Microphone calibration- GR 1559-B

[Conrad Hoffman]

It's metrology, so I'll put it here. I've wanted one of these for a while, both to calibrate microphones and to better understand reciprocity calibration. It's pretty well tuned up, save for one dial vs. potentiometer setting that's a bit difficult. Fortunately that mode isn't essential to operation. The 1965 manual that's available online has no calibration information. I'm hoping somebody might have a later manual that they could scan, if it has the info. I contacted IET, who took over the GR line long ago, and they said in the 20 years they've servicing GR equipment, they've never once seen a 1559!

I did a page on the instrument here- http://www.conradhoffman.com/gr01.htm
The 1559 is a few items down, under the hummer.

Thanks,
Conrad

[GLouie]

I don't know about the unit specifically, but the red "S" on the mic looks like the old Shure logo. You might try contacting Shure. They have a rather good archive, employing at least 2 archivists. They might have info on supplying GR. Unless someone substituted the mic on your unit.

Shure archives article:
http://blog.shure.com/mysteries-and-treasures-in-the-shure-archives/

[Conrad Hoffman]

Interesting idea. The mic is original, as the older GR catalogs show exactly that same logo on their mics. The mic is buried in the back of the PZT cavity, so the unit would likely become a doorstop before anybody went that deep to fix it.

[SilverSolder]

Interesting test gear for sure! 

By coincidence, I bought my first Genrad item the other day - a 1454-AH resistance decade divider.   After cleaning it up, a quick test drive showed that it still comfortably met its specification...  not bad, after 50 - 60 years!  Got to hand it to whoever made those resistors.

And stable! - that was the most surprising thing.  Rock solid output.  I guess when a divider is rated for 700 volts, it isn't even going to notice a modern 10V reference!

Seems to me that Genrad puts a new meaning to the term "over-engineered"...   I have never seen rotary switches this big, coated with enough silver to pay off the national debt (just about)!

Is the GR 1559-B similarly over-engineered inside?

[GLouie]

Interesting idea. The mic is original, as the older GR catalogs show exactly that same logo on their mics. The mic is buried in the back of the PZT cavity, so the unit would likely become a doorstop before anybody went that deep to fix it.

A place like Shure might know all about the device, especially if their mic was in it.

[Conrad Hoffman]

@SilverSolder- you should look at my GR pages for each instrument if you like GR stuff. The 1559 is certainly built to the GR standard, though there's no reason for big low resistance switches. They used nice ceramic wafers instead. Everything else is rock solid. You should see the insides of their 1608 bridge- a mechanical wonder. Be sure to read the GR history on the IET Labs site. They modeled themselves after Leitz in Germany and over-engineered the heck out of everything. They were also way ahead of their time in terms of employee treatment and benefits. It paid off until it didn't, like so many other classic companies.

edit- I found someone to measure the resistance of their pot at several dial settings, so I'm all "set" now.

[texaspyro]

I have a couple of GR-1567 sound level meter calibrators.   They generate a precise tone (1000 Hz, 90dB?) tone.  They are a cylindrical shape with a metal "cup" at the top where you seat the sond level meter microphone.

Caution,  the cup at the top is removable.  It is held in by three ball bearing detents under a rubber o-ring.  That o-ring can degrade over time and break...  then you get to play "find the frickin' balls".

[VintageNut]

Hello Conrad

Interesting acquisition. Great job describing the operation of the instrument on your website.

I own a calibrated capsule and some associated preamp gear.

I had the capsule calibrated at PCB Piezoelectronics. They use something like your acquired instrument but my guess is that it is more up-to-date and more automated. They may be a resource if you find a way to strike up a conversation.

The capsules are not super expensive to acquire used. Many times the capsules are 40+ years old but work perfectly if they were treated gently.

A calibration is also not super expensive.

[Conrad Hoffman]

Yes, I know PCB and they're not far away from me. I talk to them now and then, usually about accelerometers for work, though I also have a collection in the lab at home. They seem to have gotten into acoustics in a big way lately, so they're definitely worth a call. Speaking of accelerometers, I also have the GR accelerometer calibrator. Also the tone calibrator.

[VintageNut]

Yes, I know PCB and they're not far away from me. I talk to them now and then, usually about accelerometers for work, though I also have a collection in the lab at home. They seem to have gotten into acoustics in a big way lately, so they're definitely worth a call. Speaking of accelerometers, I also have the GR accelerometer calibrator. Also the tone calibrator.

An accelerometer calibrator sounds very cool. I have a small collection of accelerometers used for attaching to a loudspeaker to look at linearity of travel versus current input.

[Conrad Hoffman]

I've updated the page and included an Excel spreadsheet with some useful information. Enjoy.
CH

[Conrad Hoffman]

Emailed with Shure and, surprise, they made the mics for General Radio. I don't know for how long or when and if GR might have brought production in-house, but the GR mics are obviously a special version of the Shure 98A108A, that you can find a datasheet for online. Updated my GR 1559-B page with more into on this, photos of the adapter and other stuff. Enjoy.

[GLouie]

Nice! It's shame detailed historical information gets lost so often.

(I think there's a typo on Michael Pettersen's name)

[Conrad Hoffman]

Ooops, fix him! I also decided that I don't understand the calibration of the meter yet, so deleted that stuff from the spreadsheet. Calibration is certainly a piece of historical info that would be nice to have.

edit- 1/29 I keep uploading new versions of the spreadsheet and I think the calibration section is close now.