 crystal. They were very high gain, had a very strong mid presence with a bold, fat sounding bottom end. The high end was very clear and crisp.
As with most other Shure crystals, they didn’t withstand the test of time and they are hard to find these days at full strength. The 707A was discontinued in 1970. Some of the later models can still be found at near 100% strength although most are not. The Controlled Reluctance Transducers sound similar to these crystals if you find a good strong one, but the R7 crystals have more bottom and a crispier high end. The crystals are less gritty as well but still had great harp tone.
This is a 1940 crystal element that was removed from a model W30 (JT30 style shell). These elements were mounted with a dense foam gasket that was fitted and glued into a groove around the diameter of the crystal. These were still being made by the Brush Co. which also supplied the crystals for Shure before each company went on to patent their own particular models. The holes are much larger on the face of these early models and many did not have any cloth material underneath to protect the diaphragm.
This is the model 30 crystal that was used in the model 30 microphone, better known as the “biscuit” mic which is only slightly larger in diameter than the element itself. The biscuit mic is one of the samllest mic shells available to use with almost any good element including the Shure CR and CM elements. This model 30 crystal had a big bold sound similar to the Shure crystals. However, like most vintage crystals, finding one of these in good working order is rare these days. This particular crystal is clearly dated February 1942.
1970’s Astatic crystal elements. They were now being stamped with the Astatic logo and the MC-151 model #. They were also being stamped with an obvious date code, this one being May 1979. They were also now using a small foam button on the front to hold a screen flush to the mic grille and stopped using the phenolic piece to mount the connection tabs on.
Contrary to the caption under the picture on the left, I believe that element to be a mid or late 80’s model. Note that it does have the MC-127 cast into the back, which all the MC-151’s did have up until they went out of production. Below shows what the last of the MC-151’s looked like front and back although the stickers with the MC-151 were of various sizes.
1990’s Astatic MC-151 crystal elements. The model # is now being put on the elements with a sticker of various sizes. The front cover has a white piece of cloth behind it rather than the usual black cloth used for just about all previous elements. Many of the MC-151’s that were part of the last to be offered as replacement elements, and many that were used in the last of the Hohner Blues Blasters that came with MC-151 crystals were thin and weak sounding. The last really good MC-151’s that I purchased from Astatic were made in 1994.
This is the crystal element now being used in the current Hohner Blues Blaster mics shown front and back in the rubber gasket. It’s slightly larger in diameter than a quarter and about 1/4 inch thick. It’s a very high gain crystal but the tone is very tinney sounding and lacks low end. Not a good crystal to use with amps that easily feed back. I’ve already replaced quite a few of these with Shure CM elements. Most people don’t even want the crystals back. Hopefully Hohner will take notice and look for a quality crystal to replace it. It seems that good crystal elements are getting harder and harder to find all the time. Thank God the Shure CR and CM’s were built to last!
These photo’s show what happens to crystal elements as they deteriorate. Extreme heat, cold, moisture and physical vibration are all things that will cause a crystal to deteriorate or physically fall apart. They can absorb moisture which causes them to expand at first and eventually blow apart or crack. When this happens the mic may still work but you will notice that it has lost gain and response. They become extremely fragile when they get to this point and when they are, one good drop to the floor could be the end of it’s working days.
Storing your crystal mic in the proper type of environment can help make it last much longer. Keep crystal mic’s out of extreme hot or cold environments such as in your car or in the trunk during the hot summer and cold winter months, and keep it out of high humidity as well. Storing it at room temperature in an air tight bag or other type of container with a moisture absorbing desiccant will help your crystal mic last many years.